I have just noticed that my coolant temp readings are hovering - they go up and down all the time on my Cayenne Diesel. The oil temp changes very slowly, like it shoud, while the coolant temp readings can change as much as 10'C within 2-3s. I have switched on the digital coolant temp reading (from the Vehicle menu > Settings) and it hovers with the analog temp gauge, so it is not a question of analog gauge malfunction. More like a temp sensor malfunctioning.

I have also noticed, that when driving the car, the reading is constant at ~90'C as it should, but when I stop at the triffic lights, it can go as low as 70'C, and that is literally within seconds. It looks like the sensor has problem reading the correct temp when the engine's RPM/load are low.

Anyone had a similar problem ?

 

 

My S has a bad thermostat currently getting replaced. It fluctuated on the way up and sent error messages when it went down and took tool long to get to temp. The T-stat was sticking open. Once it gets to 201 it never moves. I've had 2 Macan S loaners that fluctuate on the way up but once at 200 they stay there.

 

If it's not too late - you might consider having the thermostat housing replaced at the same time. Yours has the glued in pipes - which have a possibility of coming unglued. At worst - they junction between the pipes/casting would be reinforced with a nice bead of JB Weld.

 

New housing with coolant pipe attachment, water pump. and I added from your earlier suggestion a new serpentine belt. Should be good to go in this area for a while.

 

Mine only does this during the first 10 minutes or so of driving. Do you have access to an OBD interface tool that you could look at the readings on your phone or something?

(The display doesn't actually show the real coolant temperature reading from the sensor.. it's a "normalized" reading to (presumably) prevent people from freaking out. The actual coolant temperature does fluctuate, especially in cooler weather. You've got the shutters up front, and a thermostat that can all change the coolant temp.)

 

Yup. BMW started the phony coolant gauge thing about 20 years ago.. for exactly the same reason. And with the newer cars - engine temperature is used for both optimizing fuel economy and emissions. I should hookup my iCarScan sometime and record the actual temperature from a cold start to fully warm..

 

If the reading is normalized, it shouldn't go from 60'C (beginning of the scale) to 90'C (middle of the scale) within 3-4s seconds, up and down all the time. Something is definately wrong.

 

Hello, did you get the problem sorted, I also have a 2012 Porsche Cayenne diesel. The engine temperature (coolant) does not reach 90, it hovers between 80 87C. Does this mean the thermostat is going bad?

Thank you

 

Seems like a thermostat.

My problem is different and unfortunately haven't been sorted out, even after repalcing the thermostat (twice) and EGR thermostat.

 

What engine code is yours Elberoth? On the MCRC(>2012) from the parts program i saw only one thermostat, in the oil housing. It is bugging me that i have the buy the whole oil housing just for a thermostat.

 

My car is 2011. I believe there was a slightly different engine on cars manufactured in 2010 and 2011 vs 2012 and later.

 

Yes, from the parts catalog the only themostat that i can find is the one in the oil housing. Thank you for the answers.
What is worring me is the fact that from my knowledge if the coolant does not reach 90C the dpf won t go in regeneration mode.
i l keep you updated.

 

Or you have the dreaded diesel coolant leak and it just started.

 

This. I had a pinhole in a coolant line and was continuously losing fluid and my gauge would fluctuate wildly when coolant got low. I think MajorPain is referring to a semi-common leak of a different type but low coolant can cause some fluctuation (regardless of the cause of the low coolant).