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I've had this problem ever since completing my MkII Cooper restoration a couple of years ago. Everything is standard 998 Cooper spec'.
Temperature gauge comes up to normal with car at idle, much as you would expect (although reading slightly above normal).
However, once on the road, the needle drops down to cold, doing around 50mph (probably less).
When I stop or reduce speed to make a turn, the needle again rises to just above normal.
I've checked the gauge and voltage stabiliser using the Smiths instrument tester, all functions correctly (fuel gauge operates correctly also verifying the stabiliser is ok.
I've replaced the sender and the thermostat for good measure, but with exactly the same results.
With the car stationary and engine speed raised to replicate road speed, the needle creeps up towards hot but never drops to cold. This should indicate that the issue is related to airflow at road speed, but why does this cause the excessive cooling effect I'm seeing ?. I'm at a loss now as to what could be causing this, any thoughts ?

Is the fuel gauge acting normally or does it also move up and down with speed?

dklawson wrote:Is the fuel gauge acting normally or does it also move up and down with speed?

That was going to be my question!

I have seen two that suffered in a similar way and was eventually worked out to be air flowing over the voltage stabaliser changing it's characteristics, Swapped them out for Solid State types and that was the end of the story.

Might be in idea to check in fact that the engine temp really isn't rising and falling. I am also assuming you have a working thermostat fitted.

Fuel gauge functions normally and thermostat is new. The old thermostat was OK anyway so wasn't causing the issue. I don't see a need to change to a solid state stabiliser, its all factory spec so shouldn't need to be altered. Stabiliser and instrument are the same parts as before restoration and they worked fine then.

I checked the stabiliser output again, steady 10 volts. I then connected the gauge to the tester and set it to normal (ie. gauge taking its feed from the stabiliser and "signal" from the tester as opposed to sender) and the needle sits bang on centre. This is all with the car stationary, I can't easily hook the tester up and drive the car. So it does appear that stabiliser and gauge are functioning correctly. I tried holding the engine at higher rpm to replicate driving however this just results in the needle creeping up towards hot, it never drops to cold. So it does appear to be linked to airflow but I can't for the life of me think why. I should also add that the water pump was also replaced during the rebuild. Stumped !

Perhaps you have a water pump rotor size/ pulley diameter and therefore fan and water pump speed problem

R&R wrote:Perhaps you have a water pump rotor size/ pulley diameter and therefore fan and water pump speed problem

All parts are standard though so this shouldn't be the case. Apart from replacing the pump (standard) everything is the same as before the restoration when this issue didn't exist.

111Robin wrote:I checked the stabiliser output again, steady 10 volts. I then connected the gauge to the tester and set it to normal (ie. gauge taking its feed from the stabiliser and "signal" from the tester as opposed to sender) and the needle sits bang on centre. This is all with the car stationary, I can't easily hook the tester up and drive the car. So it does appear that stabiliser and gauge are functioning correctly. I tried holding the engine at higher rpm to replicate driving however this just results in the needle creeping up towards hot, it never drops to cold. So it does appear to be linked to airflow but I can't for the life of me think why. I should also add that the water pump was also replaced during the rebuild. Stumped !

Are you able to try it while blowing some cooler air over the stabaliser?

Not easily. I doubt this is the issue, it's behind the bulkhead insulation on the rear of the speedometer so isn't in direct air flow.

Regarding the pump I have no idea if its the small or large impellor pump. The small impellor would have been on it originally but if I now have the larger one but with original pulley would this lead to cavitation at higher rpm that would mean the sender was not surrounded by water hence reading low ?.

The cavitation would be localized around the pump and not likely to cause air pockets in the head. However, long term cavitation can cause damage to impellers which might reduce flow. I have not personally experienced this on a car.

I do not suspect the voltage stabilizer since the fuel gauge is behaving properly. Though this will involve an investment of time and money, I suggest getting a mechanical temperature gauge and installing it at least for a short while to determine if the temperature changes are really happening or if they are something unique to the electrical gauge system. You do not have to invest a lot for a temporary mechanical gauge but I would buy one whose needle moves through 270 degrees (angular) sweep on the face so you have increased resolution in your readings.

Probably the best idea as you say, before starting to change out anyhting.