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MG MGF Technical - Overheating, but only at 70mph+


Our 96 MGF has had a few problems with overheating in the past, all mainly due to leaks in the Rad or the pressure no major chore.

A couple of weeks ago it overheated on the M61 heading into Manchester. So we had travelled at least 30 miles upto this point. I rapidly pulled onto the hard shoulder as soon as the temp started to head upwards. Once it ccoled down we carried on in to Manchester at approx 50-60mph (what a nightmare).

Now here is the strange bit. The car was happy (and has been ever since) stuck in traffic for over 2 hours, looking for a parking space, yet on the way home on the Motorway at started to overheat again.

It has occurred a couple of times since, always when the car is travelling at speed or being driven hard. Now we dare not go over 60mph.

My initial thought was a leak, but I can see no visible signs.

Does anyone have a clue ? Maybe the water pump ? Is the water pump belt driven ? (I ask this as in the mornings there is sometimes the sound of belt squeel, so maybe its slipping)

Any help would be gratefully appreciated.

The car is very much standard except for an SP Exhaust.

There's another thread from master Tim regarding connection problems with the wiring currently.
May be the radiator fan doesn't cut in as it should.

Could happen if the brown ECU sensor has poor connection, or if the whole ECU multi plug has contact probs.
http// has some data.

Another one could be the _famous_ leaking inlet manifold gasket if it's the old black coloured.


Dieter K.

I would think the rad fan kicks in as it should, as the temp stays OK in slow traffic. Maybe the engine room temp sensor is not as it should be, as it overheats at higher speeds, with a higher temp in the engine compartment.

Johan Slagter

Cheers fellas,

I agree that the fan cannot be the trouble, as it is fine pottering around town and on long runs that dont involve high speeds and/or heavy engine loads.

Can the engine room fans help when on the move, as there should be enough air flow through the vents.

How do I check/eliminate the inlet manifold gasket...this looks as if it could well be the trouble when I think about it, although the water pump bothers me slightly, and the belt noise i get on start up....could anyone tell me if the water pump runs on a belt, and if it started to slip under heavy load, could this cause the engine to overheat ?!?!?!

Thanks again.....

Sounds like it could be running a bit lean, rather than fan problems. At speed you rely on airflow rather than fans. Could be a problem with too much heat in rather than not enough out. Another possibility could be sludge in radiator reducing thermal efficiency?
Grant Vowles


>>My initial thought was a leak, but I can see no visible signs

But, are you losing coolant?

>>it is fine pottering around town and on long runs that dont involve high speeds and/or heavy engine loads.

This can happen with a small split in a hose. It will only leak when under significant pressure, caused by high engine load/heat.

Dave Livingstone

>How do I check/eliminate the inlet manifold gasket...

There are two locations where you can spot a small part of the gasket.
One on the right lower and another on the left circumference of the manifold.
The left needs access through the engine cover.
See on this picture where to look for green *pins* looking out of the plastic. If you see black only, then you got the old and should immidiately think on replacement !!
Some good pics of the jiggle valve included as well.

Dieter K.

Thats a better one. Left hand of the jiggle valve/bleed hose to the coolant expansion tank. Access without removing the engine cover.
Where to find
Dieter K.

>>Does anyone have a clue ? Maybe the water pump ? Is the water pump belt driven ? (I ask this as in the mornings there is sometimes the sound of belt squeel, so maybe its slipping)<<
It could be the water pump is not working effectively - you need to find some way of measuring flow through the radiator when the engine's running at 3000+ RPM - and you then need to know if this is within specification !
Airlocks and corrosion in coolant circuit can contribute to poor flow. I would drain the coolant, check its quality, flush the circuit and refill after measuring out the correct coolant mixture.
Even if the car is not overheating in normal use, I think you're risking a HGF.

How were the leaks in radiator etc fixed, and how long ago ?

This is classic symptoms for a sludged up radiator, but the chances of this on a car so young are very small - alternativly(check in this order):

The radiator has air locked in it, this is reducing the cooling surface, and it is fine untill you push the car. To check (with the car quite cold) open the bonnet, and the boot, take the pressure cap off, now go to the front of the car and looking in you will find a plastic bolt on the top of the radiator on your right hand side end, open untill water comes out, tighten - but not to tight, remember the bolt is plastic!.

The Thermostat is on the way out, they tend to stick either open or closed (well they are noticed more when they fail this way!), but can be sticky half way and not open fully. This is a job that could be completed by a competent DIYer, if you fancy it, buzz me back and I'll run thru it.

