1983 944 Cooling system - is this normal?

[ca44]

Hello Everyone,
I purchased this car last summer. It had a blown head gasket and a non-functioning water pump, plus a few other (not as serious) problems. Car only has 78k original miles on it.
Anyway, after fixing the 2 major problems I found an unfixable leak in the radiator and replaced it with a new rad and coolant temp sensor and hose from PP. I can not see any coolant leaks now and the engine runs smoothly.
So here is my problem. When I am driving the car the needle on the engine temp gauge is in the middle. I'm assuming that's normal. However, when the engine is idling, not moving, the needle will steadily move up until both fans kick in. The fans run for a few minutes and the needle comes back down to the middle mark.
It stays in this up and down cycle.
I have bled-out all of the air (I think....) by letting the engine warm up until the coolant is hot, loosened the top bleeder bolt and squeezed the top rad hose until no more air comes out of the top bleeder bolt and coolant starts to come out. As I am doing this I can see the level of coolant in the reservoir go down.
My question is - is this normal behavior? Shouldn't the engine temp stay in the normal range when it is idling? Is there another sensor that is involved in this behavior that I should be aware of?

Thanks in advance to anyone offering assistance.

[Volhv]

Is the temperature rating of your thermostat the same as the temperature rating of the fan thermo switch? If not, I think that might be the problem.

[rhett]

To be honest the behaviour is surely exactly what is expected. When moving, the rad is being force-cooled by the air moving through it, so the temp stays at whatever the thermostat is regulating at. Then, when not moving there is a very large drop in air-flow, so the temperature is bound to rise. This continues until the temp switch kicks in and turns on the fans. This draws air through the rad causing a cooling effect. Once the temp in the rad core drops to below the limit set by the temp switch the fans are switched off.

This behaviour then continues depending on your driving situation. It is also dependant on outside air-temp and things like blocked air intakes, clogged rad cores, etc. But, I think what you are seeing is quite normal. If the gauge regularly goes too high then you do need to look at the temp switch.

Hope that helps..

[AaronM]

Originally Posted by Volhv

Is the temperature rating of your thermostat the same as the temperature rating of the fan thermo switch? If not, I think that might be the problem.

The temp rating of the thermostat is _never_ exactly the same as the fans. The thermostat will be opening/closing all the time while the engine is running, even when the car is moving down the road. It's just that when the car is moving, the movement forces enough air past the radiator to dissipate the heat and there's no need for the fans to be running (in fact, having the fans run when the car is in motion would create turbulence and actually reduce the airflow over the radiator, at least once you're over about 35 mph). So, engineers don't want the fans to be coming on unless the car is sitting still. So they set the trigger point for the fans higher than the thermostat, slightly higher than the temperature that the radiator is expected to reach when the car is in motion.

Rhett has described the process exactly.

The reason that you don't see the gauge move on more modern vehicles is that modern cars electronically damp the gauge movement. Once the engine is up to operating temp, the ECU on most modern cars just sets the "gauge" to the center and doesn't move the needle unless there is a large swing in temperature.

The 944 gauges, however, just read what the sensor gives them, so they reflect the actual changes in radiator temperature.

[alan6272]

My temp gage stays just below midway. If it sits at idle after while the Gage will climb maybe half way the fans come and it goes back to the same as before. Maybe his sender is weak? I would try and find out the exact temp say with an inferred thermometer? That would be my next step.

[Volhv]

Awesome, I did not know that about the gauges. And as you said, the fans are only there to move air when the car is not moving.

[AaronM]

Originally Posted by alan6272

My temp gage stays just below midway. If it sits at idle after while the Gage will climb maybe half way the fans come and it goes back to the same as before. Maybe his sender is weak? I would try and find out the exact temp say with an inferred thermometer? That would be my next step.

This sounds exactly like the behavior the OP is describing. The needle normally stays in the middle, unless he's sitting still, in which case the needle will climb until the fans come on, at which point it will fall again.

The gauges will have some variance between cars, but it shouldn't be a huge issue.

[Guest765]

We should all know by now not to trust these temp gauges anyway, and really use them as an idiot light and not much more. You really want to know a true temp pull out the IR thermometer.

[mueller944s2]

how far is the needle moving up? It should be pretty stable. At idle it'll move up slightly, and when I say slightly I mean not much at all! I was always told that 944 temp gauges move up and down and it's normal, until I put in a different engine and the gauge is now steady all the time. Personally, I open the bleeder valve and blow through the expansion tank until there isn't any more air, THEN I idle it for a few minutes and put it up on ramps and rev it while bleeding. I do this for the next couple days while all the bubbles work their way out of the system. If the level in the tank is still dropping then air is in the system. Also, the fans have two speeds. Maybe the sensors are bad if you aren't getting high and low speeds.

