I85PREP
⌘ K
ASE H8 Preventive Maintenance & Inspection (Transit Bus) study materials
ASE H8 Preventive Maintenance & Inspection (Transit Bus) study resources

ASE H8 Preventive Maintenance & Inspection (Transit Bus) practice questions

One question at a time. Build your ASE H8 Preventive Maintenance & Inspection (Transit Bus) understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Vehicle Inspection, Documentation, and Preventative Maintenance Programs

In a transit bus preventive maintenance program, what is the primary purpose of scheduling inspections at set mileage and time intervals?

  1. To replace every wear part at each service visit
  2. To catch wear before it becomes a breakdown Correct answer.
  3. To reduce the total number of spare parts stocked
  4. To repair defects only after a driver reports them

Explanation

Preventive maintenance is planned work performed at mileage or time intervals so that wear, leaks, and adjustment drift are found and corrected before they cause a road failure. Intervals are set from manufacturer recommendations plus fleet operating experience, not from driver complaints or parts convenience.

Question 2Vehicle Inspection, Documentation, and Preventative Maintenance Programs

Under the federal motor carrier inspection rules that transit agencies follow, when is a driver required to prepare the post-trip vehicle inspection report?

  1. Before every trip, in place of a walk-around
  2. At the end of each day of operation Correct answer.
  3. Only when a defect is found during the trip
  4. Only when the bus is due for scheduled service

Explanation

The post-trip report is prepared at the end of each day the vehicle is operated, so defects that appeared during the day are documented and can be corrected before the vehicle is dispatched again. The next driver must review that report and sign it before taking the vehicle out.

Question 3Vehicle Inspection, Documentation, and Preventative Maintenance Programs

A driver is beginning a pre-trip inspection of a transit bus. Which item belongs on that walk-around checklist?

  1. Brake and parking brake application Correct answer.
  2. Overhead valve clearance readings
  3. Brake drum measurement history sheet
  4. Engine oil analysis laboratory report values

Explanation

The pre-trip inspection covers items the driver can see, hear, feel, or test in a short walk-around and brake application check, including service brake and parking brake operation. Laboratory results, drum measurements, and internal engine clearances belong to shop maintenance records and are not driver pre-trip items.

Question 4Vehicle Inspection, Documentation, and Preventative Maintenance Programs

During preventive maintenance on a transit bus air brake system, a technician performs a static leakage test. What is the purpose of that test?

  1. To measure the compressor's oil consumption rate
  2. To set the automatic slack adjuster stroke
  3. To confirm that pressure loss is within limits Correct answer.
  4. To verify ABS wheel speed sensors are calibrated

Explanation

A static leak test charges the system, shuts off the engine, and watches pressure drop with and without the brakes applied. Excessive drop points to a leak that must be located and repaired before the bus is released.

Question 5Vehicle Inspection, Documentation, and Preventative Maintenance Programs

What is the minimum legal tread depth for a steer axle tire on a commercial bus?

  1. 6/32 inch
  2. 2/32 inch
  3. 4/32 inch Correct answer.
  4. 1/32 inch

Explanation

Federal rules require at least 4/32 inch of tread on every front tire and at least 2/32 inch on all other tires. Many agencies set a deeper replacement point for wet traction and casing life, but 4/32 inch is the regulatory floor for steer tires.

Question 6Vehicle Inspection, Documentation, and Preventative Maintenance Programs

How long must a transit operator keep maintenance records showing inspections, repairs, and lubrication for a vehicle that is in service?

  1. 36 months
  2. 12 months Correct answer.
  3. 24 months
  4. 6 months

Explanation

The maintenance record requirement is that records documenting inspections, repairs, and maintenance be kept for 12 months while the vehicle is in service, and for 6 months after the vehicle leaves the fleet. Records must identify the vehicle and the nature of the work.

Question 7Vehicle Inspection, Documentation, and Preventative Maintenance Programs

A transit fleet's oil analysis shows a sharp rise in iron and chromium while silicon remains steady. Which condition does that pattern most likely indicate?

  1. Fuel dilution from excessive idling
  2. Air filter housing leakage
  3. Coolant leaking into the oil sump
  4. Cylinder liner and ring wear Correct answer.

Explanation

Chromium is a marker for ring and liner plating, and iron tracks cylinder liner and crankshaft wear, so both rising together points to internal wear of the liner and ring package. A coolant leak would raise sodium or potassium and show glycol, while dirt ingestion would raise silicon.

