Cooling question.

If I take my 2001 MGF on a run it seems to reach running temperature rather quickly. Then, if I am sitting at the traffic lights, it starts to overheat and the cooling fan comes on. But... if I start it from cold, not intending to drive anywhere and just sit there, the gauge goes up to the right position (just below 'quarter-to-nine') and stays there - no overheating at all. The coolant does not seem to be dropping (I have marked the low-point and the high-point on the bottle), there is no bubbling or traces of oil in the coolant, no white deposit on the dipstick. I don't think it is the dreaded head gasket failure (HG was done 50,000 miles ago). What can it be? Air blockage? Water pump failure? Any ideas would be very welcome. 

Has it been bled at all three points?

Just sitting at idle and not overheating would imply that the water pump is doing OK running at a speed where it's not pushing much flow  An outside chance is that the impeller is either slipping on the shaft or is cavitating at speed, but I doubt it. You should first bleed the cooling system at all 3 points in correct sequence check you fan belt, check your radiator doesn't  have it's airflow blocked (I had a situation on a Ford Fairlane where the thing was always running hot, I pulled the radiator out only to find it had a lovely blanket of dog fur trapped between it and the air conditioning condenser, it turns out the dogs beds were in the garage right in front of the car and every time the car was started and warmed up the fan would suck in a shit load of dog hair!)
Once you've done those checks get back here to report your findings and we will go through what comes next.

Thank you Cobber! A few things to think about. I didn't realise there was a correct sequence to bleeding the cooling system. Will do as you suggest in the new year and report back if I get a result. Just praying it isn't the HG, I have heard that a tiny crack in the gasket between water jacket and combustion chamber might produce the symptoms I described - with no other obvious outward signs. And that the engine might keep going like that for quite a long time. Very risky though because the HG is bound to eventually go - leaving me stranded somewhere no doubt!

Hi D4KGP. Thanks for your interest. No, not bled yet. I'm not able to do it over Christmas but I'm not intending use the car either. So I will do it in the new year. Fingers crossed the problem can be solved that easily!

I took another look at the MGF over Christmas and realised that the expansion tank was full to the top, and this was when cold. This is my fault, last time I topped it up I stupidly overfilled it. So I drained some off, down to the correct level. The overheating problem appears to have gone away. I went on a 20 mile run to test it and the gauge stays rock steady at the right temp, whether I am moving forward or standing idling. Could this have been the problem?

If you have correctly bled the cooling system at the 3 points, the level is ok and the temperature remains steady then you've probably aced this one. Don't forget that most head gaskets fail due to the engine being run whilst low o coolant - so if you havent done so already you must fit a low coolant level alarm. All the regulars here have one and recommend them.

CORKY wrote:

I took another look at the MGF over Christmas and realised that the expansion tank was full to the top, and this was when cold. This is my fault, last time I topped it up I stupidly overfilled it. So I drained some off, down to the correct level. The overheating problem appears to have gone away. I went on a 20 mile run to test it and the gauge stays rock steady at the right temp, whether I am moving forward or standing idling. Could this have been the problem?
 

Nah! the expansion tank being full to the top wont cause over heating, all that will happen is the coolant will expand and push out through the pressure relief cap until such time as it finds the correct level.
Way too much over thought is put into the tank level being over the max fill line.
The reason that manufacturers put so much emphasis on the max fill line is so they don't have to deal with panicked owners making warranty claims because they see a bit of coolant spilling out onto the ground, when there is absolutely nothing wrong with the car.

I must say, I would have been surprised if too much water in the system could cause 'overheating'. Doesn't seem logical at all. However, when my system was overfull and getting very hot I remember looking at the expansion tank (thinking the reason might be that the water level might have gone down). The coolant was right up to the top of the bottle but none was getting past the pressure relief cap. The whole system looked to be under a lot of pressure so obviously I did not remove the cap until it had all cooled down. I'm still going to bleed the cooling system, I guess there could still be air in there.

Yeah, the expansion tank could show as over full due to air being trapped in the system, So you must bleed the system properly., it wont take long so don't buggerise about and do it sooner than later.

I'd assumed you had already bled it in the 3 locations as advised if you are serious about sorting this out

Cooling system has now been bled and some air did come out. Not much. Topped-up to just above the seam on the expansion tank. I have also replaced the expansion tank pressure cap. Went on a 10 mile round trip to test things and temp gauge is rock steady just below the quarter-to-nine position - when moving forward or idling in traffic. Fluid level had not dropped when cooled down. Hopefully the problem is solved. Thanks guys for all your help.