Engine Cooling

Cooling Air Flow Diagnosis

Cooling Air Flow Diagnosis

Where might one begin to diagnose an ariflow 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 switchable 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 weatherstrip 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

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