Question 8Vehicle Inspection, Documentation, and Preventative Maintenance Programs

A transit bus wheel has just been reinstalled. What is the correct way to verify lug nut torque?

  1. Torque in sequence, then re-torque after the set interval Correct answer.
  2. Torque each nut once in sequence and release the bus to service
  3. Recheck torque only if the driver reports a vibration
  4. Tighten the nuts with an impact wrench until they stop turning

Explanation

Lug nuts are torqued to specification in a cross or star sequence, and the fasteners are re-checked after the manufacturer's specified re-torque interval because seating and settling reduce clamp load. Skipping the re-torque step risks loosening and wheel loss.

Question 9Vehicle Inspection, Documentation, and Preventative Maintenance Programs

Which check belongs in the preventive maintenance inspection of a wheelchair lift on a transit bus?

  1. Measure the lift's hydraulic fluid color only
  2. Confirm the lift platform is painted to match the bus
  3. Verify a full lift cycle with the interlock working Correct answer.
  4. Check the lift can be deployed while the bus moves

Explanation

The lift must be cycled through its full range, including the interlock that prevents movement unless the vehicle is secured, because the interlock is a safety function that fails silently. Fluid color alone does not confirm operation of the platform, interlocks, or roll stops.

Question 10Vehicle Inspection, Documentation, and Preventative Maintenance Programs

During PM inspection, a technician finds the gauge needle on a bus fire extinguisher resting in the red zone. What is the correct action?

  1. Top it off with compressed air from the shop supply
  2. Note it for the next service and leave the bus in service
  3. Remove it from service and replace or recharge it Correct answer.
  4. Shake the extinguisher and return it to the bracket

Explanation

A gauge in the red zone means the extinguisher may not discharge properly, so it is removed from service and replaced or recharged by a qualified provider. A bus should not be dispatched without a functional extinguisher of the required rating.

Question 11Vehicle Inspection, Documentation, and Preventative Maintenance Programs

A coolant test strip on a bus with a wet-sleeve diesel engine shows nitrite below the required level. What risk does this create?

  1. Corrosion of the aluminum wheels
  2. Liner cavitation pitting Correct answer.
  3. Freeze damage to the radiator core
  4. Air lock in the heater circuit

Explanation

Supplemental coolant additives, including nitrite, form a protective film that resists cavitation bubbles collapsing against the liner wall. When nitrite is low, the liner surface can be pitted through until coolant reaches the cylinder.

Question 12Vehicle Inspection, Documentation, and Preventative Maintenance Programs

What must be verified about the emergency exits during a transit bus PM inspection?

  1. That the exit windows are tinted to reduce glare
  2. That each exit opens freely and its alarm sounds Correct answer.
  3. That the exits are locked to prevent misuse
  4. That the exit decals are the correct font size

Explanation

Emergency exits must open from inside with the normal release force, and the audible alarm must sound so passengers and the driver know an exit has been opened. Exit decals must be present and legible, but font style is not the functional test.

Question 13Vehicle Inspection, Documentation, and Preventative Maintenance Programs

After a technician completes a repair on a transit bus, what must be recorded on the work order?

  1. Repair performed, parts used, technician ID Correct answer.
  2. Only the driver's original complaint
  3. Only the date the repair was finished
  4. Only the parts cost for the repair

Explanation

The work order must identify the vehicle, the defect or complaint, the work performed, the parts installed, and who did the work, so the repair can be traced and audited later. Partial entries break the chain of documentation the maintenance program depends on.

Question 14Vehicle Inspection, Documentation, and Preventative Maintenance Programs

A bus air system keeps collecting water in the wet tank at daily drain, and the air dryer is past its service interval. What is the most likely cause?

  1. The compressor is oversized for the system
  2. The governor cut-out pressure is set too low
  3. The air dryer desiccant is saturated Correct answer.
  4. The safety valve is opening too early

Explanation

The air dryer removes moisture before it reaches the tanks, and a saturated desiccant or failed purge valve lets water pass through. Servicing the dryer at the recommended interval restores moisture removal and protects the rest of the air system.

Question 15Vehicle Inspection, Documentation, and Preventative Maintenance Programs

A technician measures a brake drum and finds it is at the discards diameter stamped on the drum. What action is required?

