cranky

i have a 1988 928s4 that recently started running hot. took car to dealer but it performed perfectly for them(of course). humid days seem to be the common element when it does overheat( sometimes it runs just fine). today i noticed that the vents weren't open that allow the fans to cool the coolant so i used the manual control to open them. i think the problem then is with an actuator that controls the amount of opening the vents need according to conditions. if i am right is this something a novice mechanic can fix(like me) or will i have to go back to the dealer? any help will be appreciated. those vents were completely shut when the engine was running hot so i figure that has to be it-it seemed like the fans sounded funny because they couldnt get much air. thanks again

Kaz

I have an 88S4 that is serviced by Markus at German Auto Art. Mine never overheated to begin with but Markus disconnected those flaps anyway leaving them open. Now my temps stay low, even when running over the San Fernando Valley in So Cal. That place is just plain hot.

Tails

The engine cooling flaps are quite a complex system, linked into the airconditioning circuit, the engine cooling water circuit and the air inlet temperature sensor in the intake manifold. they work on a step function until they open to 100% depending on the cooling water temperature sensor.

Even Porsche found the system too complicated and removed it from their 928 GTS models even thought it improved the air drag (CD)coefficient of the vehicle.

I have checked out the circuit on my S4 and got the whole system working OK to specifications. Cooling temperature Max 87 degrees C even in summer temperatures of 40 degrees C and above with A/c full on.

Generally if the flap system is not working correctly then remove the fuse when the flaps are in the fully open position.

Simply turn your ignition off so they are in full open position, then go inside your fuse relay panel and remove fuse #23 which is the fuse for the flap motor. It will now stay open until your replace the fuse.

You can also buy a thermostat rated at 75 degrees C to enhance the cooling of the engine.

Other exponents of engine cooling also suggest water wettner additives to your cooling water.

Hope this is of assistance.
Tails 1990 928 S4

Ed Ruiz

The flaps are controlled by the Final Output Stage (the finned box next to the hood lock). It also controls the fan speed. If the FOS is bad or going bad, then the fans will not come on at the proper speed. In my case, only one fan was working. I replaced the FOS, and the car no longer overheats.

The way to test for a bad FOS is simple. Turn the ignition to on, but don't start the car. Select any cabin fan speed, then push the AC button. Check to see that both cooling fans are on. If not, the FOS is defective.