Since then the engine temperature has been all over the place rising to near boiling point sometimes and then running at normal temperature for say 10 minutes and then zooming up again to a high temp within say 30 seconds.
I had no problems until I changed the pump and the symptoms pointed to the thermostat not opening when it should as the top hose was not getting excessively hot and the fan did not switch on as the fan switch is in the top hose.
I changed the thermostat but this made no difference.
I changed the temp gauge sender but this made no difference.
The top of the rad is higher than the engine and I have never needed to bleed the system.
I suspected that I had a head gasket problem which was causing some gases to build up behind the thermostat causing it to close at random but I'm not a great believer in coincidences given that all was well before I changed the water pump.
The other possibility was a loose impeller on the pump.
After 2 weeks of back flushing /bleeding/ jacking the front of the car up/ doing burping procedures etc., I was getting nowhere so I removed the pump and fitted another one.
Bingo, all is now sorted and back to normal.
The pump that gave the problems has a cast impeller and the original pump and the one that's fitted now both have pressed steel impellers.
On inspection of the problem pump, I noted that the gap between the impeller and the body of the pump was greater than the other 2 pumps by about 2mm and other than this there appeared to be no difference.
The impeller was secure on the shaft.
So what caused the problem
I'm open to suggestions but my guess is that the larger gap between the impeller and the body was causing the pump to be inefficient allowing some cavitation to occur and that air was rising up to the inlet manifold and causing the thermostat to close and act randomly. The internal channel of these counter clockwise pumps are not the best design as any trapped air in the pump at initial fillup or due to cavitation has to travel downwards to leave the pump toward the offside of the engine. Naturally, when I saw the temp gauge rising towards boiling point I lowered the engine revs, but what was needed was some high revs to expell air from the pump. Low revs and air in the pump probably caused the pump to shift even less coolant.
Any other ideas
Regards Denis