  1. Replace the drum; it has reached its discard limit Correct answer.
  2. Machine it to the next oversize and reuse it
  3. Reduce lining thickness to compensate for the size
  4. Reinstall it; the stamp is a manufacturing dimension

Explanation

The stamped dimension is the maximum allowable inside diameter, beyond which the drum loses strength and cannot dissipate heat safely. A drum at or beyond that limit must be replaced, and it must not be machined further.

Question 16Vehicle Inspection, Documentation, and Preventative Maintenance Programs

During PM inspection, a technician finds a bus battery with a bulging case and electrolyte weeping from the vents. What is the correct action?

  1. Clean the terminals and continue the inspection
  2. Slow-charge the battery overnight and retest
  3. Add distilled water and return the bus to service
  4. Remove the battery from service and replace it Correct answer.

Explanation

A bulged case with electrolyte weeping indicates internal damage or overheating, and the battery can fail without warning or present a safety hazard. It is removed from service and replaced rather than topped off or recharged.

Question 17Vehicle Inspection, Documentation, and Preventative Maintenance Programs

Which condition found on a drive belt during inspection calls for replacement?

  1. Dust from normal belt wear
  2. A part number printed on the back
  3. Light contact marks from the pulley
  4. Cracks running across the ribbed side Correct answer.

Explanation

Cracks across the ribs mean the belt material has aged and lost flexibility, and rib material can separate and throw the belt. Normal dust, printed markings, and light pulley contact marks are not reasons to replace a belt.

Question 18Vehicle Inspection, Documentation, and Preventative Maintenance Programs

Why do transit agencies set chassis lubrication intervals in both miles and months?

  1. Because lubricants expire on a fixed date
  2. Because lubricants and seals age even at low mileage Correct answer.
  3. Because the shop calendar drives parts ordering
  4. Because mileage-based intervals are no longer accepted

Explanation

Grease and seals degrade from heat, oxidation, and contamination over time as well as from use, so a low-mileage bus can still need service on the calendar interval. The program applies whichever limit comes first.

Question 19Vehicle Inspection, Documentation, and Preventative Maintenance Programs

A steer tire on a transit bus shows a feathered tread pattern with sharp edges along one side of each rib. What does this most likely indicate?

  1. Incorrect toe adjustment Correct answer.
  2. Excess wheel balance weights
  3. Underinflation
  4. Worn shock absorbers

Explanation

Feathering occurs when the tread is dragged sideways as it rolls, which is typical of toe-in or toe-out error. The sharp edge faces the direction of the scrub, and the fix is an alignment check rather than a tire change alone.

Question 20Vehicle Inspection, Documentation, and Preventative Maintenance Programs

A rear air spring on a transit bus shows cracked rubber and a bubble on the sidewall. What should be done?

  1. Lower the ride height to reduce the stress
  2. Patch the air spring with a tire repair kit
  3. Clean the area and monitor at the next PM
  4. Replace the air spring before dispatch Correct answer.

Explanation

A bubble means the fabric or rubber has separated, and the spring can rupture at any time, dropping the suspension and affecting brake and driveline geometry. Air springs are replaced, not patched, and the bus stays out of service until the repair is done.

Question 21Vehicle Inspection, Documentation, and Preventative Maintenance Programs

How should a technician verify the turn signals and four-way flashers during a PM inspection?

  1. Check the fuse panel for blown fuses and relays only
  2. Ask the driver whether the lamps worked
  3. Look at the dash indicator lamps only
  4. Operate each function and walk around the bus Correct answer.

Explanation

Each lighting function is operated and the technician walks the bus to confirm every lamp at each corner actually flashes, because dash indicators and fuses do not prove the lamp is lit. This catches failed bulbs, bad grounds, and corroded sockets.

Question 22Vehicle Inspection, Documentation, and Preventative Maintenance Programs

A fleet wants to extend engine oil drain intervals on its transit buses. Which approach provides the best evidence that the longer interval is safe?

  1. Trending oil analysis results at the longer interval Correct answer.
  2. Following the interval used by another agency with different routes
  3. Extending intervals only on the newest buses without testing
  4. Comparing the cost of oil against the cost of engines

Explanation

Oil analysis at the proposed interval shows whether viscosity, wear metals, soot, and additive levels stay acceptable in actual service. Cost comparisons and borrowed intervals from other duty cycles do not show what is happening inside this fleet's engines.

