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MG MGF Technical - OVER HEATING 1.8i WITHOUT THE HEATER ON

Can anyone shed any light on a problem we have? We have asked all the experts we can find and now turn to our knowledgeable public!
The engine temperature will only stay at normal when the heater knob is turned to hot (Blower off). If you turn the heater back to cold the temperature rises and will fall as soon as you turn back to hot. T4 shows the temp gauge is reading correctly.
So far we have changed the cylinder head for a new one, both temperature sensors, then the radiator, thermostat, water pump and finally all new cylinder liners with a reconditioned cylinder head.
The same problem still occurs.
We've checked the routing of all coolant and heater hoses and the system is bleed correctly.
We have been working on Fs for years but never found one that has given us this much grief!
Any thoughts, however off the wall, would be welcome.
Thanks in advance.
William de la Riviere

The coolant hoses from the inlet and outlet for the heater are part of an H shape assembly. Sounds like the central block (which acts as a bypass path) has become blocked, and turning off the heater blocks all bypass current.

The thermostat is on the input side of the engine, it requires a hot water bypass/heater circuit to feed the back of the thermostat with hot water it then cools by adding cold water from the rad, no, or very little hot water means no or very little cold.

You could remove the stat and fit a PRT thermostat instead, this would replace the hoses and give a much better solution for cooling.
Will Munns

If Will is right I'm glad I'm not paying that garage bill !
sgg

I agree with Will and also doesn't the PRT setup get rid of the jiggle valves ?
Jon Baker

nope, the jiggle valves solve a diffrent problem, they exist to allow air that has got into the engine (thru coolant changes or gassing) to escape into the header tank, they don't play any part in cooling.
Will Munns

Will,
Thanks for your help. We found a ball in the narrow section of the H pipe below the jiggle valve this was blocking flow. New jiggle valve at about £5 plus a rebleed did the job. Having already replaced an intact valve this must have been from a previous valve that must have melted. Something to look for next time.
Fortunately it's one of our sales cars so there is no poor customer paying the bill. Just us picking up the tab!
Any one want a low mileage (40K) 2001 Flame Red F with £1000's of pounds worth of new bits? Asking price £5,495.
Thanks again for your help.
William
William de la Riviere

Glad to be of help, but I can't help but feel slightly guilty it was this simple after all the expense!
Will Munns

No guilt required. We spoke to so many experts in the trade and none could offer any concrete solutions. Hence the string of components changed. We'll put it down to a learning exercise.
William de la Riviere

This thread was discussed between 25/04/2007 and 06/05/2007

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