Fuel, Electrical, Ignition, and Exhaust Systems Diagnosis and Repair
Fuel, spark, and exhaust flow
This domain ties driveability symptoms to measurable fuel, ignition, and exhaust conditions. Confirm fuel delivery with both pressure and volume - a pump can make rated pressure at idle and still starve the engine under load. Evaluate injectors with a balance test, or by watching fuel trim and misfire counters as you disable cylinders one at a time.
Ignition diagnosis starts with a spark tester set to a realistic gap, then a secondary waveform when one is available: firing line and burn time tell you about mixture and resistance. A coil-on-plug misfire that moves with the coil when swapped is a coil fault, not a compression fault. Read spark plugs as a system report - oily, sooty, blistered white, or one clean plug each tells a different story. On the exhaust side, a backpressure gauge is the fastest restriction test; converters, crushed pipes, and collapsed muffler baffles all show up as elevated pressure plus low power and low vacuum.
Traps: a misfire code does not equal ignition parts. Use relative compression or a cylinder balance test to separate mechanical from electrical misfires. Do not substitute a resistance check for a voltage-drop test - an ohmmeter will pass a cable that cannot carry current. Do not condemn an oxygen sensor for accurately reporting a mixture fault created upstream. And remember that a vacuum leak is unmetered air on a speed-density system and a lean shift on a mass-air system.
Rule of thumb: exhaust backpressure at idle should generally stay under about 1.5 psi and below roughly 3 psi at 2,500 rpm; anything higher means restriction.