Low Compression and Blue Smoke in One Cylinder of a Three-Cylinder Petrol Engine
A three-cylinder 800 cc petrol engine has run 48,000 km. The owner reports blue smoke at idle, an oil consumption of about 0.5 litre per 1,000 km, and poor pickup on inclines. A dry compression test gives 10.2 bar, 8.4 bar and 10.0 bar on cylinders 1, 2 and 3. When 10 ml of engine oil is injected into each cylinder and the test repeated, the readings become 11.8 bar, 12.1 bar and 11.6 bar. The spark plugs are removed: cylinder 2's plug is oily and black, while cylinders 1 and 3 are light brown. The engine idles unevenly, and a faint blue haze appears after idling for two minutes. There is no external oil leak, the PCV hose is clear, and the coolant is clean. The workshop manual gives a minimum compression of 9.5 bar and a maximum difference of 1.5 bar between cylinders.
1. Interpret the dry and wet compression readings and identify the most likely faulty cylinder and components. Explain the reasoning, including why the wet test changes the result.
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Cylinder 2 is faulty: its dry reading is 8.4 bar, below the 9.5 bar minimum and 1.8 bar below cylinder 1. After oil is added, cylinder 2 rises to 12.1 bar, a 3.7 bar increase, while cylinders 1 and 3 rise by 1.6 bar. A small rise is normal because oil temporarily seals the rings. A large rise points to pressure escaping past the piston rings or cylinder wall, because the oil film seals that path. If the loss were at an intake or exhaust valve, the oil would not seal it, so the wet reading would stay low. The oily black plug and blue haze support oil reaching the combustion chamber. The most likely parts are worn or broken piston rings and a worn or tapered cylinder 2 bore, with valve guides possibly adding to oil consumption.
2. A mechanic proposes replacing only the valve stem seals because the plug is oily. Evaluate this proposal.
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Valve stem seals can cause blue smoke after idling and an oily plug, so the suggestion is not unreasonable as a secondary repair. But it does not explain the low dry compression or the 3.7 bar wet rise in cylinder 2. Stem seals do not affect compression; compression is lost past rings, the head gasket or valves. If only the seals are replaced, the engine will still have low compression, poor pickup and blow-by, and oil consumption may continue. A leak-down test should be done first: air escaping from the oil filler or crankcase breather confirms ring or cylinder leakage; air from the exhaust or intake confirms valve leakage. Valve guides should be checked while the head is off, but the primary repair is likely rings and cylinder work.
3. Describe the measurements and repair decision you would make before authorising a rebore.
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With the head and sump removed, measure cylinder 2, and the others for comparison, with a bore gauge at three heights and in two directions at 90 degrees to check taper and out-of-round. Compare with the manufacturer's wear limit and piston-to-bore clearance. Check the ridge at the top of the bore and the ring end gap in the bore. If the bore is within limits, hone the glaze and fit new rings after checking ring groove wear. If taper, out-of-round or clearance exceeds the limit, rebore to the next oversize and fit matching oversize pistons. Also check the crankshaft, bearings and oil pump, because low oil pressure or contaminated oil may have caused the wear. After assembly, run in with proper oil and re-check compression.