Oil Consumption After a Top Overhaul: Reading Compression and Leak-Down Together
A 1.2-litre four-cylinder petrol engine is brought to your workshop with the complaint of rough idling and blue smoke. The owner says a 'top overhaul' was done at another garage 3,000 km ago: the cylinder head was skimmed, the head gasket renewed and the valves lapped. Since then the engine has misfired, oil consumption has been about 1 litre per 600 km, and blue smoke appears mainly when the car coasts downhill with the throttle closed. Coolant level is steady and there are no external oil leaks. Your readings: compression (dry, throttle open, warm engine) 11.2, 11.0, 8.1 and 11.1 bar on cylinders 1 to 4; with 5 ml of oil added, cylinder 3 rises to 10.9 bar. A cylinder leak-down test on cylinder 3 shows 35 per cent leakage, with air escaping from the oil filler neck. Plug 3 is oil-fouled; the others are normal. No bubbles appear in the radiator.
1. Interpret the compression and leak-down results. Which components are at fault, and how do these results exclude the head work that was just completed?
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Compression readings: cylinder 3 at 8.1 bar against 11.0-11.2 bar on the others is roughly a 27 per cent loss, outside the usual 10 per cent cylinder-to-cylinder spread. That tells you cylinder 3 cannot seal, but not where the leak is.
Wet test: adding 5 ml of oil raises cylinder 3 to 10.9 bar, nearly matching the others. Oil temporarily seals the gap between rings and bore. Pressure recovering like this points to the piston rings and bore, not to a valve, because oil in the cylinder does not seal a burnt valve or a leaking head gasket.
Leak-down test: 35 per cent leakage with air heard at the oil filler neck means the air is escaping into the crankcase, past the rings. A leaking inlet valve would be heard at the throttle body, an exhaust valve at the tailpipe, and a head gasket or cracked head at the radiator or adjacent cylinder. None of those is present, and there are no coolant bubbles.
Conclusion: worn rings or a worn/tapered bore on cylinder 3, plus likely glazed or worn bores generally. The top overhaul addressed the head only, so it could not have fixed this. The misfire is the oil-fouled plug shorting under load, and the closed-throttle smoke is high manifold vacuum drawing oil past the rings (and possibly valve guides) on the overrun.
2. The owner wants the previous garage to redo the head under their 'warranty'. Give your advice, with reasons, and state what you would put in writing.
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Advice: a second head overhaul will not fix this engine. The evidence shows the pressure loss is past the rings into the crankcase, so re-skimming a head or re-lapping valves changes nothing about the 8.1 bar cylinder. The owner risks paying twice and losing the use of the vehicle again.
What to put in writing (factual, not personal):
- Dry compression figures for all four cylinders, with the test conditions (warm engine, throttle wide open, all plugs out).
- The wet-test figures and the volume of oil used.
- Leak-down percentage for cylinder 3 and where the air was heard.
- Plug 3 photograph or description as oil-fouled, others normal.
- Statement that no coolant loss, no bubbling and no external leaks were found.
- Conclusion: the leak is past the piston rings into the crankcase; the cylinder head is not the source.
Also useful: check whether the original complaint was oil consumption before the head work. If it was, the earlier diagnosis was incomplete, and the owner can raise that with the other garage using your report rather than your opinion. Do not promise a rebore until the bores are measured - it may need only a hone and rings, and saying otherwise exposes you to the same criticism you are making of the other workshop.
3. What measurements would you take before quoting, and what would decide rebore versus hone-and-rings?
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Measurements before quoting:
- Bore diameter with a bore gauge or internal micrometer, at top (just below the ring ridge), middle and bottom, in two planes 90 degrees apart, on all four cylinders.
- Taper and ovality calculated from those readings and compared with the manufacturer's limit - typically around 0.05 mm taper and similar ovality is the practical ceiling, but always use the workshop manual figure.
- Piston-to-bore clearance by measuring the piston skirt at right angles to the pin and subtracting from the bore, or checking the feeler gauge pull between skirt and bore.
- Ring end gap measured with the ring squared in the bore, and ring groove side clearance.
- Top ridge depth - a pronounced ridge means the bore has worn most at the top and usually points to reboring.
- Valve guides and stem seals, since closed-throttle smoke can also come from guides; and bearing condition on the big ends and mains while the sump is off.
Decision rule: if taper, ovality or piston-to-bore clearance exceed the manual limits, or there is a heavy ridge, rebore to the next oversize and fit matching oversize pistons and rings. If the bores are within limits and only glazed or lightly scored, hone to restore the cross-hatch, fit new rings and check the ring gaps in the cleaned bores. Quote both possibilities to the owner with the measurement sheet attached, so the final bill is not a surprise.