Overheating

[andrew96]

Hey guys,

I recently(3 weeks ago) posted about a bad water pump and how my car was over heating. Well I got my hands dirty and did the timing belt and water pump job but still have an issue...im still over heating. Before I had no coolant running up to my heater core, hard pipe, or breather hose going to the expansion tank and no heat. After I replaced my TB and water pump I was able to get flow up through the heater core, hard pipe, and well every where. I also now have heat so I know the new water pump is functioning. In the fall I replaced my Thermostat and Jplug, a month ago I replaced my turbo coolant feed line from the block to the turbo. I did a full coolant flush with distilled water and filled up with new clean coolant after the TB. Before with the bad waterpump my engine would shoot to the H really quickly, now it seems controlled.... to some extent. If I'm boosting at all the temperature gauge starts going up to about 3/4. I keep cranking the heat and that helps cool the engine down to 12 o'clock but once I slow down, 15-20mph or slower, the heat goes back up.

The good...
Water pump seems to work fine
T-stat seems fine as no coolant flows UNTIL the engine reaches full operating temperature
I have heat
I bled the system by bleeding through the bleeder on the hard pipe and bleeder on the heater core.
I can't find any leaks
Oil consistency is fine so I think my head gasket is all good

The bad...
Over heating, but doesn't go past 3/4, but I also haven't let it, as in I crank the heat to help cool down the coolant.
My coolant light will come on when it reaches 3/4s but coolant level is between MIN and MAX

Possibilities?
My Haynes repair manual suggests
Insufficient coolant in system(level looks good)
Drive belt defective(just did it so that's fine)
Radiator core blocked- any suggestions on how to check?
Thermostat faulty(seems good and only 8months old)
Cooling fan not functioning- Maybe?
Pressure cap not holding pressure-seems fine but I haven't had it checked
Defective water pump- just replaced and working
Improper grade of engine oil-No, I know thats right
Inaccurate temperature gauge- possible

Any and all suggestions will be helpful!

And also for anyone who does normal repairs and needs to do a TB, don't let it intimidate you. I did mine and it was quite easy but took 8 hours. Actually, I enjoyed it enough I'll be helping a buddy soon with his.

[old guy]

From all the information provided I can think of two possibilities.

Overheating a 1.8T can cause head gasket issues rather quickly. Are you blowing off any coolant?

Another possibility is that your fairly new thermostat is sticking. Do you have VCDS? If so you can monitor how then thermostat and thermostat heater are functioning and determine if it is opening properly.

I don't think it is your cooling fans since you mentioned that the overheating condition occurs while the car is moving at 20 mph. That should be adequate speed to flow enough air through the radiator. If you let your car sit at idle can you eventually hear the fans cut on? If not, the fans could be the problem. if the fans do cut on I would look closely at the thermostat.

[andrew96]

When you say blow off coolant do you mean steam coming from the engine? Have yet to see any. Found coolant on the transmission so I replaced the rear flange. Seems the previous owner replaced it before very poorly. Still over heating with a new rear flange. Fans come on. VCDS is Vag com? I don't have one but should invest. Any other why to figure out if it's opening? Possible that I didn't get the water pump in correcring? I made sure there was an O ring and lubricated with fresh coolant before install. Torqued the Allen bolts too

[old guy]

By blowing off coolant I am referring to losing coolant from the overflow on the bottom of the coolant reservoir. That would be an indication of a bad head gasket. VCDS is what used to be called Vag-Com. (VagComDiagnosticSystem) If you had VCDS you could confirm the temperature when your thermostat was opening.

You mentioned that your fans do come on. With the fans working and no coolant being blow off it sure sounds like your new thermostat isn't operating properly.

There is one other possibility. You have a defective rear coolant temperature sensor. Did you replace the CTS when you replaced the rear flange?

[andrew96]

In a panic when I first found my water pump was dead and the engine began to over heat I slowly released the coolant reservoir cap and coolant began to flow out the bottom relief. Not sure how good of a decision that was but seemed fine. When running it'll never blow off, only if I open the cap when coolant is hot. The valve is suppose to open when it reaches 22-23 psi, correct? Assuming you have a bad head gasket and it blows off coolant, is that because the blown head gasket is pressurizing the system???

I replaced the CTS with the rear flange. I drove the car last night and it seemed it would run at 12o,clock, then start to heat up, cool down, heat up and cool down again. Keeping my heat on helps make a huge difference with the temperature. Possible I have a large air bubble trapped in the system?

Maybe I should take it to a shop who has a VCDS so I can check out the t stat

[old guy]

Opening the coolant reservoir when under pressure didn't cause any issue. And you are correct, the cap blow off pressure is around 22 psi (1.5 bar). That's a good sign that you aren't experiencing any blow-off when the temperature goes past the 12:00 position. A head gasket leaking pressure into the cooling system will tend to over-pressurize the system.

Since you replaced your rear CTS and the same symptoms persist it sure sounds like you have a defective thermostat. Especially because of the cyclic nature that you are reporting. I really don't think it is an air bubble causing your problem.

VCDS can log the actual coolant temperature exiting the block along with the thermostat heater duty cycle to monitor the thermostat performance. When the rear CTS reports excessive temperatures the ECM can apply power to the thermostat heater and thus forcing it to open at a lower temperature. When the temperature drops the ECM cuts off power to the heater. That could explain the cyclic nature of your overheating issue.

My money is on a defective t-stat.

Good luck!