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My 2006 VW Passat 2.0T is losing coolant in the past few months. I have added 2qt distilled water for a 7.6qt system. Less than 1000 miles driven.

  • No visible leaks.
  • No combustion leak detected (tested 3 times with the Autozone kit).
  • Hoses and gaskets seem fine (no pink/white residue, which happens for G12/13 antifreeze).
  • No overheating. Coolant temperature always is constant during operation.
  • No white fumes from the exhaust.
  • (update) All spark plugs look the same with some deposits, suggesting no coolant burning in the chambers
  • (update) Few times, I had a hard time opening coolant reservoir cap due to the high pressure

Final Update I pressure tested the coolant system and found out the leak happens at 15 PSI and somewhere near the water pump.

3 Answers 3

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The tester you referenced only checks for combustion gases in the coolant. But some head gasket failures only result in coolant being sucked into the combustion chamber and being sent out the exhaust. No combustion gases are being pushed into your cooling system if the tester result was negative.

If you replace your distilled water with actual coolant and take your vehicle to a shop with an Exhaust Gas Analyzer, they should be able to detect coolant in your exhaust. In my experience you can sometimes smell the coolant in the exhaust but that depends on how good your nose for that is and also how much coolant is being lost.

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  • Should I be able to detect this if I check my spark plugs?
    – Ramyad
    Mar 22, 2022 at 16:07
  • Maybe. Sometimes you will see one spark plug that looks like new and the others have some deposits on them. If you have a borescope you can also look into the cylinders. One that is "burning" coolant will usually have a very clean piston.
    – jwh20
    Mar 22, 2022 at 17:01
  • No sign on the plugs. All of them look the same with deposits. I guess I need to check other possibilities.
    – Ramyad
    Mar 22, 2022 at 21:20
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The coolant could be escaping from the coolant expansion tank overflow while driving.

There is a pipe that comes off the top of the coolant reservoir that coolant could be escaping from. Trace where the pipe exits the engine compartment. You could stretch a balloon (or something similar) over the end of the pipe which will then indicate if coolant is coming out while driving.

If it is exiting there, report back.

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  • I have a hard time finding that pipe, or any other pipe exiting that system for coolant overflow. But, you gave me an idea to check. The reservoir itself might be cracked and only under high pressures lets the coolant (or the cap might be faulty allowing higher than 15PSI in the system).
    – Ramyad
    Mar 22, 2022 at 21:34
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I pressure tested the coolant system and found out the leak happens at 15 PSI and somewhere near the water pump.

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