I chased a temperature problem for 4-5 years. Sometimes with a leak and sometimes without. When I put in my 3rd engine the problem vanished, and I have NO idea what it was. I would tighten one thing and a leak would pop up somewhere else. All the hoses were new, radiator, heater core was new. Eventually I got the chance to start over when engine #2 blew. I guess everything got retightened and checked? Who knows.

[CHICKS]

Eh', sounds normal to me.

[Tom944]

Sounds normal to me too. The absolute position of the needle in the gague will depend on how good your grounds are. Dirty grounds means the gague reads lower.

[Gawernator]

Also depends on your ambient temperature and what temp thermostat/ fan switch combo you have. My car is about the first mark on the highway. And at idle in the middle or below the top mark. I also have a new sender unit and cleaned cluster and grounds

[mattdavis11]

Sounds right to me, when it starts steaming, I'll pull over. Yet it never has.

[ca44]

Thank you everyone for your responses.
I am starting to feel better about the situation and am driving the car more often.

When the car is moving slowly the temp needle will stay almost exactly in the middle. When I driving on the highway it stays just below the middle.

As I mentioned earlier, at idle, when the car is not moving, the needle will slowly rise up to the 3/4 mark and then the fan will start. I was not aware that the fan should run at 2 speeds. I am pretty sure I have only heard 1 speed, high. Should my fan have 2 speeds? When should the 1st, slower speed come on?

Thanks again to anyone wanting to respond.

[AaronM]

Originally Posted by ca44

Thank you everyone for your responses.
I am starting to feel better about the situation and am driving the car more often.

When the car is moving slowly the temp needle will stay almost exactly in the middle. When I driving on the highway it stays just below the middle.

As I mentioned earlier, at idle, when the car is not moving, the needle will slowly rise up to the 3/4 mark and then the fan will start. I was not aware that the fan should run at 2 speeds. I am pretty sure I have only heard 1 speed, high. Should my fan have 2 speeds? When should the 1st, slower speed come on?

Thanks again to anyone wanting to respond.

Fan operation differs among the various iterations of the 944 series. For the early 944, the fan should come on at low speed when the coolant temp reaches 92 degrees centigrade at the radiator. High speed comes on later, but I've not been able to find a specific temperature listed as I have for the later cars. Note that in early cars with A/C, there are two fans, while early cars without A/C have only one fan. In the slow-speed operation, only the driver's side fan should run in cars with A/C. In high-speed operation (or when the A/C is on) both fans should run in high speed mode.

In the later (post-1985.5) cars, the low-speed fan still comes on at 92 degrees centigrade, but the high-speed is engaged when the coolant temperature reaces 102 degrees centigrade. In later cars, both fans run, either at low or high speed depending on the coolant temp. Also, these later cars have a pressure switch tied into the A/C system, so the fans only run in high speed if the pressure in the high side of the A/C refrigerant lines exceeds 14 bar (about 200 PSI), so it's possible that on a cool day the fans might not come on high speed immediately with the A/C if the car's at idle. This is only for late cars though, the early cars simply force the fans to high speed any time the A/C is on.

For more information about the operation of the 944 cooling fan circuits, look here: Cooling Fan Operation and Troubleshooting

On an early 944, 92 degrees is just slightly less than halfway between the middle and rightmost white tickmark on the temp gauge. On the later cars, 92 degrees is just slightly more than halfway between the first and second white marks.

For information about what mark corresponds to what temp, look here: Water Temp Gauge

Summary:

Stock Thermostat: Opens at 80 degrees centigrade.

Low-speed fans come on: 92 degrees centigrade.

High-speed fans come on (Early cars): "High temperature condition" (no temp specified).

High-speed fans come on (Late cars): 102 degrees centigrade.

[Gawernator]

Very good info^

[Rasta Monsta]

^But wrong.

"Low speed" only occurs in early cars with ignition off. The switch is one temp only. . .it doesn't ramp up as described.

[jeffrsmith]

Did you test or replace your thermostat?

If you have a sticking thermostat it can cause the temp to rise higher than normal. Yes, the swing on the temp gauge will happen, but if you are going to three quarters and then it starts to come back down it sound like a sticky thermostat.

Pull the thermostat and test it, if it is not an OEM thermostat replace it - Stant thermostats, when put in a Porsche are junk, they just don't work properly.

[AaronM]

Originally Posted by Rasta Monsta

^But wrong.

"Low speed" only occurs in early cars with ignition off. The switch is one temp only. . .it doesn't ramp up as described.

You're right. I misread the description of how the early cars work with A/C. The Clark's description of the non-A/C cars was clearer.

Looking at the diagrams it's clear that in early cars if the ignition is on, the fan only runs in high-speed mode.

[porschetub]

Originally Posted by CHICKS

Eh', sounds normal to me.

Count yourself lucky your system is working as normal,.