A/C causing overheating

[Yoni_d]

Hi Guys

When I start my A/C the engine tends to overheat when the car is idling. Usually the car drives fine and there are no overheating problems.
The electric fan works when the A/C is on.
The auxiliary air valve is not the original one - see attached pictures but I guess that it is not important since it only opens an air way - and it does that.
The engine idle drops a little when the A/C is on (from 900 to 800 ~850)

The question is what can be wrong?
I assume that when the A/C is on the fuel mixture gets very lean and thats what causing the phenomenon..
Do you have any ideas?

Thanks!

[924CarreraGTP]

I bet it's a bad thermostat. Are you sure it's only when the A/C is on? Have you let it idle with A/C off? My '83 did this too for a while. I pulled the thermostat and it was sticking. I put in a used one that I got for free out of an '85 928. It tested good and it runs exactly where my '84 runs now. A little bit over the first mark.

[Mrmerlin]

If after swapping out the thermostat it still runs hot,
then you need a new fan clutch,
you can also add some water wetter to the coolant

[Yoni_d]

It is a very interesting point but when I am driving the car on the highway, the engine heat is between the 1st white line and the middle so if the thermostat was bad, the car should have overheated on the highway right?

[dcrasta]

The factory pusher fan is pretty high volume. If your replacement is not high flow then that could be your problem.

If the car heats up when standing still then is ok on the highway its usually a fan issue not radiator.

If the car heats up on the high way its usually a flow issue (Radiator blocked)

Of course , there are exceptions however this helps to determine where the issue is.

You may want to replace the thermostat and the orings in the thermostat housing. Often people do not replace the rear thermostat oring, this leads to issues.

Also I would wash the radiator and the area between the radiator and AC condensor.

I still would bet its that 'pusher' fan. Dont use some corney Import autozone fan on there.

Also is your shroud in good condition ? Running without a shroud is not going to keep the air flowing where it needs to go.

(wshew. Coffee makes me type fast..)

[Danglerb]

Temp OK at cruise, not ok sitting still at idle, very good chance it is air movement, bad fan clutch.

[JK McDonald]

Hey Yoni - Your low speed overheating problem sounds like an issue often associated with a failing mechanical fan clutch. The best solution is to replace the fan clutch or alternatively refilling it with the proper type and quantity of silicon fluid. There are several good write-ups on the procedure. Like others have also mentioned, for the best possible cooling performance check your thermostat and add “water wetter” to the radiator.

In addition to correcting the performance of the mechanical fan clutch, you might also want to add a manual toggle switch to the circuit that turns on the electrical cooling fan mounted on the front side of the radiator.

Normally the electric fan is turned on by either the A/C Refrigerant Temperature Switch (mounted on the A/C dryer) or the Radiator Coolant Switch (mounted on front left lower corner of the radiator). Once the temperature for either switch has reached the trigger point, a ground is applied to relay 18 (A/C cooling fan relay) which directs power to the cooling fan motor. By adding a toggle switch ground in parallel with the two temp switches, you can apply power to the electrical cooling fan as soon as you see the temperature gauge begin to climb. This can be especially helpful if you find yourself sitting in heavy traffic.

I typically mount the new toggle switch in the parking brake handle housing on the left side of the drivers seat.

Good Luck, Michael

[Yoni_d]

Hi Guys,

Thank you very much for the advice!
I checked my mechanical fan and consulted with other mechanics and by all opinions it seems to work fine.
I do have quit old electric Push Fan and my thermostat is a big question mark.
I think that as a first step, I will replace the thermostat and see if there is any change. If not, I will replace the electric fan with a newer version.
The cooling radiator is clean and the air movement seems to be fine as well.

Its a bit frustrating... :\

[Mrmerlin]

the fan that needs replacement is the belt driven fan clutch hub

[stepson]

Agreeing with Mrmerlin here. Chances are your viscous fan is NOT working properly.

Check also that your evaporator fan turns in the correct direction when it is on. I have heard (never seen in person) that if the fan in front of the evaporator core is wired backwards somehow, that it will work against the viscous fan and allow the car to overheat. I've never seen it in person, but it's worth a 10 second check

[Yoni_d]

Thanks guys!
I will follow your recommendations

[924CarreraGTP]

I switched out my fan first because I had one, but it turned out not to be the fan. Now I have a later plastic fan though so its an upgrade in some way. It's surely lighter than the original '83 aluminum fan. The fan should have a little resistance when you first turn it, but it should free up at about half a turn or so. At least both of my fans do this and they cool both cars properly.