Coolant System acting up. Got logic?
 

I've been chasing the "continue to bleed the coolant system" tail and I'm getting dizzy. Have an 87 924S that I'm getting ready to hand over to my son for the summer, so I'd like to nip this before he gets it.

The temp gauge on the dash moves regularly and always hovers in the 3/4 & higher range. So I've been continuing to bleed the air out of the system (with a raised front end)- although it seems all I get is a little burp then coolant.

Thermostat
New Behr 80 deg thermostat installed with the reman waterpump while engine was out.

Not getting anywhere I've approached the following:
Check temp gauge
Set ohms resistance based on Clark's article at 40, 105, & 120 degrees C. The gauge read correctly, within a needles width, at all three settings.

Checked the temp sensor
This one was weird. I placed the sensor in 212 deg F water (boiling) and measured the resistance. It measured 67 ohms. Per spec at 220 deg F it should measure 33.6 ohms. So that tells me the sensor is measuring cooler than actual - right? And I should be seeing slightly cooler temps on the gauge. Not.

Thermo switch
Fans have been operating fine, but I wanted to checked the temp it was operating at. I checked the themo switch today. removed it and checked in water. It's a 75/70 deg C switch and opened at 165 deg F.

Conclusion?
If I'm reading Clark's coolant flow diagram correctly, that means the fan is coming on too early (at 70 C) so the 80 C thermostat never opens - and the coolant doesn't cycle for optimum cooling. I trying to understand this correctly before just throwing new parts at it.

Is that conclusion sound? If not - what do you suggest?

TIA,

Basic Question: When you bleed the cooling system, do you turn the heat completely on?

fan switch on the radiator is shot.

<<If I'm reading Clark's coolant flow diagram correctly, that means the fan is coming on too early (at 70 C) so the 80 C thermostat never opens - and the coolant doesn't cycle for optimum cooling. I trying to understand this correctly before just throwing new parts at it.>>

Not correct. Porsche uses a double acting thermostat. The thermostat controls both flow to the radiator and flow through a bypass passage. At any point in time, it's both "open" and "closed"...either flow is directed to the radiator or to the bypass. If flow goes through the bypass, coolant circulates around the block until it's hot enough to cause the thermostat to open the passage out to the radiator. If your fan is on at too low a temperature, then you "running on the thermostat", meaning that the thermostat is constantly cycling. It's unlikely that this is the source of your problems.

I'd definitely replace the sensor, and rather than trust the gauge, use an infrared thermometer to measure temperature at the outlet. Did you open the air bleed that's on the upper hose connection?

I have the same setup. 75*C thermofan switch & 80*C thermostat.

Same symptoms a few weeks ago. Replaced the thermofan switch with a new 75*C .

My temp needle will go to just about half and the fans kick in dropping the temp to just above 1/4.

With the AC on temp needle stays at about 1/3 constant.

matt is correct.

BUT-------

When you put the thermostat in, did you use the 2.5mm thermostat gasket or the larger 4.5mm? This is important and may the reason why you overheat>

Per Clarks/Fletch read the thread:
Clark's Garage Message Board :: View topic - Thermostat installation

GL
John

did this start all at once or did something happen? If you continuously get air then the head gasket could be starting to compromise. That would be the only way you would consistently be getting air. I use a "spill free funnel" that locks onto the coolant tank and keeps coolant above all other points of the cooling syestem while filling this removes all of the air.

Thanks for the feedback guys.

When bleeding, the heat is on full and I use the bleeder at the top of the inlet at the front of the motor.

John, you may be on to something. I recall having problems with the thermostat, gasket & oring. The oring was to thick to fit and the snap ring took up all the space in the groove. Its been many months since I put that in since it was done while the engine was out.

I read the link you provided, but I'm still not clear how the spacing of the thermostat flange with a gasket or oring affects the thermostat opening properly.

Also, there wouldn't be any benefit for me to replace the thermo-switch if mine tests and operates properly. Or is there something I'm missing?

Thanks again,

the wrong gasket will let water bypass the thermostat and can contribute to overheating. It needs to be a good tight seal.

Bad,

Thanks for the feedback. I'm not questioning you personally, but I've got to press this further. How can water bypassing the thermostat lead to overheating, when the thermostat is designed to open at a temperature below overheating? So that means the only time water would be bypassing is when the thermostat is closed - at a temperature well below overheating. There must be more to it than that.

Agreed. That should ground at 212 unless im confused about its function. Either way. Wouldn't cause this problem.

You said no overheating problem before you changed it?

the thermostats purpose is to hold water in the engine until it reaches temperature, if the water can bypass the thermostat then it doesn't stay in the radiator long enough to be sufficiently cooled. Thats why many times removing the thermosat completely makes an engine run hotter, this was always an old hillbilly trick. People thought removing the stat would make the engine run cooler but this isn't always the case. On circle track cars on alcohol which run cooler than gas we use a restrictor to slow the flow of coolant as well

I spent an entire lifetime trying to figure out 944 cooling systems. In the end, it wasnt a relay, or thermostat, or water pump, or radiator, or bleeding the system, or thermo fan switches... It was a lousy $5 radiator coolant tank cap that had a bad seal which was not letting the system pressurize properly. Buy a new one and try that before going insane diagnosing all the other stuff. It sounds too simple to be true.. but eliminate it 1st, before going crazy overthinking other stuff.

Seems that I read somewhere that the fan switch and the thermostat should be the same temp range.
Could be wrong.

Most t stat are around 175. Most cars will have a 212 switch to trigger the fan. If they were the same temp then the fan would run everytime the t stat cycles. This way the fan runs only if the system gets behind. On most cars this would even be called the 212 switch. Somebody confirm or deny?

+1 on pressure check. Most newer cars will overheat without a pressurised system. Not sure why this is.

+1 on an open t stat causing overheat. Also on no tstat sometimes causing hightemps. The water in the radiator needs time to cool for effect.

Not a bad suggestion.

Keep in mind, most late cars use the 150 KPA or 21 pound cap. Usually more than $5 ;)

The early cars use the 110 KPA or the 16 pound cap. This is the cheap cap.

Owners should check.

John

You test the cap with any radiator pressure tester there is an adapter in there. If it opens up before the correct pressure on the gauge it's bad. Plus they're cheap, we have cars at the shop on occasion the overheating is due to bad cap

Plus 3 on the cap. Just dirt or dried coolant around the cap seal could cause pressure loss.

just to eliminate something i've seen some shops do

make sure you have a bleed screw and not just a bolt in there - otherwise, you'll never get the air out

also, you are waiting to bleed the system until after the engine has reached the thermostat point, right?

I checked for details on my rebuilt water pump and thermostat.
Water Pump
This is an updated late model (non-turbo) pump. It does not have the extra port for the turbo, but it does have the new guard rail (as did the one it replaced). The diameter of the roller the belt drives on is approx 58mm.

Thermostat
Is the Wahler 80 C (056-121-113A)

Clarks page doesn't indicate whether this is for the older or newer style pump, or what gasket/seal is required.

Since I have the radiator mostly drained already I'm going to drain the block and pull the thermostat. I'll measure the groove and its location form the seat and perhaps I can determine what parts I need from there.

Keep ya posted.

I am not familiar with the Wahler thermostat 056 121 113A. Pelican shows it is for a VW/Audi.

Pelican Parts - Automotive Parts and Accessories - Porsche & BMW

I do not know if it will interchange.

John