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MG MGF Technical - Intermittent coolant loss / overheating

Hi,

Please help me. I have an R reg ('98) VVC MGF. It has been owned by family since new and MG serviced.

There seems to be an intermittent cooling problem (steam came out the back). I stopped it ever going into the red though.

The garage replaced the fan fuse, but said it "might be a bit stiff"? Since then I know the fans seem to work becasue I run the engine stationary until they come on (no overheating).

I replace the cap with a halfords one as a precaution.

Twice now, I have had it on the motorway (about an hour) then after slowing down the temp has gone up and coolant escapes from the back. Last time I got the RAC to pick me up and he said HGF likely because of observed turbulence in tank.

However MG garage said they tested in twice and the HGF passed? I am hoping it is OK, but don't believe the garage that there is nothing wrong. They replaced my cap with an "official MG" one. What do I do now?

Could it be themostat or airlock (I asked garage to bleed it properly and replace coolant but don't beleive they did it - they mentioned topping it up!).

Please help.

Thanks,

Phil
b bob

When the steam came out of the back I presume you stopped and took a look. Werre you been able to see any signs of coolant anywhere within the engine bay, when looking in through the boot grille? Do you see any drips of coolant on the ground?

You say it's happened after slowing down after being on a motorway. The slowing down - is that considerably slower after coming off the motorway - even stopped in traffic jams or at traffic lights for example?

I'm thinking about a split hose that is only leaking under pressure i.e. when the engine is hot. A constant speed with plenty of cool air flowing over the rad will keep the engine reasonably cool compared with when the car is hardly moving. It's possible for a hose to leak only when idling or slow moving and not leak at all when cruising with plenty of airflow.

However, I'd have expected the RAC man to pick this up. His comment about turbulence in the tank (expansion bottle) implies boiling due to being depressurised. This points to the expansion cap, but you've had that replaced twice - right? Unless the depressuring is due to the split hose! But, then I would expect coolant to be leaking visibly at that point.

The fact that your getting steam means that coolant is escaping and evapourating on the hot engine. The trick is to find out where it is leaking. There are limited possibilities

* from the head gasket (worst case) - look just above the alternator on the right hand side of the engine as you look from the back of the car.
* from expansion bottle cap, hoses, clips, underfloor pipes, radiator bleed screw, rad itself etc. (all easily dealt with as long as air doesn't get in the system and produce HGF as an after effect!)

HTH
Dave Livingstone

Thanks

>>Werre you been able to see any signs of coolant anywhere within the engine bay, when looking in through the boot grille? Do you see any drips of coolant on the ground?

I believe it was escaping from the tank through the cap. Isn't that the point of the pressure release valve? By the time we looked it was all over the place but that's because it was escaping and condensing.
The MG garage said there are no leaks (Not that I trust them).

>>You say it's happened after slowing down after being on a motorway. The slowing down - is that considerably slower after coming off the motorway - even stopped in traffic jams or at traffic lights for example?

Yes - considerably slowe - the first time it was going slow on a small road, the second time we have to practically stop beacuse of an accident on the motorway.

>>His comment about turbulence in the tank (expansion bottle) implies boiling due to being depressurised.

Yes but when we were inspecting it, we were stationary with the engine running but the cap OFF. Hence it would be depressurized, so probably not a very useful test.

>>This points to the expansion cap, but you've had that replaced twice - right?

Yes but the MG garage said the one I had replaced from Halfords (looked up the part for the car - it was yellow) was not correct. However they probably say that because it isn't an offical Rover/MG part?

>>Unless the depressuring is due to the split hose! But, then I would expect coolant to be leaking visibly at that point.

I agree but could see no visible leaks anywhere when it was hot.

The interesting point is that the air blowers were not hot when it was overheating! What does that mean?
b bob

Cold air from heater when overheating = head gasket failure in my experience.
qwerty

>>The interesting point is that the air blowers were not hot when it was overheating! What does that mean?

If the heater is blowing cold then either the water is cold or there is no water and it's air. In your case it would seem to be the latter! So, if air is causing the overheat then maybe the expansion bottle cap is just doing its job and blowing under pressure. But, why would this happen when you slow down after a motorway run? I guess the extra heat build up due to slow moving just adds that final pressure to make the cap blow.

If this is the case then you could be damaging the cylinder head - there is very little coolant flow here and a small amount of air could easily cause overheating and a blown head gasket/warped head etc.

I think it is definitely worth bleeding the system straight away. Instructions here if you want to have a go
http://www.mgf.ultimatemg.com/group2/common_problems/hgf_pages/coolant_bleed_procedure.htm
Dave Livingstone

Check the radiator for corroded cooling fins = impaired cooling.HTH
mike

Also might be worth checking the radiator fan fuse again (particularly fuse#15 in the passenger compartment fuse box) - if the fan "is a bit stiff" the initial draw current maybe higher than the specified rating on the fuse. You'll have a 10amp fuse there - but this can be safely replaced with a 15 or even a 20amp fuse (see Dieter's contributions on this topic in the archives) if it has failed once again.
Rob Bell

Sounds like what happend to my F and sure enough the head gasket had gone.
D Knott

This thread was discussed between 11/01/2005 and 12/01/2005

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