Overheating and blowing cold

[vtraudt]

What can cause the following symptoms:

- sometimes overheating (fans turned on with switch), but vents blowing cold; easy coasting on highway
- a few minutes later, temperature drops (even below operating temperature), and vents blowing hot
- water pump (German Graf) has 6k miles. Removed for inspection, impeller not slipping on shaft
- no coolant in oil
- no oil in coolant

Notes:
- rule out stuck thermostat (stuck close (overheating) would still blow hot air)

History:
- SNOW water/meth solenoid leaked water into intake pipe overnight, engine start hydrolocked
- 20 psi lower compression on cyl 2
- found one slightly bent exhaust valve in cyl 2, replaced.
- piston no real signs of damage. Removed and new rings installed
- cyl 2 still shows higher leak down numbers (currently sent out again for inspection)
- APR head studs

Any suggestions?
Could head gasket leak cause the above symptoms?
Crack in block?
Crack in head?

[Seerlah]

Throwing jabs, but it has me thinking if it is the t-stat if it is a B6. If not, then I'm stuck. Thinking it closes, you extinguish the heat capacity of the coolant in the heater core, you lose heat, coolant temp increases with t-stat closed (or semi-closed), then the aux fan kicks on, t-stat opens back up, coolant temp drops when coolant can circulate through the radiator again and back through the heater core, where you get your heat back. Could also apply to the B5, but these are usually fully stuck open or close. Just a theory and could be completely wrong.

If it were my personal car I would replace the t-stat and coolant temp sensors, then take it from there. It is not good to throw money on parts at the car, but that is how I would have looked at it and went from there. If it fixes the issue, then the theory was most likely the reason. If it doesn't, then money wasted.

[vtraudt]

Throwing jabs, but it has me thinking if it is the t-stat if it is a B6. If not, then I'm stuck. Thinking it closes, you extinguish the heat capacity of the coolant in the heater core, you lose heat, coolant temp increases with t-stat closed (or semi-closed), then the aux fan kicks on, t-stat opens back up, coolant temp drops when coolant can circulate through the radiator again and back through the heater core, where you get your heat back. Could also apply to the B5, but these are usually fully stuck open or close. Just a theory and could be completely wrong.

If it were my personal car I would replace the t-stat and coolant temp sensors, then take it from there. It is not good to throw money on parts at the car, but that is how I would have looked at it and went from there. If it fixes the issue, then the theory was most likely the reason. If it doesn't, then money wasted.

06A engine.

Not the thermostat. If stuck open, heater would blow warm (engine temp drops below normal with fans turned on). If stuck closed (overheating), heater would blow HOT (it actually blows COLD, not even warm). Also: in summer, I remove the thermostat (improved flow for track days).

Not CTS: see above, bad CTS does not effect how your heater blows. Also measuring blocks in engine and instrument cluster show correct temperature.

Something is prevent coolant from being circulated through the engine. Obviously, bad water pump (impeller spinning on shaft) was obvious first choice. But after removing the pump (German Graf pump, 6k miles), no indication for that.

[ChiliS4]

I’m having similar symptoms, how was this resolved?


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[vtraudt]

After ruling out stuck thermostat and blown head gasket, only left the water pump.
Took pump out and found impeller spinning on shaft. Yes, good quality German pump. Still happens.

[ChiliS4]

How did you rule out thermostat? Is there a way to know if it is opening and closing when it’s in the car?


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