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Fault guide · from a real jobVauxhall Mokka 1.4 Turbo losing coolant: how I tested for a head gasket.
A 2016 Mokka 1.4 Turbo losing coolant and overheating after the thermostat, hoses and tank were replaced. The tests that pointed at the head gasket.
The car
A 2016 Vauxhall Mokka 1.4 Turbo, petrol, about 66,800 miles. Coolant loss, smoke from under the bonnet, and the overheating warning several times. The thermostat, some hoses and the expansion tank (after it cracked) had already been replaced.
What I tested
| Check | Result |
|---|---|
| Cooling system pressure test, cold | Coolant leaking at the seal between the thermostat housing and the water pump housing. The hose into it looked swollen. |
| Combustion leak (block) test | The test fluid went from blue to yellow-green: combustion gas in the coolant. |
| Engine running | Coolant bubbling hard into the tank, frothing, and rising until it overflowed. When the engine stopped, the level dropped straight to the bottom. The exhaust smelt sweet. |
| Drive belt under pressure | Squealing, with steam from the belt area. |
| Engine oil | Level right. No milky residue on the dipstick or filler cap. |
| Temperatures | About 98 C at idle with the fan coming on late, while the radiator sensor read 15 C. |
| Fault codes | P0171 (lean) stored. No misfires. |
What it means
Together, those tests strongly point at a failed head gasket, letting combustion gas into the cooling system. Some froth reached the tester, so a cylinder leak-down test was advised to confirm before any repair. The 15 C radiator reading needs checking too: the sensor, the thermostat, and whether coolant reaches the radiator once the engine is fully hot.
The advice on the day: don't drive it, have it recovered to a garage, confirm with a leak-down test, and fix the thermostat housing seal, the hose and the belt with the repair.
If your car keeps losing coolant after the parts are changed
Test before you spend more. A block test and a pressure test take minutes, and they tell you whether you're chasing leaks or something bigger. If it's the head gasket, as the tests on this one strongly suggest, replacing parts around it won't fix it.
How I got these readings: on the job, with a pressure tester, a combustion leak tester and a full-system scanner.
I'm Kieran, mobile diagnostics round Hemel, from £60.
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