My overheating dilemma.
 

I started posting my problem on an old thread I started, but I'm hoping to get more help starting a new one. This is the situation... I'll try to make a long story short. My coolant system filled up with engine oil because my oil cooler went bad. I drove the car a long time like this and it made this really thick liquid that was like the consistency of a milkshake. I finally replaced the oil cooler and flushed out the coolant system about 5 times which seemed to have got almost all of it out. Now I have a problem with my car overheating. The overheating is inconsistent and happens when it feels like it. I can be driving and suddenly the engine temp starts rising slowly to the next line past the middle and hangs there for a little bit and usually goes back down a little or even back to normal. I know it's not the CTS because when the needle rises, I can hear the water starting to boil out of the expansion tank, so it really is overheating. Also, the oil temp rises above normal when this happens. The water pump ant t-stat have about 40k on them. This problem is really worrying me because I need to add water twice a day before I go to work and before I leave work.

Any help is greatly appreciated.

If you're having to add fluid every day, then you must have a leak somewhere. Did you check the coolant flange in the back of the engine? Has the expansion tank and cap ever been replaced? Those are common culprits.

Btw, I hope you are adding a 50/50 mix of G12 coolant and distilled water, and not just 100% plain old tap water. If you just have water in there, it's gonna boil.

I'm 99.9% positive I don't have a leak. I can hear the water boiling out when the overheating occurs, so I know that's where it's going. The coolant flange and reservoir with cap were all replaced this year. After the first 2 flushes, I added the 50/50 coolant and purified water mix, but now that I remember, I added it to an already hot engine. Maybe the coolant boiled out before it had a chance to get through the system. I guess I'll buy another bottle and pray that It'll fix the problem by adding it when the engine is cold.

By the way, does the whole front of the car have to come apart just to change the thermostat? I know you have to in order to change the water pump.

You could always buy a little tester that would tell you how low your coolant protects your engine to. That would give you an indication on how good your mixture is. If it's only good to 20 degrees F, then you probably need some more straight G12.

Hmm, on the 1.8T I'm not sure about changing the T-stat. On the 2.8 the car needs to be put into service position to get at it.

Ok, thanks for the advice. Today the car was magically running better and the needle pretty much stayed in the middle all day. I'm going to do one more flush and then add some fresh G12 with filtered water. I'm thinking that the T-stat was getting stuck from that crap in there and now it's finally freeing up. Oh, and I think I saw where it's located and it looks like an easy job to replace if I have to. It looks to be right above the oil pan above the big sensor on the driver's side.

I went and bought a new thermostat, but I think I'm going to hold off on it to see if I even need it. I think I figured out the problem which seems to be that the coolant reservoir was still a little clogged. I never noticed that there were these small passage ways inside it and thought it was just all open. There was still sludge in there stuck to the walls so I sprayed almost a whole can of engine degreaser and rinsed it out trying to get it all out. I got most of it out but there's still a little more stuck in there. After doing that, the car didn't overheat at all driving to work and back today and the oil temp stayed low too. I might just go get a new reservoir if this is the problem.