Coolant Loss With No External Leak
A petrol engine with 140,000 km has been losing about 500 ml of coolant every 400 km. There is no puddle under the vehicle, the radiator cap is new, and the cooling fan operates. The top hose is firm when hot. A combustion gas test on the expansion tank shows a colour change indicating exhaust gases. The engine oil on the dipstick looks normal, but there is a faint white deposit on the spark plug of cylinder 3. A compression test gives 12.5, 12.4, 9.8 and 12.3 bar across cylinders 1-4. A wet test on cylinder 3 raises compression to 12.2 bar.
1. Interpret the test results and identify the most likely fault, explaining each piece of evidence.
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The evidence indicates a head gasket failure between cylinder 3 and a coolant passage.
- Coolant loss with no external leak suggests internal loss: combustion chamber, oil gallery or heater matrix. The firm top hose and working fan rule out simple boil-over.
- Combustion gas test positive: exhaust gases in the expansion tank confirm a breach between combustion and coolant.
- White deposit on cylinder 3 plug is consistent with coolant entering that cylinder and burning off, leaving silicate deposits.
- Compression: cylinder 3 at 9.8 bar against 12.3-12.5 bar for the others is a clear low cylinder.
- Wet test raises cylinder 3 to 12.2 bar: this points to ring or bore sealing loss, but combined with the gas test and plug deposit, the more likely explanation is that coolant was present and the wet test temporarily improved sealing. A cylinder leakage test and cooling system pressure test would confirm.
- Oil looks normal makes a major oil-gallery breach less likely, but the oil should still be changed and checked for emulsion.
Conclusion: remove the head and inspect the gasket and head flatness.
2. Describe the checks you would make after removing the cylinder head, and state the acceptance criteria you would apply.
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Post-removal checks:
- Gasket inspection: look for a blown track between cylinder 3 and a coolant hole; note the firing ring condition.
- Head flatness: use a straight edge and feeler gauge along diagonals and across the centre. Typical limit is 0.05 mm over the length; if exceeded, skim within the manufacturer's maximum removal.
- Cylinder head cracks: pressure-test or dye-penetrant check around valve seats and injector bores.
- Valve seats and guides: check for recession and play; lap or replace as needed.
- Block deck: check flatness and for corrosion around the coolant passages.
- Cylinder 3 bore: check for scoring and taper; the wet test result warrants a close look.
- Coolant system pressure test after reassembly, plus an oil and filter change to remove any contaminated oil.
Acceptance: flatness within spec, no cracks, correct torque and sequence for head bolts, and a successful pressure and combustion gas retest.
3. The customer asks whether a chemical head gasket sealer would be an acceptable alternative. Give a reasoned answer.
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A chemical sealer is not an acceptable repair for this fault, though it may be a short-term expedient in limited circumstances.
Reasons against:
- The breach is significant: 500 ml per 400 km plus a 2.5 bar compression loss on cylinder 3 indicates a real gasket failure, not a minor weep.
- Sealers can block the heater matrix and radiator cores, causing secondary overheating.
- Debris can contaminate the coolant and damage the water pump seal.
- The underlying cause (possibly a warped head or a cracked head) is not addressed, so failure will recur, often worse.
- Coolant may still enter the cylinder, risking hydraulic lock and bent con-rods.
When a sealer might be considered: a very small external weep on an older, low-value vehicle where the customer accepts the risk. Even then, the cooling system should be flushed afterwards and the customer informed in writing. Here, the correct repair is gasket replacement with head flatness checks.