Coolant Temp gauge failure after coolant fittings fix
 

I had my coolant fittings fixed last summer after having a fitting blow out on the track last summer. I had them all welded properly and everything seemed to be working fine except it always seemed to take longer to reach normal operating temperature.

Occasionally on cool mornings, the coolant temperature gauge would fail after driving a few miles but would then come back on and stay on once the car approached normal operating temperature. Sometimes it never really reached operating temp when driving fast on the freeway. Recently on cold winter mornings (35-45 F--don't laugh, this is cold in San Diego!), not only would the temperature gauge fail, but also the CEL would come on. I brought it to the mechanic who reset the fault lights and replaced the heat sensor. Unfortunately, it has not worked and the temp gauge is still periodically failing.

Has anyone else had this problem and if so what was the fix? I hate to keep throwing money at this nagging problem.

If changing the sensor didn't solve it, suspect the guage. Wiring and grounds etc should be checked also.

+1

I'd be interested to know which sensor he checked / changed.

there are a couple of them that I remember including one that would be basically impossible to get to without major disassembly that's on the coolant housing that goes into the oil cooler under all of the mess of the manifold.

almosts sounds like the thermostat has failed(stuck open)....

what happens when you take it for say a 20minute drive then let it sit in one spot (at idle engine-speed, cabin heater off) for say another 5-10minutes?

you know...talking / thinking about what sensor could be bad I completely looked past the obvious. Your explnation absolutely makes sense given the extremely long time to warm up and then getting cold again while moving. I'm going to +1 this suggestion.

These thermostats generally don't fail.

My suspicion would be if the coolant temperature sensor was removed prior to welding (threaded location in the photo below). I'm sure the heat from welding could have damaged it.

https://lh6.googleusercontent.com/-k...o/s640/116.JPG

Sensor is right above the blue tape in this pic and difficult to reach with the intake manifold on the car. Was this the one that was replaced?

https://lh5.googleusercontent.com/-f...U/s640/189.JPG

I agree the sensor could have been damaged if not removed during welding, but it was replaced after the fact...could be the new sensor is faulty or wiring not clipped in right or damaged, but the symptoms point to the engine not actually not getting hot, thermostats are replacement items and the car is a ~decade old now...:icon107:
will be interesting to find out what the actual problem is :cheers:

Just saying from experience. My friend's shop has never had to replace a thermostat on a GT1 engine, and they service a lot of 996/7 turbos and GT3s.

Not saying a failure is impossible - just not common. I replaced the waterpump when I built my 4L engine, but did not bother with the thermostat.

:cheers: good to note

There is another temp sensor on the housing just under the oil cooler on the other side of the motor too. Maybe it's oil temp but I swear it was in a coolant pipe. Either way both of them are very hard to get to

... But just as the sensors could have been damaged if left in the thermostat could have been too if not removed from the water pump housing or possibly while out of the car. Lots of possibilities but the thermostat is likely the easiest to check at this point. To note I also didn't change my thermostat when mine was apart.

to me it's a good $50 preventative maintenance item, but glad they are commonly not failing :burnout:

Thanks everybody for the info. I will forward this thread to my mechanic to see if it helps him.

My mechanic had checked the thermostat as well as the connection under the carriage by the trans and both were OK. The temp sensor he checked had a faulty ohm reading so he replaced it with sensor part #996 606 410 00. This sensor was not particularly hard to replace, there was only about 1 hour of labor for it. Despite all this, the car still has the same problem.

I will update this if and when he fixes the problem, meanwhile let me know if anyone has any other ideas on it.

Stole Serge's picture to point out the other sensor I was talking about. Right next to one of the fittings on the base of the oil cooler that has to be welded.

http://i136.photobucket.com/albums/q...ps4cb58e77.jpg