Where might one begin to diagnose an airflow problem? When driving in the city with the air conditioner at full blast (which really doesn't seem like much, but that's a whole other story..), the car gets hot after about 1/2 hour. When driving with the a/c on, on the highway, I can go for about an hour before its above the upper white bar....

What do you suggest?

You can pretty well divide 928s into two categories where cooling air flow is concerned - splitting between '86 (early) and '87 (late).

Early Cars
Early cars have a belt driven fan, with a thermostatic clutch. The clutch on many cars is getting weak. There is not really a good test for the clutch, but one quick and dirty check is to open the hood on a hot engine, rev the engine to 2500 RPM, and hold it there for perhaps ten seconds. If there is not a pretty strong blast of air from the fan, the clutch is slipping too much. A new clutch is expensive - between $280 and $300. There is a procedure to refill the silicone fluid that sometimes helps, sometimes doesn't - it costs about $10.

Early cars have one electric cooling fan. This fan is triggered by one of three sensors: a thermostatic switch in the lower left forward face of the radiator, which measures coolant temp; a thermostatic switch on a stem on the A/C receiver/dryer, which measures Freon temp; and a thermostatic switch in the intake manifold, which measures manifold temp, primarily for after-run cool-down (not on the earliest cars). The radiator switch can be replaced very cheaply ($6) with a 75 deg C unit, which will cut the fan in sooner. The Freon switch isn't too effective, in my opinion, and I was tempted to replace it with a switch able relay driven from the compressor circuit. The inlet temp switch is primarily for cool-down after shut-down, and really doesn't affect overheating.

It is important that both fans be fully functional. A fully functional clutch on the belt-driven fan, and a 75 deg switch on the electric fan will take care of most problems.

Late Cars
The late cars have no belt-driven fan, but instead have two computer-controlled electric cooling fans. The sensors are similar to those on the early cars, except that the A/C switch actually is a Freon pressure switch, not a thermostatic switch, so the fans run anytime the A/C is on.

The fan control is fairly complex. Checking consists mainly of ensuring that the fans are both running at full speed anytime that the car is really hot, especially if the A/C is on. The final stage control (the finned black box on the right front header panel behind the bumper) occasionally gives problems.

If your car has the movable air flaps behind the grille, you might consider pulling the fuse while the flaps are fully open. Slightly more drag, slightly slower warm-up, but eliminates one possible problem area.

All Cars
Recirculation of hot air will cause problems. Inspect the area around the radiator, and ensure that there are no air leaks around the radiator. What you are trying to avoid is hot air from the engine area sneaking around and going back thru the radiator, especially while sitting still in traffic. If your car has no spoiler, and no belly pan, recirculation is almost certain in traffic.

(Incidentally, while it is not a factor in engine overheating, if the weather-strip that runs between the back of the hood and the firewall is missing, very hot engine compartment air goes straight into the A/C air inlet, so you are trying to cool 200 deg F air, not 80 deg F air - quite a difference!)
Check the front face of the A/C condenser for bugs, dirt, and bent fins.

Check the space between the A/C condenser and the radiator for leaves, bugs, dirt, and bent radiator fins. A high-pressure spray from a self-service car wash will help here.

And finally - remember that, while poor air flow is the most common cause of low-speed overheating, it isn't the only cause.

Wally Plumley
928 Specialists

At 09:36 AM 10/10/01, res062vo wrote:
Can anyone tell me why my electric cooling fan as decided to stay on well after the car has been turned off??? The other night it stayed on until the battery finally died on the car. The temperature has been cooling off. 35-65, and I just had my fuel pump replaced.. the car has not been running perfect (slight roughness) since the fuel pump replacement... now this? I'm at a loss, unrelated? Totally related? or direct cause info would be appreciated.

Thank you.
Bill Frisbee '89S4

On the later cars, the fans should run on at low speed after shutdown if the engine is very hot. The switch is on the top of the intake plenum. The switch should be open (infinite ohms) below approximately 87 deg C., and closed (zero ohms) above that.

If the fan is running on when it shouldn't, unplugging the switch will usually stop it.

Wally Plumley
928 Specialists



Dave Roberts was spot on, just disconnect the fuse (ignition off). I was surprised to learn from my friends in Germany that the factory mechanics recommend disabling them. I can see how they can help in cold climates, there is excess cooling and the A/C isn't on. I guess they were just too elegant of a solution that caused more problems then benefits.

I was surprised to see the flaps working on my '87 when I purchased it.
Give our long hot season I decided to leave them in the open position.



My '88 928s4 still has those nose flaps working. I check them sometimes when I think the temp is a little higher than normal.
Strangely enough they are not always open in the off position, and I catch them sometimes in the half-open or closed position when I expected them to have opened fully. But then again: the system seems to work correctly since they do open when the fans come on or when I switch on the AC. Its obviously designed to speed up the warming up process specifically in cold climates and/or wintertime. At temps of -10C (14F) I understand Porsche's intentions and it does help. There's also a goal of improving the cw value I think.

I doubt if the flaps are default in the open position when ignition is off. Its easy however to take the rubber off from the motor, turn the knob, and *then* take out the fuse (or indeed: unplug the device as someone mentioned).

During the summertime I usually take the fuse out when I feel there's no point in quickly warming up the engine but I put it back in the winter.



>When I changed the coolant this summer, I decided to try a 75 C thermostat in place of the stock 85 C thermostat -- the fans and flaps were functioning >normally. With the old thermostat, the temp gauge was almost always halfway between the lower and upper bars -- since then, the temp gauge ranges >from just a hair below the lower bar to slightly above the lower bar. Well, it was 35 F here in southern Wisconsin when I drove the shark out of the >garage this morning, and I am wondering whether I am putting the ECU into too cold/rich of a condition?

>Thanks in advance. Bill Schmaal 87 S4 a/t

I am not sure you are doing yourself or the engine any favors with the lower temperature. The reason the flaps were part of the S4 design was to allow the car to heat up quicker in cold weather. Here in sunny California we happily remove or lock open the flaps, but getting an engine to warm up fast in cold climates is important to longevity. A cool engine - one not up to operating temperature - will wash raw fuel down on the rings causing the lubrication to be diluted wearing out the rings and engine prematurely.

Yes, Porsche deleted the flaps on the new cars, but this was probably more due to common failure than a failure of concept. If I lived in a northern climate and was foolish enough to drive the 928 during the winter, I would activate the flaps and put in the higher temperature thermostat. The concern during the winter is generally not overheating unless the front of the car gets clogged up with snow and ice. The concern is getting and keeping the engine warm enough to get the highest efficiency.

Unless you have chronic overheating problem - and if you do it should be addressed - then I would change to the higher temperature thermostats in the winter and back to the lower temperature ones in the spring. Remember they come in pairs and should be matched.

Dan the Pod Guy
Portia's Parts