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Model Specific Questions and Info => MK6 Golf/Jetta, Beetle, Sportwagen => Topic started by: HPmpU94n3e on March 02, 2020, 05:38:45 pm

Title: 2015 Jetta TDI Overheating
Post by: HPmpU94n3e on March 02, 2020, 05:38:45 pm
Hey all,

Grabbed an inexpensive 2015 Jetta TDI just after the holidays last year. Itís got right around 56k miles on it.

Background: It was definitely in an accident involving the front end and passenger front corner. I don't see anything mechanically wrong under the hood so far from that. From the records Iíve got before we picked it up, the heater core and thermostat were changed last August, a couple thousand miles ago at most. There is something going on behind the dash that I need to dig into: with the fan on, the blower is hitting something, the driverís heated seat doesnít work, at all (lights on the switch do come on, though), TMPS reset button doesnít work, and there something else I canít remember right now.

Problem for this post: itís overheating. A lot. It runs really well, but with any kind of load on the engine the temps climb high quick. I have found that leaving the overflow cap a little loose help. With the cap tight, itíll overheat just sitting idling, with it loose the car will at least idle. If youíre driving down a completely flat road, you can idle without it boiling itself, but any hill or rise at all and the temps go up. When the temps get on the higher side, the heater stops blowing warm air (guessing due to air in the system at that point?).

Work so far: I have verified that the fans work, and they do come on during normal operation. I do not know if the shutter assembly works, but I removed it and it made no difference. I have not found a good procedure for testing the electric auxiliary pumps yet, if anyone has that it would be appreciated. I have pulled the radiators and flushed both with fresh water, they appear to flow fine with nothing unexpected coming out of them. They did not appear to leak, either.

Any suggestions for next steps would be appreciated. Iíd rather not just start throwing parts at it, but I need to get something moving pretty soon. Itís been sitting in the garage for too long now. Any common issues with these that anyone knows of? I have another 2015 TDI that I bought new in 2017 after the scandal stuff was subsiding, thatís been rock solid for ~53k miles so far.

Title: Re: 2015 Jetta TDI Overheating
Post by: ORCoaster on March 02, 2020, 06:44:25 pm
It almost sounds like the water pump is not working.  Does it pump back into the reserve tank at a good amount and change if you rev it up?

Filling it up may introduce an air gap.  You may need to park it pointed downhill so that the reserve tank is the highest point in the system. 
Some have luck putting a vacuum on the cap and pulling air out that way.  Just go easy and only have the tank about half full doing it.

You may need to also pull the thermostat out and verify it works.  Just because it is new doesn't mean it works.  Plenty of cheaply built VW parts these days.
Title: Re: 2015 Jetta TDI Overheating
Post by: HPmpU94n3e on March 02, 2020, 11:30:11 pm
Water pump and thermostat are my leading theories right now, I'm just avoiding that one until it's my last option. I've been meaning to pull the supposedly new thermostat and test it, just haven't had a chance. Hopefully that tests bad and it's a done deal, but I'm not banking on it.

I have the vacuum kit for filling the system, it works pretty slick. I did a drain a refill on it using distilled water to test, it didn't make any difference at all. I really can't tell if it's flowing in and out of the reservoir much. I'll take a look at my other car and see what it looks like, see if I can tell the difference.

Thanks for the thoughts!
Title: Re: 2015 Jetta TDI Overheating
Post by: HPmpU94n3e on June 18, 2020, 10:39:38 pm
Here's what we ended up with:

Upon opening the cooling system, it had clearly been filled with regular green antifreeze after the previous repair. We replaced the thermostat (previous one tested fine), water pump, and timing set. We flushed everything possible with water, both radiators and heater core flowed well and clear. The old water pump spun fine, but the sleeve on it was pretty stiff. It did not slide in and out well at all, and I think that binding was part of the problem.

So far everything appears to be good. It has taken quite a few flushes once everything was back together to truly clear out the system. I have not found a good way to completely drain/flush the system, short of pulling the lower hose and back flushing as much as possible. We're now able to drive it extended distances, even on some very steep hills, without overheating.

Thanks again for all the help.