temporary overheat

[wellskint]

after about 4 miles when my renault diesel warms up the temperature guage continues to rise to the point where the water overheat light comes on.

sometimes after about 15 seconds it goes out and the indicated temperature falls back down
sometimes i turn the engine off the second the light comes on and this seems to return the temperature indication to normal
sometimes when you open the bonnet and look at the expansion tank there is evidence of water having been expelled (throught the tank lid) as it is below the minimum mark and when you release the expandion tank lid if gushes out under pressure.

anybody got any ideas please i'm wellskint

[littleredrooster]

Assuming the temperature sensor is in the cylinder head, that would suggest that the Thermostat is getting lazy and not opening at the correct temperature, or not opening quickly enough.

[GreenV8S]

Suggest you find a point in the bypass circuit where you can measure the temperature using a contact or IR thermometer. Any metal spigot/joiner/pipe that has bypass flow through it and is not solidly connected to anything cold should do the trick. If the stat housing is accessible that might be a good option. This will let you measure the actual coolant temperature so you can see whether it is actually overheating.

Also find a spot on the radiator or top hose that you can touch safely. This will let you feel when the stat opens.

I don't know what the designed stat open temperature is on your engine but mid 80s to mid 90s C would be typical.

If the engine is overheating and the stat is staying closed, replace the stat.

You may find you have some other fault such as a blocked radiator, failed cooling fan or just a leaky pressure cap. Check the actual temperatures before you try to guess what's wrong.

[TooMany2cvs]

Thermostat is the obvious cause, if the temp really is doing what the gauge suggests. The gauge could be lying, though.

IR thermometer - less than a tenner off eBay. If the stat is slow opening, then the rad will still be cold when the coolant in the engine is roasty-toasty.

A new stat is cheap. Change the coolant at the same time, and give it a bloody good clean through.

[wellskint]

i was thinking the thermostat but cant help wondering if its the head gasket becuse ther is loads of opressurein the expansion tank when you release the cap - so much so loads of coolant gushes out. is tht normal?

[GreenV8S]

If you release the pressure cap when the engine is hot, it will be under pressure and spray out steam and hot water. That's bad for you and bad for the engine.

You should only release the pressure cap when the engine is cold.

Edited by GreenV8S on Monday 5th February 11:01

[E-bmw]

HMM, now where have I heard/seen that before?

https://www.shutterstock.com/image-photo/radiator-...

https://www.myg37.com/forums/private-classifieds/2...

Oh, i know, it is on every radiator/expansion chamber top I have ever seen!

[Mr2Mike]

Could be a head gasket problem, easy enough for a garage to check this by testing for exhaust gasses in the coolant. Could also be caused by a simple airlock, or by a failing water pump (impeller loose on shaft).