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MG MGF Technical - overheating MGF

Help please
My '95 MGF overheats after about 4 miles. It has had new head gasket, thermostat, numerous pipes and the expansion tank cap.
It ran lovely until the duel metal water pipes leaked, these were replaced and I have not been able to cure this overheating problem since.
It has been bled several times, but the expansion tank still keeps bubbling over.
Any suggestions please as it is now getting very expensive?
Carol

Carol, I have the exact same problem.

Recently I had a new radiator fitted and things are much better, but the problem still persists.

HC Jones

Is it being bled from all 3 bleed points? One on radiator, one on heater matrix, and one in the engine bay which is all-too-often overlooked, and that's the most important one... Sounds very likely that the bleeding isn't being completed correctly, leaving air pockets in the system.
bandit

Expansion tank bubbling over indicates water under pressure either because it's not cooled enough or there is mounting pressure in the water circuit. My thought process concludes muck in the coolant circuit, or a defective radiator. Also check radiator fans work - but they wouldn't normally come on in just 4 miles, unless you've been thrashing the motor!

When the thermostat opens, hot water can flow to radiator for cooling and pressure is relieved - it's not likely to be a stuck shut 'stat problem as you've changed it already.

The pressure cap is new, which should keep coolant "in" at standard heat water, but not water under massive pressure or over heated water. You've had HG changed, so it's unlikely to be the allowing the engine to pressurise the cooling system.

As you think the system has been bled free of air that leaves over heating as the cause - or more accurately under cooling. I guess that the corroded underbody pipes have let go some rust into the radiator and/or elsewhere giving rise to impaired radiator cooling function. A blockage there or elsewhere being the cause. You should be able to "feel" cold spots in a blocked radiator. Flushing the system through to remove accrued sediment may fix the problem.

Suggest the above to your mechanic, or search the archive for "how to" if you fancy some DIY!.


Jeff
J Lennon



I have had garages mixing the two hoses that connect to the underfloor pipes before.This effectively swops the cooling circuit around. Could be one to check?
Gavin King

> Also check radiator fans work

That's almost the first to be checked at old MGF.
Either the wiring is interupted or short circuit to ground, or the fan has got sticky and has blown the to small fuse. Change fan fuse from 15Amp to 20Amp in any way. That's got standard at later MGF since 2000.

However, the front fan should kick in after latest 45 Minutes idle run of the engine.
If not than search for the failure from fan connector back over the fuse, the relais to the brown coloured coolant sensor at top rear of the engine.
Dieter

I've recently re-built my engine any was finding that the pressure was building up in the system. Every thing is fine now but I found a number of issues which contributed to the over heating:

1) I had accidently swapped the rubber hoses leading from the engine to the under body pipe. Confirming Gavin's point.
2) The Jiggle valve in the inlet plenum was stuck, a bit of pocking around with a piece of wire sorted that!
3) Although the pressure cap looked OK it was duff.

When bleeding the system it can be useful to jack the car up to force air bubbles towards the radiator or expansion tank.

Everything is now hunky dorey.

Tim
Tim

This thread was discussed between 22/06/2009 and 31/07/2009

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