Question 23Vehicle Inspection, Documentation, and Preventative Maintenance Programs

A technician replaces the brake shoes on a transit bus. What must be done before the bus returns to service?

  1. Back off the slack adjusters to prevent drag
  2. Road test and check brake performance Correct answer.
  3. Paint the drums and release the bus
  4. Disconnect the ABS to prevent fault codes

Explanation

After a brake reline, the technician performs a road test and checks that the brakes apply and release properly, adjust correctly, and pull evenly. Shoes also need a burnish cycle so the lining seats against the drum.

Question 24Vehicle Inspection, Documentation, and Preventative Maintenance Programs

Who must certify that a reported vehicle defect has been repaired or that repair is unnecessary before the bus is operated again?

  1. The parts room clerk
  2. The driver who wrote the report
  3. The dispatcher on duty
  4. The maintenance technician or shop supervisor Correct answer.

Explanation

The carrier must certify the disposition of a reported defect, and that certification is made by the maintenance technician or supervisor who inspected and repaired the vehicle. The certification appears on or with the driver's report so the next driver can see the status.

Question 25Vehicle Inspection, Documentation, and Preventative Maintenance Programs

A transit fleet sees a high rate of repeat road calls for the same fault within a week of repair. Which change most directly addresses the problem?

  1. Increasing the parts inventory for that component
  2. Adding more buses to the spare ratio
  3. Reviewing root cause and verifying each repair Correct answer.
  4. Shortening the PM interval for every vehicle

Explanation

Repeat failures usually mean the root cause was not identified or the repair was not verified, so a root-cause review plus a quality check at release targets the actual cause. Adding spares or shortening intervals treats symptoms and raises cost without fixing the repair process.

Question 26Vehicle Inspection, Documentation, and Preventative Maintenance Programs

A transit agency assigns a group of buses to a downtown circulator route with frequent stops, heavy passenger loads, and long periods of low-speed operation. How should the preventive maintenance inspection intervals for these buses compare with those of identical buses running a low-stop suburban route?

  1. They should stay identical because the buses are the same model
  2. They should be shortened to match the harsher duty cycle Correct answer.
  3. They should be extended to reduce shop visits and labor hours
  4. They should be set by the calendar only, regardless of route

Explanation

Frequent stops, heavy loads, and sustained low-speed operation are severe-duty conditions that accelerate component wear and fluid degradation. Manufacturer PM schedules typically publish separate intervals for severe duty, so the operator should shorten the mileage or time intervals accordingly. Route assignment and duty cycle, not the bus model alone, should drive the interval choice.

Question 27Engine and Engine Support Systems

When checking the engine oil level on a transit bus during a pre-trip inspection, the technician should:

  1. With the front of the bus raised on a lift to reach the dipstick
  2. With the engine idling at high idle and the dipstick removed
  3. Immediately after shutdown before oil drains back from the upper engine
  4. Engine off, wait for drain-back, and check on level ground Correct answer.

Explanation

Oil drains back to the pan within a few minutes after shutdown, so the dipstick reads accurately only when the engine is off and the vehicle is level. Overfilling aerates the oil, while underfilling can starve the oil pickup. Checking while running or on a slope gives a false reading.

Question 28Engine and Engine Support Systems

Which condition found during a serpentine belt inspection requires belt replacement rather than continued service?

  1. Multiple cracks across the ribs with a glazed surface Correct answer.
  2. Belt width wear of 0.5 mm compared with a new belt
  3. A light dust film on the ribbed side
  4. Belt dressing residue on the belt edges

Explanation

Cracks that run across the ribs and glazing on the sidewalls show the belt is aging and slipping, which can lead to a roadside failure. Minor surface dust or edge wear within OEM limits does not condemn a belt. Belt dressing masks slip and is not a repair.

Question 29Engine and Engine Support Systems

The air cleaner restriction indicator on a transit bus has latched at the maximum allowable restriction. What is the correct action?

  1. Replace the element and reset the indicator after installation Correct answer.
  2. Reset the indicator and continue until the next scheduled PM interval
  3. Replace the air filter element immediately regardless of mileage
  4. Clean the element with compressed air and reinstall it

Explanation

The restriction indicator latches at the highest restriction seen, so it must be reset after the element is replaced or it will read falsely. Paper elements are not cleaned with compressed air because that can rupture the media. Replacing an element that has not reached restriction wastes money and can increase initial dust passage.

