I posted a week ago about my 87 S4 overheating probs. I got a lot of good responses and tried most of the ideas that i didn't try before. Here's whats happening now, as I check if the cooling fans are working or not with the hood open standing outside the car, they seem to be working fine and the car stays cool as normal. But the minute I get in the car to take it for a test drive the fans only come on for less than a second and the car starts to overheat again. Does anyone know if their is a sensor in the drivers seat that might be malfunctioning and if so how do I repair or disconnect this sensor. Any help with this situation wood be greatly appreciated.

 

Just sitting in a stationary car ? Don't sit - stand
on the door sill.

You must be driving it or how do you know it overheats?

need nore info.

G'luck.

 

if you removce the coolant resivour cap do you see bubbles with the engine running? have you replaced the cap on the resivour? have you replaced the thermostae and the sealing ring that is press into the housing behind the thermostate?

 

The hood switch alarm contact I thought was supossed to turn off the fans when the hood was open at least that was what I read. But it sounds like yours is working different from that.

 

The only seat switch was option 007 in an Aston Martin.
Seriously, what is the condition of the cooling system ? Do the fans come on with the AC ? Are the flaps functioning properly ?

 

it would help to have continued the previous thread so that we can all see what has been suggested and tried before.perhaps you could provide a link.

Ok found it .

You could have a look at some of the suggestions here
http://www.nichols.nu/cat22.htm

You need to be methodical and work through everything . Don't trust a new thermostat. Stick it on the stove with a thermometer and make sure it opens at the right temprerature. Its easy to put them in upside down.
If everything checks out ok and you have done a pressure test of the water system then you might need to think about the worst scenario. ie plastic impellor slipping or off the waterpump spindle. (very expensive.)
Let us know what you have actually done and the observed result. good luck

 

heres a good discription of the cooling system of the various models courtesy of Wally Plumley
http://www.nichols.nu/tip729.htm
Engine Cooling

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

 

I have disconnected the flap fuse so that they stay open, & the thermistat is about 9mo. old and have not ckd. as of yet. As i said with the car running w/hood open(hood switch connected or disconnected, tired both ways)the fans & flaps come on and off in which seems in a normal manner. The gauge reads normal in about the center range. But when i close the hood and get in the car i watch the gauge and can hear that the fans are not coming on, and the gauge will slowly start to rise past the the white line to the red. I tried this 3-4 times because i didn't believe this was happpening in this manner. But it happened every time. I turn off the engine to let it cool down leaving the ignition in the on position, get out of the car, open the hood and the fans will restart when the car cools down to about the top white line. I start the engine and the fans again start operating normaly, until i get in the car again. Any ideas and thanks again

 

The final stage that controls the fans is the finned device in nose of the car on the right side. Check your connections (thry're bolted down) and make sure there's nothing wrong. It's got to be the closing of the hood that's causing the problem.

 

well you might try cycling the switch for the hood its next to the hot post under the hood on the passenger side run the car with hood opened when the fans come on try pressing the switch see if the fans go off when the switch is pressed

 

Rob, I had similar overheat problem-- fans would stay on or not come on, right to red zone and coolant spill over. Check the fan fuse (mine was all melted), but not sure, as I had thermostat done at the same time. Shawn

 

I recently had the same problem with my 1984 and by replacing the thermostat it did the trick. Prior to replacing the thermo, the lower radiator hose never got really hot (which is probably why the fan switch never turned on), while the temp sensor area of the block (temp gauge) was extremely hot to the touch. The car would overheat after several minutes of running. When I put the old thermo in a pan of hot water, and boiled it to death...it didn't even budge. Unfortunately, this was my last failure point as the prior owner replaced the thermostat only 1 years ago (3,000 miles) so I guessed it was the least likely to fail. Of course, this was after: radiator removal and clean out, new fan clutch, new fan relay, new radiator mounted fan switch, and new radiator hoses. Car runs very cool now.

 

Note the above mention of the O ring for thermo is WRONG - it goes in FRONT of the thermostat, under the outlet part;
On another make I had a thermo go bad on me - would overheat in very short distance, and when shut off, make loud banging noises as the pressure rose and fell under the thermo.
As somebody else suggested, something is changing as you ope/shut the hood, or get inside somehow....
jp 83 Euro S AT

 

Your description requires a two step determination.

1) with the car running and the cooling system functioning properly with you outside of the car and the hood open..... ONLY close the hood and watch from outside the car....... overheat implies the hood position causes the change (maybe hood switch, may be something else the hood touches, but it's in there somewhere)

2) with the car running and the cooling system functioning properly with you out and the hood open..... ONLY get into the car and leave the hood open....... overheat implies it's NOT the engine compartment (not sure what it would actually implicate in the passenger compartment, though)

 

If you are referring to Stan's (MrMerlin) mention of the "sealing ring", he is right. There is a sealing ring that goes behind the T-stat and a large O-ring that goes in front, as you mention. So, you're both right.