Mystery Overheating

My water pump belt broke, so husband replaced it. Next time I drove fast, temp warning light came on. Hubby checked water, no water lost but belt lose, so he tightened it. Got light on again when under load, belt not loose this time, and no water lost. Temp light always goes out when I drop my speed and turn the heater on.

Changed thermostat and temp sender, still got same problem. Tightened belt more, still got problem. He has checked the fan comes on when idling, he has taken out the water pump and says it is fine - what else can it possibly be!!

Could it still be the pump belt is too loose? A mechanic checked it and reckoned it was fine. Don't want to over-tighten and break water pump.

mystery car too tracy :)
assuming its a k12 ? it sounds like the the coolant is,nt circulating properly eh

Yes, it is a K12, didn't know that myself till you asked, but that is what the owners manual says!! Would the heater still work if the coolant wasn't circulating? The heater is definitely working fine.

I would be going with an air lock in the cooling system.
Check to see if it has a bleed valve to get the air out, Otherwise you will need to remove the radiator cap (When cold) and run the car until it warms up and continue to top up with water as the air comes out. Make sure the heater is on while doing this.

Thanks, we are going to try this and see what happens. Will let you know how we get on.

How long did you drive it before you noticed the water pump belt was gone.

You might have cooked the motor and as a result warped the head creating a head gasket failure.

Failing that bleed the coolant system as you might have an airlock in it from taking waterpump off and also thermostat.

I would be saying bleed it first if that does not help you could be up for a gasket change. If the belt is reasonably tight then you will be fine. If it's to loose it will squeal under power normally. If the mechanic checked it and said it's fine then it's fine.


Bleed bleed bleed.....

one symtom of head gasket failure is that the coolant pipes can go very hard , despite the engine not being warm , as the engine pressurises the coolant system . worth a check ?

may even pump coolant back into the overflow bottle as a result ?

try also ruuning the engine and check for bubbles rising into the filler cap opening with the rad cap off .

the OP twice stated no coolant loss tho eh, if the heater is working ok then cant be a pump/belt problem imo.
i would check there,s a decent flow out of the top hose personally

Thanks for all of these. Hubby putting water pump back on today, so we'll try bleeding first. It was driven for about 10 mins after the red light came on when the belt broke (my elderly mum was driving so she just wanted to get it home!) We have been thinking about head gasket, but would expect to be losing coolant. Pipes don't seem to be pressurised, but maybe it's only leaking a little bit. If bleeding doesn't work, I think we're going to have to go this route.

just might be worth checking the thermostat still works ok before stripping the block down , it may of got overheated and now sticks .
May not be opening fully ??

We think the head gasket was slightly gone, and has now gone completely because the coolant suddenly disappeared when he has bleeding it! He put Radweld in and the pipes were definitely softer after that. Hubby took thermostat out, and we don't get the red light as long as the heater is on.

Thanks for all your help but I think it is a head gasket change for us!

be carefull when running the engine if the water is getting into the bores , it may lock the engine when starting it , may even bend a rod , also the water will wash the oil off the bore causing piston wear and its resulting piston slap .

Did you fix the overheating and what was the problem