Hello guys,

I've just got myself a lovely S-Type V8 4.0 (2000) and I have the strangest problem with it over heating (gauge reading) but it's not over heating at all.

This is where it gets really strange! Here is the problem I have and I'm getting baffled ......... Okay, I start the car up from cold, drive it up the road, 15 minutes later, the temp gauge jumps from normal (half way) to maximum HOT within 5 secs, now here is where it gets strange because while I'm driving it and the temp is right over in the red, (not stopping the car) I put my heater on inside the car, air blows out really hot as it should, then my temp gauge goes back down to normal again and stays there!

When I first see this happen, I was naturally pulling over and turning the car off but as I've discovered there is no water loss, engine is not over heating (even though gauge says so) I just continue to drive it.

I have changed the thermostat and changed the temperature sensor. I've also tested the head gaskets (compression test) so it's not that either. It's had a new water pump as well fitted 1 year ago. Bottom radiator hose is hot, so thermostat is working as it should. I don't lose any water from the system, expansion tank is always on max.

Now my only guess is an air lock in the system, hence switching the heater on in the car stops the temp gauge shooting up. I have looked everywhere for the correct way of bleeding the air out of the system but can't find it. There seems to be one on the V6 but that's not the V8 which is totally different.

Does anyone know the correct way of bleeding the air out of the system on these V8 engines or does anyone have another idea on what it might be?

Thanks.

 

Download a copy of the JTIS it is the workshop manual. If you buy yourself an elm 327 you should be able read the actual temperature of the engine because the temperature gauges are not all accurate mainly there for show but don't disregard as it may be showing that you have a cooling problem.

 

I had a similair issue with my X type a few years ago. The guage doesnt show actual engine temperature per-say. It shows what the ECM thinks is the appropriate temperature range, I think someone refered to it as a "messaged" signal. That is to say that when the guage is rock solid in the middle, the engine is somewhere in a specified temperature range and the ecm sends a signal out to the guage that the temperature is perfect. As soon as it starts to overheat, the ECM sends a panic signal to the guage to indicate full-hot.

This scared me the first time it happened on the interstate in my x type halfway to work. I pulled over, checked the engine over, and decided it wasnt too hot to finish the drive. I later found out that by shutting the engine off for a few seconds (sometimes while driving on the interstate) the guage would settle back down immediately to the middle. It turned out to be a failing thermostat, an the engine really was over heating (but only a few degrees over the maximum "ok" temperature band).

 

I've just attempted to bleed the system to get the air out. I was running the car with heater on blowing hot, all was running normal, heater was hot, lots of coolant movement so water pump is doing its job. Now when the engine was running, I turned the heater off from inside the car and the water level was rising immediately. Now I was standing there thinking it could be the expansion tank pressure cap. If the cap is faulty and the water is rising, the water has no where to go unless I turn the heater on inside the car which relieves the water. Or am I just going mad lol

 

Work on the basis that it is overheating and do not let it do it again as the 4.0 cannot cope with it. It may be a stuck thermostat, bad engine fan module, airlock or whatever but be aware you can easily destroy the engine and it will not tell you until it's far too late.

 

+1 on bad fan module. Is the fan running at temperature? Should be on low then should increase in intensity as the temp climbs. Turn AC on and fan should run full speed.

And yes, get this sorted very, very, very quickly!

 

Just a quick update .... Whatever I did the other day with attempting to bleed the air out must of done the trick! ... I've just been out there with the car ticking over for about 30 mins with no heater on inside the car and it's not overheating now. The gauge is sitting there just before the half way and just as it started to move dead on half way, the fan came on and put it back down to just under half way, .... So it must be cured now, it never did this before, normally jump from half to the red in seconds. I'm now a happy bunny and can enjoy my lovely S-Type until winter time (icy conditions) because if it's anything like my old S-type 3.0 it was the worst car I've ever driven in the ice and snow!! Hopefully the new tyres I have on this one might cure that problem as well (Pirelli ).

Thanks guys for all your help and information ;-)

 

i have this same issue, threads all over the place, done what i needed to do and yet it persists?random(guage panic) over heating that is not overheating

 

Have you have a scan tool. You should be able to see the actual temp.

 

a scanner will not read the correct temp via obd2 if the temp sender unit is the root cause .
also it looks like we are talking about an xk8 here in the s type forum ?

 

Exactly - that is why OBD should be used.

If it shows the same as the gauge then a bunch of things can be ruled out and a different bunch if it's different.