Low Compression After a Timing-Belt Replacement
A 1.2-litre four-cylinder petrol engine has run 68,000 km. It cranks at normal speed and occasionally fires, then dies. The owner reports a recent timing-belt replacement at another shop. A dry compression test gives 6.2, 10.8, 11.0 and 10.5 bar; the specification is 10.5–12.5 bar. A wet test on cylinder 1 raises it to 9.8 bar. Spark is strong at all plugs, fuel pressure is 3.2 bar against a specification of 3.0–3.5 bar, and injector pulse is present. The timing marks show the camshaft pulley two teeth retarded relative to the crankshaft. Valve clearances are within specification. The workshop has a leak-down tester and a borescope but no cylinder-head removal equipment on site.
1. Interpret the dry and wet compression results. Does the timing-belt position explain all the readings? Give your reasoning.
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Cylinder 1 is far below specification at 6.2 bar, while cylinders 2–4 are within the lower part of the range. The wet test raising cylinder 1 to 9.8 bar is a large gain, which points to poor ring-to-cylinder sealing rather than a valve that is not seating. If a valve were bent or leaking, adding oil would not usually raise compression by 3.6 bar. The cam being two teeth retarded can affect all cylinders, but cylinders 2–4 are still near normal, so the belt position alone does not explain the single low cylinder. The likely picture is two separate issues: incorrect cam timing and a cylinder-1 mechanical sealing fault. A leak-down test is needed before blaming the belt alone.
2. Which next test would you perform, and what result would confirm cylinder-1 ring or cylinder-wall damage?
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Perform a cylinder leak-down test with cylinder 1 at top dead centre on the compression stroke. If air escapes into the crankcase, heard at the oil filler or dipstick tube, that confirms ring or cylinder-wall leakage. If air escapes at the intake or exhaust, that points to valve sealing. Use the borescope to check for scoring, glazing or a damaged piston crown. Also verify the cam timing with the belt marks and a dial indicator if available. The wet-test rise already suggests rings, but leak-down localises the leak and prevents unnecessary head removal. Record the percentage leakage and compare it with the other cylinders.
3. The customer wants only the belt re-timed because it is cheaper. Explain the risk and the correct repair recommendation.
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Re-timing the belt may correct the cam position, but it will not restore cylinder 1 compression if the rings or cylinder wall are damaged. The engine may run unevenly, consume oil, fail an emissions test and damage the catalytic converter. If the belt has jumped, there is also a risk that valves have contacted pistons; the leak-down and borescope results must rule that out. The correct recommendation is to correct the timing, then repair the cylinder-1 mechanical fault, which may mean piston and ring work. The customer should be told that a belt-only repair is a partial fix, not a reliable repair, and that continued driving can increase the damage.