Temperature gauge "failure mode"

[agdroptop]

Having a problem with the temp gauge on my 04.

When it is cold out (below 40 F) after driving my 996 for a short while (10-15 minutes) the needle will drop to 0 (actually 100 on the gauge but that's the bottom) and the red warning light will start to flash slowly and the dash will display failure mode,  but not always a CEL.

After continuing to drive for another 5 minutes or so the gauge will return to normal operation.

Doesn't happen all the time and when the ambient temp is warm it never happens, only when the air temp is cold.

Occasionally it will throw a CEL, The code was P0128 (Coolant temp below thermostat regulating temp).

 

In the owners manual is reads that a steady red light indicates excessively high temp and a flashing red indicates too low a coolant level. So my symptom would indicate too low a coolant level, except the warning light goes off after a few minutes. And the reservoir is showing coolant between min and max at rest, even checked right when the light is flashing.

In the Bentley Service manual it describes a slow flashing light (1/2 sec cycles) as low coolant or engine compartment temp too high. But coolant level is good and it only happens during cold ambient temps, and not when air temp is hot. I wouldn't think the fact it is cold out would negatively impact the engine compartment fan and after the compartment temp warms up the light would go off, unless possibly a temp sensor that actives that engine compartment fan is failing cold.

A rapid flash indicates the temp sensor faulty. I'm not seeing a rapid flash. more of a slower flash. And with the rapid light the needle should be pinned to the right, mine is to the left.

Coolant level looks fine. Last year I had the reservoir and cap replaced (as a preventative measure) and a complete coolant pressure flush. Cleaned the grounds, the contacts on the cap on top of the reservoir and the contacts on the temperature sensor. Temperature sensor was also replaced last year when I first encountered this problem.  Also just replaced the coolant level "float" in the reservoir but problem continues. No coolant leaks from the system that my tech or I can find.

Replaced the thermostat shortly after that first instance last year in case that was contributing to the problem, no change. It was changed from a 160 to a 180 and has since been replaced with a 160, same problem continues.

At a loss for next step.

 

Anyone ever encounter this? Thoughts?

[Lykanthrop]

Dear agdroptop, It looks like I am experiencing a similar issue with my 2002-4S, Tiptronic. I am wondering whether you found the root cause yet.

Is your a Tiptronic as well?

 

Thanks

 

Martin.

 

 

[Silver Fox 64]

I had a similar problem with my 2002 C2 Tip. Fitted a new genuine coolant temp sensor and all is good again. I had to delete the fault code after fitting the new sensor. No issues now ;-)

[Lykanthrop]

Problem is solved. The vacuum system was leaking ( broken pipe behind alternator ) ...

Due to the missing vacuum, the coolant control valve of the Tiptronic was always open. 
Therefore, the engine could not get to temperature within a certain time and the warning lamp came on to inform the driver.

To bad, the Porsche service center couldn’t find the problem...

After fixing the vacuum pipe the engine is at 80degC within 5 minutes.

 

Thanks,

 

Martin

 

[Loren]

2 hours ago, Lykanthrop said:

Problem is solved. The vacuum system was leaking ( broken pipe behind alternator ) ...

Due to the missing vacuum, the coolant control valve of the Tiptronic was always open. 
Therefore, the engine could not get to temperature within a certain time and the warning lamp came on to inform the driver.

To bad, the Porsche service center couldn’t find the problem...

After fixing the vacuum pipe the engine is at 80degC within 5 minutes.

 

Thanks,

 

Martin

 

Glad you got it sorted.