Losing lots of coolant but no smoke

[Beemerdan]

My 1991 M42 is losing pretty muck all of the coolant in the expansion tank within 500 miles. No smoke, on startup or while driving. The profile gasket was replaced around 20,000 miles ago.
Any ideas what it may be or where to look?

Thanks

[audiquattrot]

under your dash wet? in the footwells?

[Beemerdan]

Nope, totally dry.

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[jlevie]

If you can't find any leaks with a pressure test, there is the possibility of head gasket leak that is pumping air into the cooling system. Eventually the pressure will get high enough to lift the cap, which will dump coolant overboard. That may only happen while driving and leave no trace as to what happened.

The definitive test for this is to pressurize each cylinder with 180psi air for several minutes while monitoring cooling system pressure with a 0-5psi gauge. If the test is negative on a cold engine it must be repeated with the engine hot.

A DIY test is to remove the cap to release any pressure once the engine has completely cooled down when you park the car for the night. The hoses will be soft at that point. Then drive the car and allow,the engine to completely cool down. If the hoses are firm/hard then, air is being pumped into the cooling system.

[Gerta]

My 1991 M42 is losing pretty muck all of the coolant in the expansion tank within 500 miles. No smoke, on startup or while driving. The profile gasket was replaced around 20,000 miles ago.
Any ideas what it may be or where to look?

Thanks

Dan, I'd pressure test the cooling system first. I had a similar issue where a hose had a pinhole leak that only leaked when fully pressurized. I think you can rent fairly inexpensively a pressure tester from your local Autozone / O'Reilly's etc ...

Just a thought.

[TobyB]

I once had a water pump that failed that way too, now that I think about it...

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