A friend has a 1999 MG MG-F, left hand drive, an import from Germany, with about 200k on the clock. Those have the Rover 1.8 K series engine of overheating/blowing head gasket fame. The engine is mounted in the middle of the car, behind the seats.
The car has had at least two replacement engines and one engine rebuild (don't ask), and is a total shed, but he has a sentimental attraction to it. Recently it bust a rear wheel bearing, fixed with a used replacement wheel hub from eBay UK. But he has spent thousands of euro on it over the years. The phrase 'money pit' would be inadequate to describe this motor. It spends more time off the road than on it.
Recently it started boiling over and ejecting coolant from the pressure relief valve in the cap of the expansion bottle located under the rear bonnet. The car was fine during motorway driving but in stop-start driving it would boil over with the temp gauge going into the red.
He was keen not to wreck the engine again so asked me to help. Using info found online, we found fuse F6 in the fuse box under the front bonnet blown and replaced it. This supplies power to the electric cooling fan. The fan is also fed by fuse F15 under the steering wheel (why?!) and this was ok. Once we'd done this, the fan started operating as expected when the engine temperature reached a certain point at a standstill.
We removed the engine cover plate and located the coolant temperature sensor which seems to be ok and have clean connections.
The car still boiled over during stop-start driving, and we were puzzling over it during a motorway run when on a whim I tried turning the cabin heater on. The car's located in the Canary Islands, so I need not say that the heater never gets used.
The vents blew cold air at first which was puzzling and started me thinking about a blocked heater matrix or seized valve, but started blowing hot air after about ten minutes, and the car has been fine since then in stop/start traffic with the heater off.
Is it likely this could just have been an airlock in the cooling system, which was dislodged by turning on coolant flow through the cabin heater matrix? Seems a bit far-fetched to me but would welcome opinions. Ta.