God I hate cooling problems also. A good way to tell if your thermostat is working properly is to feel your radiator hoses. if the thermostat is open both hoses will be hot. If not, the bottom one will be cold turkey. This also identifies if your radiator could be clogged too.

In my case the fans just weren't coming on so I changed the fan switch, nothing. Looked up and what did I see? a severred harness! Make sure that your harness going into the fan switch isn't cracking or wires aren't showing. Get a lower temperature fan switch and do a low temp thermostat. I believe this may help it. But if your radiator fans still aren't coming on, there must be an electrical issue on the board. I wish I could say I could help you with that. best of luck to you.

 

Incessant - That's one thing my car was doing. Temperature would get high at a stop light, then it would drop quickly when I started moving again. I'm not sure if replacing the fan relay fixed that yet.

 

I have to say I too, did the Clark's garage test when mine started to run hot. It failed and I bought a new thermo relay switch. Guess what? The new one failed the same test. So, I thought, I'll buy a new radiator, any other car that I have owned I would START with a new radiator, so why not try that.

Even with a new radiator, I have the same problem. At this point I feel that in my car the fans just do not come on soon enough. As soon as the temp gauge is about to hit the third line the fans kick in and bring it down to just above the second line, where it has been the entire time that I have owned this car, until this problem started.

Crap, it just hit me. Could it be that on my car 'low' is not working and 'high' is and it needs to get to high before the fans come on?

Rennlist you're the best.

 

Sure sounds like your fans aren't coming on; when you're moving air is pushed through the Rad so you get cooling. When you stop, no airflow, hence the need for fans.

In fact I'd say if you only get problems when stationary, this is very likely the problem. If it was the Rad, thermostat etc, the problem would likely not go away. I say likely as, as mentioned elsewhere, overheating can be a sod to cure unless you go on a parts replacement spree.

Simple to check the fans. Start her up, leave it running and watch the temp gauge. When it gets hot, the fans should come on. If they don't, you can then check a) is there a live feed to the fan b) is the temp sender providing an earth (try earthing direct from the fan earth side to check by removing the wire from the sender and earthing it ) - if the fan comes on, its your sender c) if not repeat the test by earthing from the fan end - if it comes on then you have a wiring problem, d) if not then if you are getting live and an earth, then its the fan motor.

Hope it makes sense, its just a process of elimination. Be careful when you're doing this - you don't want it to get so hot that you blow a head gasket!

If you find the problem, let us all know.

 

Have you pressure tested the system? a small leak, cut hose or a bad clamp may produce the same results.

I chased down an overheating problem on a Toyo- turned out to be a headgasket.
Double check oil/ coolant mixing.

EDIT Make sure the temp sender is correct- is it boiling over or is the temp guage going to red? Boiling over may be an air bubble in the system.

I agree- coolant issues are infuriating.

 

Only the automatic 944 radiators are a bit larger. Standard 944/951 radiators are the same size.

Thermostat would be my first guess.

If the fans are operating erratically, in addition to the relay, I would check the fan circuit resistors. They're located beneath the plastic cover between the windshield and the firewall, on the battery side. There are 2, and they're 944S2 specific.