This car arrived with suspected head gasket failure. Before we start stripping down a cylinder head we will firstly check the diagnosis. So cue the combustion gas checker, and diagnostic check. There was clear evidence of combustion gases in the cooling system. The error codes we read PO300 & PO304 denoted a general misfire and a misfire specifically on number 4 cylinder. Once we started stripping the engine apart, it was clear to see that the gasket had failed on both oil and coolant, and combustion seal. Number 4 cylinder was very oily and this would explain the misfire.
We took the head to the engineering machine shop for a skim. Upon closer inspection we could see one of the exhaust valves on number 4 had lost a bit of it’s head. This is probably the main reason why the misfire on number four was occurring, rather than the oil ingress into the cylinder. indeed, the two can be linked because presence of engine oil will reduce the air/fuel ratio, making the mixture leaner. A mixture close to Stochiometric (14:7 ratio) at wide open throttle could easily burn out a valve.
After we reassembled, we flushed the cooling system with a proprietary cleaning product. Drain and then flush again and run up temperature and finally fill with new coolant of the correct mixture and grade. In this instance a Red coolant which isn’t miscible with Blue. A quirk of the Zafira design is no coolant temperature gauge inside the car. On the diagnostics you’ll also discover another quirk – the Thermostats on these are set to open at 105 Deg C !!! The faulty stat wasn’t opening properly and it got to 113 Deg C. So you can imagine anyone not knowing this would assume many a good 16v Vauxhall Ecotec was overheating, when it is in fact working as normal.