Question 30Engine and Engine Support Systems

During inspection of the engine mounts on a rear-engine transit bus, which finding indicates a failed mount?

  1. Slight rubber compression under engine torque during a loaded acceleration test
  2. A thin film of oil mist on the rubber surface
  3. Light surface rust on the mount bracket and bolts
  4. Rubber torn away from the metal plate Correct answer.

Explanation

A mount whose rubber has torn away from the metal allows the engine to move excessively, stressing hoses, wiring, and the driveline. Surface rust or oil mist alone does not condemn a mount. Normal compression under torque is expected behavior.

Question 31Engine and Engine Support Systems

Why must water be drained from a diesel fuel/water separator at the recommended interval?

  1. Water improves fuel lubricity at low temperatures
  2. Water can damage the high-pressure injection pump and injectors Correct answer.
  3. Water raises the fuel cetane number above specification
  4. Water increases fuel filter life by keeping the element wet

Explanation

Water is denser than diesel and collects at the bottom of the separator, where it is drained before it reaches the high-pressure pump. Water provides no lubricity and causes corrosion and seizure in pumps and injectors. Cetane and filter life are not improved by water.

Question 32Engine and Engine Support Systems

Which check confirms that an engine block heater is functioning during a cold-weather inspection?

  1. The heater element glows red when viewed through the coolant port
  2. The block heater circuit breaker trips when power is applied
  3. Coolant temperature rises slightly after several hours Correct answer.
  4. The heater cord feels warm to the touch immediately after plugging in

Explanation

A functioning block heater warms the coolant so the engine starts easier and reaches operating temperature faster in cold weather. A cord may be slightly warm from current flow, but that does not prove the element heats the coolant. A tripping breaker or a glowing element indicates a fault, not normal operation.

Question 33Engine and Engine Support Systems

When the malfunction indicator lamp is illuminated on a transit bus, the technician's first step should be to:

  1. Disconnect the batteries to clear all codes and road test
  2. Clear all codes and release the bus if the lamp goes out
  3. Retrieve stored fault codes and freeze-frame data before clearing Correct answer.
  4. Replace the sensor that is easiest to access

Explanation

Stored codes and freeze-frame data record the conditions at the time of the fault and are lost if memory is cleared first. Clearing codes before diagnosis can hide an intermittent problem. Parts should not be replaced without confirming the fault.

Question 34Engine and Engine Support Systems

Coolant level on a transit bus with a pressurized surge tank should be checked:

  1. By opening the radiator drain and measuring the volume recovered
  2. With the engine at operating temperature and the pressure cap removed for a visual check
  3. Only when the low-coolant warning lamp is on
  4. Cold, with the coolant at the marked tank level Correct answer.

Explanation

Coolant expands when hot and the system is under pressure, so the cap must never be opened at operating temperature. The cold level at the marked line confirms the system is filled correctly. Checking only when a warning lamp is on allows an overheat to occur.

Question 35Engine and Engine Support Systems

To obtain a representative engine oil sample for trend analysis, the technician should:

  1. Draw the sample from the oil gallery at operating temperature Correct answer.
  2. Take the sample from the drain plug after the oil has cooled overnight and settled in the pan
  3. Collect the first oil that drains from the pan at the start of an oil change
  4. Sample from the top of the oil filter housing before the engine is started

Explanation

Oil analysis is most representative when the oil is at operating temperature and has been circulating, which suspends wear metals and contaminants. The first oil out of the pan carries sludge that skews results. Cold, settled oil does not represent the circulating fluid.

Question 36Engine and Engine Support Systems

A refractometer shows a coolant freeze point of -12 C (10 F) in a fleet that specifies -34 C (-30 F). What is the correct action?

  1. Record the reading and continue service because freeze protection is present
  2. Correct the concentration with the specified premix and recheck Correct answer.
  3. Add pure antifreeze until the reading reaches specification without measuring the mixture
  4. Drain the cooling system and refill with water only

Explanation

A freeze point warmer than the fleet specification means the coolant is over-diluted with water and will not protect the engine in cold weather. The concentration should be corrected with the specified premix and rechecked with a refractometer. Adding pure antifreeze without measuring can push glycol above the recommended maximum.

Question 37Engine and Engine Support Systems

Which observation on an automatic belt tensioner indicates it should be replaced?

