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Temperature Gauge bouncing?

01-24-2003, 12:09 AM
Hi everyone. I know I am a newbie here and everything and this is my first post, but I am wondering if anyone can solve this problem. Here is the specs:
1986 Porsche 944 - manual - 84000 original miles

Here is the problem -
On typical driving the temperature gauge is steady at 1/4 on the scale.
When driving, sometimes more so in traffic, the temperature would begin to rise quickly, sometimes all the way up into the redzone. Ive tested it by just starting up the car in the driveway and the temperature gauge would not stay at a quarter, it would go up to full hot, at the point and only on the full red, the cooling fan THEN turns on and it cools down. But it is weird though, because the gauge when its "hot" it bounces like its unsteady at hot, and at times im driving and it bounces at hot, then juts down to being steady at a quarter. If anyone has any clue of whats going on, please respond.

What I have done to rectify the situation - replace the thermostat/flush the radiator - put new coolant in = problem still happens

My only guess left is possibly the temperature sensor?
Thank you for your time.

12-18-2004, 03:42 PM
I have the same problem without the overheating. I changed the temp sensor on the radiator because the primary fan would not turn on. I followed the standard troubleshooting in the manual, jumped the wires and it turned on. Replaced the sensor and it is cooling fine except as it heats up to the 1/4 mark and the fan turns on the needle bounces, stablizes and is fine. I did notice some corrosive discoloratioon on the wire terminals at the sensor so I am trying that to see if it fixes the problem. If anyone has any ideas, please post.

12-19-2004, 12:00 AM
Check to be sure system is properly bled, clean all connection, and make sure the thermostat and fan switch match in temp. If they don't fans may come on too soon or not come on in time. (note: the temps are on the side of the switch and the thermostat in Celsius)- Also check for immeadiate pressure in system-could be a sign of a bad head gasket. Worn or leaking water pumps are also a cause of weird temp readings.

12-19-2004, 10:36 AM
When you say fan switch, do you mean the temp sensor on the radiator? It is the lowest at 74 degrees Celsius. I do not know what the thermostat temp is set at. No leaks or water pump problems. How do you check for an immediate pressure in system to look at the head gasket?

12-19-2004, 11:28 AM
The "temp sensor" in the radiator turns the fans on/off. 74 is, I believe the lowest one, I think 85 is the normal one. You can check for pressure in the system by using a radiator pressure guage or simply start the car and go squeeze the upper hose if it is rock hard right away then you may have a problem.

12-28-2004, 11:54 PM
When I bought my 87n-a I was looking at an overheat problem that possibly was a head gasket. I found a vaccuum line disconnected at the heater core water valve. Thus when everyone and their sister was trying to bleed the coolant system..... they never had the valve opened to purge the air from the heater core. Nobody was able to figure this out..... that as hard as they tried they were'nt able to bleed all the air out of the engine.

01-22-2005, 02:35 PM
This may sound pretty shade tree, but hit the top of your dash right over the guage and see if it causes the needle on the guage to jump. I had a problem on my 951 that I fixed by repairing a solder conection on the back of the cluster. I can't remember if it was on the back of the temp. or gas gauge but it affected both. Also see if the gauge moves relative to battery load, like turning the fan to high or hitting the flash to pass, if so your problem is one of the three grounds on the back of the engine. L.C.

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