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MG MGF Technical - Overheating - anyone had this weird one before?


2001 standard 1.8.

Scenario:

The missus gives me a call to say the engine started to overheat, but thankfully she noticed before it got to 2/3rds and shut it off. No coolant!

I get there and top it up a little above the halfway mark on the expansion tank using proper MG coolant and water, and she starts off again. We get about 200 yrds in a heavy stop/start traffic jam and it spews steam so she pulls over again. I can see coolant on the floor and around the engine bay, but no obvious signs of a leak. It's lost about an inch from the level.

Let it cool down, top it up again, and then drive the next mile or 2 to her work no problems (out of the traffic jam now). She parks it up and no coolant has been lost.

While she's in work I give it a couple of tests. Oil is fine, coolant is fine and clear. Start her up and can see the coolant circulating out of the top hose into the expansion tank. Continue to let it heat up, and then..... Bam.

As soon as the radiator kicks in the coolant level on the expansion tank shoots up, fills the expansion tank completely, and starts to squirt out of the expansion tank cap. This cap is fairly new having been replaced at the last service too.

Anyone know why this happens?

Thanks in advance.

D Forber

Sounds like a bl***y great airlock in the radiator can you try bleeding it and then all of the other points? I haven't had one before; but they are not a good thing by reputation!
C.R.B. Simeon

Forgive me if you think I'm insulting your intelligence but when you say the fan kicks in, is it definitely the radiator fan and not the fan in the engine bay (which is considerably more audible)? The symptoms seem to point to an inoperative radiator fan, commonly caused by the fuse going - worth a check. The radiator fan should have kicked in long before the pressure rises to the point where the expansion bottle cap vents, so that implies the engine was already too hot, which would have triggered the engine bay fan.

HTH
Mike Hankin

Well up until you've questioned it I was fairly sure it was the radiator fan, but I guess I'll have to run the test again now to be certain.

The car normally runs a little below half way on the temp gauge, with the radiator fan kicking in when its a little above half way.

Its at this 'little above half way' point when a fan kicks in and fills the expansion tank up, so its not overly hot.



D Forber

The radiator fan should be operating before the gauge rises above its 'normal' position, and if all is present & correct then the gauge shouldn't move, ever. Also worth checking the condition of the radiator, you could get these symptoms if half the fins have rusted off, which is quite possible even on a 2001 car 8-(

Fingers crossed that a 20amp fuse is all that your car wants for Christmas ;o)
Mike Hankin

Definitely worth checking the condition of the rad - you will need to get right down and look under the car preferably with a strong light to get a visual idea of the rads corrosion status. I agree that the fan(s) should kick in before the temp gauge rises above normal.
Robert Matthews 1

What about a faulty thermostat?
Mark Clayton

The fact that the engine doesn't overheat when it was moving suggests that the thermostat is opening OK. Or at least, it did that time :o/

What news, Mr Forber..?
Mike Hankin

Not really much more to tell, except I've booked it into an MG garage for Wednesday.

When cold, the coolant level is half way up the expansion tank.

Under normal driving everything is fine, temperature gauge stays a tad below half way, and coolant is circulated through the expansion tank and system. This says to me the thermostat works and the water pump works. The radiator can't be blocked or the temperature would rise here.

When stood still the temperature rises, and when the gauge on the dash gets a fraction over half way the radiator fan kicks in. Once again this sounds normal. The temp sender is working and the cooling fan kicks in as pre-determined by the temp sender.

This is where it all goes wrong though. As I've said earlier, the volume of water being pump into the expansion tank is greater than the volume being taken out, the expansion tank fills, and under high pressure forces coolant out through the cap.

Stuffed if I've ever come across this before, and I've restored quite a few 70's and 80's Fords.

Suppose I will have to wait and see what the garage says, and I'm not overly impressed with the quality of Rover-trained mechanics.....


D Forber

So everything's fine while cold air is passing through the radiator, but goes pear shaped when the car isn't moving? That all points to either inoperation of or lack of airflow from the radiator fan. On both my Fs and all previous ones I've owned, the radiator fan would operate (barely audibly) before the gauge showed any rise above 'normal'. The fan I'd expect to hear operating when the gauge is above 'normal' is the engine bay one, and the last time I heard that was when the fuse for the radiator fan had blown... That was the last time I saw the gauge go above 'normal', too...
Mike Hankin


Hi,

I had a similar problem vith my car (MGF, VVC 1998). Everything was okay under normal driving, but when standing still temperature was rising almost 2/3 up to red.. Coolant level rised about an inch when opening expansion cap (with a cold car).

The problem was caused by air in the radiator. The air got into the coolant system from a small (almost invisible) leak where the underbody coolant pipes connects to the hoses from the engine. I got the hose-clamps changed and the coolant system bleed at the garage. The coolant system and temperature now appears to be okay (but I keep my fingers crossed).

The car spend the winter at a heated garage, but I started up today for a normal check. The coolant temperature raised to normal (little below half), the radiator fan started after 25 minutes and there was temperature in both underbody coolant pipes.

However, I´m not sure if my problem is the same as mentioned above, but I think it is very important to report any observations about the coolant system..

Michael
Michael

Hello again

Just thought I'd let you know what the apparent resolution was. I'll say apparent as I'm going to use it for the next few days to get some confidence (honest - not because I enjoy driving it) before giving it back to the missus.

It's been back into the garage - an ex MG dealership (now Fiat but still with the Rover mechanics) - this morning and they've found a leak in the expansion tank. This allowed air in and prevented proper pressure retention. Relieved to say everything else tested OK, and a further pressure test found no problems.

Thanks for all your input.

D Forber

This thread was discussed between 19/12/2006 and 27/12/2006

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