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MG MGB GT V8 Factory Originals Technical - Overheating..but only for a moment?

This is a topic well covered but I couldnt find this bit in the archives. I have a 3.5 with new water pump 14 inch push fan, new V8 rad. Done about 300 miles in it and found that the temperature creeps up to normal over a few minutes, sits there for a while, then rockets up to the top of the gauge over about 30 seconds then immediately falls back down to normal where it happily resides. The fan does not come on at all while driving and only come on whle idling for long periods. I have changed the 88 degree thermostat (twice just in case)

Anyone seen anything like this before, I thought it might be an airlock but I can not work out the best way to 'burp' the baby...I have the bleed hole on the thermostat at the top to try and get the air out.
frank swinton

Yup, me too. I fitted a digital temperature guage - very accurate and very quick response. In degrees celsius the guage hits 96 or even 98 and then falls back very quickly and seems to run at a pretty steady 86. As low as 85 and as high as 89 in traffic. All in all, I'm happy with this and I'm not bothered about the high blip although I've no idea what causes it. At high speeds I've seen the guage get to 92 and I wouldn't mind dealing with this as there is definitely a much nicer feel at 85.
Americans will have to divide or multiply by 5/9 and add or subtract 32 - or something like that.


I would say fairly normal on first heat cycle for guage to go above normal and then drop back to N as the thermostat fully opens. The materials used for a B thermostat I would assume are the same as for V8 but V8 can generate probably twice the heat in same time giving rise to a lag. Also likely to see lag if cruising and then floor the beast.

Depends what you mean by top of guage which I assume is white region - this could be a faulty guage- could try some temp strips from Demon Tweeks to measure top temp.

The 82deg thermostat may be a beater for these cars, perhaps 88deg in winter.

Mark - does 92 depend on ambient temp is so not really a problem for UK climate, although I thought you had Ally rad now. From your stats you have an 88deg thermostat so worth trying 82deg.

Have you details for digital temp guage.


Just been for a test drive and turned heater on full from the start and the gauge rose to normal and stayed there during normal 25 minute run (with some heavy acceleration, low speed, high speed etc.). Is this an indicator of what the problem might be? I might try putting a joiner pipe between the heater in and out so that I can turn the heater off. I am not sure what having the heater on would fix though.
frank swinton


The heater valve is at back of engine and controls flow of water to Heater. If you run into cooling problems always turn fan and heater on at max to keep engine as cool as possible. I assume no air lock in Heater system then. Is this first time you've had heater on? if so the system may have settled itself.


Paul, I do have an ally rad and that is what has helped to bring the entire heating problem under control. I agree with you that I should run a cooler thermostat. Ambient temperature doesn't seem to make any difference at all but I'll know better in January.

The digital guage comes from Demon Tweeks - very pricey but includes also a voltmeter as well. I have one or two criticsms about the backlighting but in principle it is really first class as you can see exactly what is happening and it gives a sense of confidence that the thing is telling the truth. It updates every couple of seconds. Of course it is totally not "in keeping" to have a digital guage but I'm happier with the functionality and fits perfectly in place of the standard temp guage.
It records maximum temps and voltages and you set alarms for low volts and max temp limits. - It's a bit frightening to see what happens to the volts when the fans come on.
I think that it is even possible to get the user settable max temperature warning to pulse out to the fans so that the fans are controlled by the temperature at the block rather than at the rad.

I've ordered a similar dual oil pressure/temperature guage for the other side.

I keep my heater switched off using an in-line cock-stop. Doesn't affect running temp at all but much easier on my athlete's foot! Saving a fortune on anti-perspirants too.


No, when I got this going a couple of months ago the heater was on constantly (fun in summer) and there were no problems. then I fixed the heater shutoff valve and here come the problems.
I think it may have something to do with returning hot water from the top of the block (at the back) to the front of the engine and up into the water pump. and yes the thermo is round the right way.
frank swinton

The temp gauge going skywards during warm-up is a symptom of a blocked steam pipe between thermostat housing and manifold on factory V8s. Mine got as high as 60psi on the oil gauge on one occasion!

Paul Hunt

This thread was discussed between 29/08/2001 and 30/08/2001

MG MGB GT V8 Factory Originals Technical index

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