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Discussion Starter #1
I have to replace the water thermostat in my TDI and I am thinking if it would have an adverse effect, if I were to replace it with a 165F, I believe the original thermostat in the car got to be either a 180 or 195F? what do you think?
 

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No, 165 is too low, 180 is a little low but the preferred solution for overheating is a larger radiator. The ALH thermostat is a 195 but normal coolant temps may be +/- 10 degrees F. There are some parts that you can use generic parts, but from what I've read, the VW thermostat seating surface is slightly different. I would use a genuine VW thermostat. ECU control can do all sorts of funny things when you do things such as fully disable the EGR cycle without tricking the computer first. I would think that all the engine maps are optimized for regular operating temps.

I think you could gain mileage by blocking most of the front grille and putting in a larger radiator to make up for lost airflow but the thermostat still needs to be at regular temps.
 

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I don't think the 180F will damage anything but I wouldn't since the engine is designed for 195. From the bentley : it opens at 185F and is fully open by 221F. 7mm min movement.

2nd on the VW thermostat, when you are removing it you're supposed to turn it counterclockwise 90 degrees. When putting it back, turn it clockwise 90 degrees so that the stamped fin is vertical. Be careful when tightening the flange, a common issue with 0-rings is that they get squished and twisted if they're overtightened or dry. The FSM suggests 11 ft - lbs, 15 NM for the coolant flange nuts. I don't suggest using gasket maker here. People can use too much sealant because they think more is better. Sometimes you find orange bits clogging the coolant passages. Bad for a turbo engine since the turbo is water cooled.
 

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Discussion Starter #4
My next question is, when changing the thermostat i will lose some coolant, what is the mixture ratio for destilled water and g-12 coolant, is it 50, 50? thanks for your replies
 

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You can go 40-60 or 60-40. I suggest 50-50, bentley says it will provide anti-freeze until -34F, -37C.

40% coolant COULD lower your operating temps much like a lower temp theromstat, maybe 5 degrees as a guess. 60% coolant will raise your temps but give better anti-freeze protection.

Edit: In the end, coolant temp is still regulated by the thermostat, but if your coolant system isn't up to par, a higher percentage of water will help keep high temps under control.
 

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The coolant temp is controlled by the thermostat, only change the ratio if you're overheating or in very very cold temps.

I would just get a stock replacement because 165F is way too low, even for a gas car. A gas car should run at about 180-190F, a diesel has no problem running at 190-200F, and that's what my car runs at.
 
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