Low Compression on Cylinder 3 After an Overheat
A 1.2-litre four-cylinder petrol engine (71 mm bore, 76 mm stroke, 9.5:1) has covered 42,000 km. The owner reports about 500 mL of coolant loss every 300 km, white vapour from the exhaust for the first minute after a cold start, and the temperature gauge climbing to three-quarters on a 6 km climb. A dry compression test at normal operating temperature with the throttle wide open gives 13.0, 12.9, 7.8 and 13.1 bar against a specification of 12.5-13.5 bar and a maximum cylinder-to-cylinder difference of 1.0 bar. Repeating cylinder 3 after adding 10 mL of engine oil raises it to only 8.1 bar. A leak-down test on cylinder 3 at TDC shows 55% leakage, and bubbles appear continuously in the radiator neck with the cap off. The workshop manager wants to order only a head gasket and hand the job to the apprentice because 'the head is aluminium, it never warps'.
1. Interpret the dry, wet and leak-down results. What does the small rise after oiling tell you, and what does the radiator bubbling confirm?
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Dry test: the average of the three good cylinders is 13.0 bar; cylinder 3 at 7.8 bar is 5.2 bar below that and far outside the 1.0 bar spread limit, so cylinder 3 is the faulty one.
Wet test logic: oil temporarily seals the gap between rings and bore. If the rings or bore were worn, adding oil would push the reading up towards 12-13 bar. Here it rose only 0.3 bar (7.8 to 8.1), so the piston rings and bore are sealing reasonably - the leak is above the piston: head gasket, valves or a cracked head.
Leak-down: 55% leakage (normal is under about 10-20%) on cylinder 3 at TDC. If a valve were leaking, air would hiss at the intake or exhaust port. Instead, continuous bubbles in the radiator neck with the cap off prove combustion pressure is escaping into the water jacket.
Combined with the symptoms (500 mL/300 km coolant loss, white vapour on cold start, overheating on a climb, no rise on wet test), the diagnosis is a gasket breach between cylinder 3 and a coolant passage, or a cracked head in the same area. It is not worn rings and not a valve-only fault.
2. Justify what must be checked on the head and block before a new gasket is fitted, and describe what can happen if only the gasket is replaced.
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Checks before assembly:
- Head flatness with a straight edge and feeler gauge (typically 0.05 mm in 150 mm and about 0.10-0.15 mm overall, per the manual).
- Crack test of the head by dye penetrant or pressure test, especially between valve seats and around the fire rings.
- Block deck flatness and condition of the fire-ring seats; no scratches or erosion.
- Whether the head has already been machined near its minimum thickness.
- Gasket mating marks showing which cylinder breached, and whether the head bolts pulled threads.
If only the gasket is fitted on a warped head: clamp load is uneven, so the new gasket will leak again within a few thousand kilometres. Worse, coolant can enter the cylinder overnight and hydraulic-lock the engine on the next start, bending a connecting rod; coolant in the oil destroys bearings and cam surfaces. The 500 mL/300 km loss also means the cooling system has been running low and hot, so the head is the most likely part to have distorted.
Therefore the head must be resurfaced or replaced as measured, pressure-tested, and refitted with a new gasket and correctly torqued bolts. The apprentice may assist, but the decision and the measurements belong to a competent technician.
3. Describe the correct tightening sequence and torque strategy for the head bolts, and explain why the old bolts should not be reused on this engine.
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Sequence and strategy: clean the threads and blow out the bolt holes, oil or seal the bolts as specified, then tighten in stages from the centre outwards in a spiral/cross pattern so that each pass spreads the load. For example: 35 Nm, then 70 Nm, then a final +90 degrees angle, in three passes, engine cold, using a calibrated angle gauge. The exact figures come from the manufacturer's manual, not from memory.
Why: progressive symmetrical loading seats the gasket evenly and prevents local distortion of the aluminium head and gasket shim. Tightening one bolt to full torque before its neighbours squeezes the gasket sideways and creates a leak path.
Why not reuse the bolts: torque-to-yield bolts are designed to stretch plastically during the first tightening and then work in their elastic range. Once used, they have already passed yield; reusing them gives less and inconsistent clamp load, and they can break. The manual will state either 'replace' or a maximum free length - measure the bolts against it.