I had a bad thermostat and it's only indication was the temp rising when going up hills under sustained load.
I had this year ago, thermostat was stuck closed, so coolant couldnt go to big circle and into cooler, it went up after 10 minutes of highway driving up to 115° (obd temps) on dash it wasnt that much
I used the genuine part as there is no desire to redo it again too soon.
It has been two weeks since the repair without issues, so I am going to call this one solved. If I was paying for this out of pocket in the future, I would only replace the thermostat and not the pump as that is what was closing and causing the issue.
I did the timing belt water pump thing on my MK4 last year, and shortly after the test drive with fresh genuine VW antifreeze and while patting myself on the back for a job well done it occurred to me that I hadn't changed the thermostat. Sure enough a month or so ago when the first of the really cold whether started around here the temp gauge would only get to about halfway to normal during a trip to town and back. I tried the ol' cardboard in the grill routine for a couple of days until I got a chance to change the thermostat and that brought things back to normal.
Thermostat and temp sensor were just replaced with OE Mahle/Behr. P0128 showed up after car didn’t get up to temp. Heat is slightly affected until reaches operating temp.
I’ve been using Germany-made Mahle thermostat for years. Now it throws a code that the heater doesn’t work (so it won’t open electrically, but mechanically it is working just fine), but no leaks.
1st Mahle. The engine only reaches up to 65 degrees and it drops further if I drive under 2000RPMs on highway for example or turn on the heating.
car in winter cant even reach optimal temperature in idle, reaches optimal temperature only uphills on RPMS higher thatn 2500
After getting mine back together, and filling it back up with coolant, i found the new thermostat from the dealership was stuck wide open and the car wouldn’t get over 110*F.
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