  1. The pulley spins freely with the belt removed and only slight drag
  2. The tensioner pointer is outside its wear range Correct answer.
  3. The tensioner arm moves slightly while the engine idles with the belt installed
  4. The tensioner housing shows a light film of dust

Explanation

The wear indicator shows how much the tensioner has moved to take up belt stretch; once the pointer leaves the range, the tensioner can no longer maintain tension. Some arm movement at idle is normal as the tensioner damps load changes. A pulley that spins with slight drag is also normal.

Question 38Engine and Engine Support Systems

A closed crankcase ventilation filter on a transit bus is being serviced at the recommended interval. What is the consequence of leaving a loaded CCV filter in service?

  1. Higher turbocharger boost pressure
  2. Crankcase pressure rises and can force oil past seals Correct answer.
  3. Increased engine oil pressure at the gauge
  4. Lower crankcase pressure and reduced oil consumption

Explanation

The CCV filter traps oil mist before crankcase gases reach the intake. When it loads up, blow-by cannot escape and crankcase pressure rises, forcing oil past seals and carrying oil into the charge air system. This can cause oil consumption, leaks, and aftertreatment problems.

Question 39Engine and Engine Support Systems

During inspection, a coolant hose feels soft and spongy when squeezed and shows slight swelling near the clamp. The correct action is to:

  1. Wrap the hose with tape and note it for the next inspection
  2. Replace the hose and pressure-test the system after refilling Correct answer.
  3. Tighten the clamp and return the bus to service
  4. Apply a sealant to the hose and monitor it at the next PM

Explanation

A soft, swollen hose has lost its internal structure and can fail suddenly under pressure and temperature. Tightening the clamp will not restore a degraded hose. Sealants and tape are not acceptable repairs on a pressurized cooling system.

Question 40Engine and Engine Support Systems

A radiator cap is tested on a cooling system pressure tester and relieves pressure well below its rated value. What does this indicate?

  1. The relief spring is weak and the cap must be replaced Correct answer.
  2. The water pump is cavitating at high engine speed
  3. The coolant mixture is too rich in antifreeze
  4. The thermostat is stuck open and is overcooling the engine at cruise

Explanation

The cap spring sets system pressure; if it relieves below the rated value, the cooling system will not hold pressure and the coolant boiling point drops. A cap that fails its bench test must be replaced. Thermostat, water pump, and coolant mixture are separate checks.

Question 41Engine and Engine Support Systems

A DEF refractometer reading shows a urea concentration of 32.5 percent. What is the correct interpretation?

  1. The concentration is within range and the DEF is serviceable Correct answer.
  2. The DEF is too dilute and will freeze at a higher temperature
  3. The DEF is contaminated with coolant and must be drained
  4. The DEF is too concentrated and must be diluted with water

Explanation

Diesel exhaust fluid is specified at about 32.5 percent urea by weight, and a refractometer reading near that value confirms the fluid is serviceable. Diluted DEF causes poor NOx reduction and can trigger derates. Contamination would show as an off-scale or cloudy reading.

Question 42Engine and Engine Support Systems

After replacing both fuel filters on a diesel transit bus, why must the fuel system be primed before cranking?

  1. Trapped air can prevent starting and damage the injection pump Correct answer.
  2. Priming raises the cetane rating of the fuel before combustion
  3. Priming cools the injectors before the first combustion event
  4. Priming removes water from the fuel/water separator

Explanation

Air trapped in the fuel system can stop the engine from starting and can damage the high-pressure pump because diesel fuel is its only lubricant. Priming fills the filters and lines before cranking. Cetane, injector cooling, and water removal are not the purpose of priming.

Question 43Engine and Engine Support Systems

Valve lash adjustment on a heavy-duty diesel engine is typically performed:

  1. At the OEM interval with the engine at the specified temperature Correct answer.
  2. Only after the cylinder head is removed and reinstalled
  3. Only when the operator reports a noise complaint
  4. Every oil change regardless of engine model

Explanation

Valve lash changes as components wear, and incorrect lash affects valve timing, compression, and noise. The adjustment is made at the OEM interval and at the specified engine temperature because thermal expansion changes the clearance. Waiting for a noise complaint allows damage to develop.

Question 44Engine and Engine Support Systems

A transit bus with an electronically controlled fan clutch overheats only at low speed and idle. Which test best confirms the fan clutch is the cause?