Other cooling problems, you say that in the past you have had Rad leaks or pressure cap failures, just to check- all the pipework is rover? the pipework has a restriction in it which makes sure only a small amount of water goes thru the bypass (if this is missing then you'd get your cooling problem).

The water pump is belt driven, but if the belt were slipping then you'd be in much more trouble as it also drives the cam belt!!! Water pumps tend to die by killing the bearings, which causes a squeel which if ignored causes the pump to lock solid and strip the belt (very big ouch and new engine!).

If the car has had a HGF in the past and not flushed out properly than this would cause the same problems, but as you don't note it, and I assume you drove all summer I guess I can rule this out.

The Inlet manifold would not cause this, it would cause water loss.

The fans would be pointless at this speed, especially in this weather!
Will Munns


Thanks for all your technical know-how.

It happened again yesterday. Drove all the way to the Lake district on the A-roads, never went above 55mph, and it behaved. On the way back, got as far as Lancaster, got stuck in traffic after having a clear run, then it started to boil up. Left it for 5 minutes, started up again, and it behaved fine for the remaining 20 miles in traffic !

I've had a look everywhere for signs of a leak, but to no avail. It doesn't seem to be losing coolant, only tiny amounts that I'd put down to boiling off.

That is until I washed the car! The paint laquer looks as if it has started to bubble on the front bumper, in front of the lower grill !!!!!! Could this be coolant spitting out of the rad, when under greater load ??????

>On the way back, got as far as Lancaster, got stuck in
>traffic after having a clear run, then it started to
>boil up.

This is as expected, you got the engine hot, then removed the cooling of the fast airflow, and the fan couldn't keep up (as only half the rad had water in it).
In your case you should have turned on the heater full blast, rather than stopping the engine, that way the coolant still circulates, and you heve less chance of boiling the coolant in the head.

If the bleed screw is leaking then it is possible that it's pusing out a little coolant at high pressure, and sucking in air when the system cools (although this would go against your symptoms, in that the rad would be full of water and so not give you high speed problems)
Will Munns

This is a wierd one!

Water pump could be a suspect, but typically, as the bearings fail, it makes noise, so I'd have expected that you'd have reported this... Might be worth checking the pump for leaks too - another sure-fire sign of a failing pump.

The fact that heat becomes a problem with higher engine/road speeds seems to negate any problem with the cooling radiator fan. I'd suspect that the radiator is fine too - in fact, I wonder whether Grant's suggestion of the engine running lean might be the problem? Pop out the spark plugs and inspect. The the engine is running lean, the sparkers will give you a clue as to what is happening - see:

Weak mixture could be caused by a failing fuel pump or inadequate fuel rail pressure (need to replace the fuel pressure regulator)
Rob Bell

Hi Ben,

You need to get a coolant pressure tester. This will pressurise the system and force coolant out making it fairly easy to find.

I worked on a MR2 turbo with similar symptoms, OK when driven gently, but overheated when driven hard. The tester showed coolant loss which we couldn't see at first, then we took the spark plugs out! This confirmed HGF. At high load the combustion gasses were passing the head gasket and boiling the coolant.

I have also had the gauge go off the scale, but this was due to a faulty sensor, not sure how you test this other than replacing it, or plug into an Emerald.

Does the front fan come on when it overheats? This is controlled by the ECU which uses a seperate coolant sensor to the guage sensor.



Alastair McLeod

Alastair, I fear that you could be right with the HGF diagnosis - as I have heard of this 'overheating at speed' problem occuring on cars that have suffered an HGF that was otherwise not imediately apparant... :o(
Rob Bell


The front fan is working as it should, but in certain conditions when the engine has been worked hard, it doesn't seem to cope. On one occasion is started to get too hot, so I slowed right down, and the temp returned to normal ! This could add weight to the theory that the combustion gases are seeping past the head gasket under load. But again, the added heat if the engine is running lean would do the same, as too would the Inlet manifold gasket issue.

I'm going to take it in tonight after work.....

I'll let you all know the outcome !

Thanks again.....

Did you try bleeding the rad?
Will Munns

I am sorry to say however this is mostly likly caused by HGF... It will just get worse as the exhaust gases mix with the coolant causing higher pressure and heat.

It will only happen at higher revs however as the HGF becomes worse the revs will reduce mine went from 5K down to about 2K, had the exact same symtoms, I though refused to believe it however had to accept it in the end...:(

Sorry buddy, I'd get the gasket done and the head checked ASAP

Cheers Steve

This thread was discussed between 23/01/2004 and 02/02/2004

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