Low Compression and Tight Valve Clearance on One Cylinder
A four-cylinder 1.2-litre petrol car has covered 82,000 km. It idles roughly, lacks power uphill, and the exhaust note is uneven. A dry compression test at normal operating temperature, throttle wide open, gives 11.8, 7.0, 11.5 and 11.2 bar; the manual specifies 11–13 bar with a maximum difference of 2 bar. A wet test with 5 ml of engine oil added gives 11.9, 7.2, 11.6 and 11.3 bar. A leak-down test on cylinder 2 at TDC shows 35% leakage, and air is heard escaping at the throttle body. Cold valve clearances on cylinder 2 are intake 0.05 mm and exhaust 0.25 mm; the specification is intake 0.20 ± 0.03 mm and exhaust 0.25 ± 0.03 mm. The spark plug in cylinder 2 is dry and light brown. The customer wants only an oil change and says the car 'just needs tuning'.
1. Interpret the compression, wet, leak-down, and clearance results. What is the most likely cause?
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Cylinder 2 is the outlier. Its dry compression is 7.0 bar against a specification of 11–13 bar, and the difference from the other cylinders is well over the 2 bar limit. The wet test raises it only from 7.0 to 7.2 bar. A large rise after oil is added would point to piston rings or bore wear; this small change suggests the pressure is escaping past a valve or head gasket rather than past the rings. The leak-down result supports that: 35% leakage is excessive, and air heard at the throttle body means the leakage is into the intake port, so the intake valve is not sealing.
The clearance measurement gives the mechanism. The intake clearance is 0.05 mm against 0.20 ± 0.03 mm. A tight clearance can hold the valve slightly off its seat when the engine is warm, causing compression loss, rough idle, and a lean misfire. The exhaust clearance is within specification. The dry, light-brown plug does not suggest oil fouling. The most likely cause is an intake valve held open by insufficient clearance, possibly with carbon or seat damage that must be checked after adjustment.
2. What repair should be authorised, and why is an oil change or 'tuning' not sufficient?
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The first step is to adjust the intake valve clearance to the specified 0.20 mm cold and recheck compression and leak-down. The cam lobe, rocker, and adjuster should be inspected for wear. If the clearance can be set correctly and leakage falls to an acceptable level, the immediate fault may be resolved.
If compression remains low or leak-down still shows intake leakage, the head must come off. The intake valve face and seat should be inspected for burning, pitting, or recession, and the valve guide checked. A damaged valve should be replaced and the seat refaced or replaced according to the workshop manual. Carbon buildup on the valve stem or seat should be cleaned. The head gasket and head bolts should be renewed as specified.
An oil change cannot restore a valve that is not seating, and 'tuning' the ignition or fuel system would only mask the misfire. Continued driving with a tight valve can burn the valve and damage the seat, increasing repair cost.
3. How should the repair be verified before the car is returned?
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Verification should be evidence-based, not a short idle. After the clearance adjustment or head repair, set all valve clearances cold to specification. Run the engine to normal temperature, then repeat the dry compression test on all cylinders and compare the spread; cylinder 2 should be within 2 bar of the others and within the 11–13 bar range. Repeat the leak-down test on cylinder 2 and confirm that air is no longer heard at the throttle body.
Check for misfire with a scan tool or by cylinder balance, confirm stable idle, and road-test the car under load, including a hill or uphill section. Listen for valve train noise and check for leaks. Record before-and-after readings in the job card. The customer should be told that the oil change is maintenance, not the repair, and that the valve clearance or valve/seat repair was the root cause. If compression remains low, the car should not be released as repaired.