  1. Check the air filter restriction indicator and replace the element if it is tripped
  2. Measure the coolant freeze point with a refractometer
  3. Command the fan on and watch fan speed and coolant temperature Correct answer.
  4. Replace the thermostat and road test the bus

Explanation

Commanding the fan on with the diagnostic tool verifies the clutch, wiring, and ECM output in one test, and coolant temperature should drop if the fan engages. Overheating only at low speed is consistent with a fan that is not engaging. Thermostat and air filter faults do not explain the speed-dependent symptom.

Question 45Engine and Engine Support Systems

A transit bus sets an underboost code and the driver reports a hissing noise under acceleration. The most likely cause is:

  1. A stuck-open thermostat keeping the engine cool
  2. A leaking charge air cooler or boost hose connection Correct answer.
  3. A restricted air filter element causing high intake vacuum
  4. A faulty coolant temperature sensor

Explanation

A hiss under acceleration with an underboost code points to pressurized air escaping between the turbocharger and the intake manifold. Charge air cooler cores and hose joints are common leak points. A restricted filter would cause low power but not a hiss.

Question 46Engine and Engine Support Systems

Inspection of the crankshaft vibration damper reveals a rubber ring with deep radial cracks and an outer ring that has shifted relative to the hub. The correct action is to:

  1. Mark the alignment and monitor it at each PM interval
  2. Replace the damper before the bus returns to service Correct answer.
  3. Clean and repaint the damper and continue service
  4. Replace the crankshaft pulley bolts and retorque

Explanation

A damper with cracked rubber and a shifted outer ring can no longer control crankshaft torsional vibration and may separate completely, damaging the crankshaft or accessory drive. Painting or monitoring does not restore function. It must be replaced.

Question 47Engine and Engine Support Systems

During a charge air cooler leak test, the technician pressurizes the cooler to the specified value and the gauge drops slowly with no audible leak. Which additional step is most appropriate?

  1. Raise pressure to the maximum turbo boost level and listen for a pop
  2. Spray soap solution on the core and connections and look for bubbles Correct answer.
  3. Condemn the turbocharger immediately and order a replacement
  4. Replace the intake manifold gaskets and retest the system

Explanation

A slow pressure drop without an audible leak may be a small external leak or an internal leak past the core. Soap solution applied to the core, tanks, and connections will produce bubbles at the leak point. Overpressurizing can damage the cooler, and condemning the turbocharger is premature.

Question 48Engine and Engine Support Systems

An injector leak-off test shows one injector returning significantly more fuel than the others. What does this indicate?

  1. Excess return flow indicates internal wear or a sticking injector, so replace the unit Correct answer.
  2. The return line is plugged and no action is needed
  3. The fuel filter is restricted and all six injectors should be replaced as a set
  4. That injector is delivering more fuel and the engine will run rich

Explanation

Leak-off measures return flow from each injector; an injector returning much more than the others has internal wear or a sticking valve, which lowers rail pressure and can cause a misfire. Replacing the fuel filter or all injectors is not supported by this test. A plugged return line would reduce flow, not increase it.

Question 49Engine and Engine Support Systems

A transit bus in urban stop-and-go service shows a rising exhaust backpressure trend and repeated DPF regeneration cycles. Which condition best explains this?

  1. The DEF tank level is low
  2. The engine coolant thermostat is stuck open
  3. The turbocharger wastegate is stuck closed and overboosting the engine at cruise
  4. Ash has accumulated in the DPF and it needs cleaning or replacement Correct answer.

Explanation

Ash is non-combustible material left after regeneration and cannot be burned out, so it gradually restricts the DPF and raises backpressure. Stop-and-go duty with frequent regeneration accelerates ash accumulation. Low DEF, a cool thermostat, and a stuck wastegate do not explain a rising backpressure trend.

Question 50Engine and Engine Support Systems

During a road test, a transit bus loses power and the engine derates after coolant temperature rises above the protection threshold. What is the correct interpretation?

  1. The ECM is derating to protect the engine, so diagnose the cooling fault Correct answer.
  2. The transmission is slipping and the engine is not at fault
  3. The ECM has failed and must be replaced
  4. The derate is a normal response to the load and the bus can continue until the next PM

Explanation

The ECM derates power when a protection threshold is crossed, such as high coolant temperature, to prevent major damage. The derate is a symptom, not the root cause, so the cooling system must be diagnosed. Ignoring it can lead to a roadside shutdown.