I thought I was almost done with maintenance projects on this car till the spring. Maybe not.

I noticed yesterday the coolant was down half an inch or so in the expansion tank. It has not lost any coolant for months, but may have been losing a little early in the summer. No evidence of coolant leaks anyplace in the engine compartment. Hoses all look good. New radiator in May. Expansion tank cap is not cracked. Heater control valve does not have any sign of leakage. No coolant drips on the garage floor. No coolant odor inside the car when driving and no sign of moisture on the carpets. I just bought replacement heater hoses and valve (after-market parts from FCP Groton - valve is all plastic and different shape than the original part, but is supposed to be interchangeable). Was planning on installing these in the next week or two and then suspending maintenance operations until it gets warm again.

Last week was a little chilly here, so we are using the heater in this car more than recently. I am thinking maybe we have a slight leak in the heater core that is somehow evaporating before the liquid gets a chance to seep out anywhere.

Any thoughts or advice would be welcome. Is there any simple way to confirm if the heater core is leaking without ripping out the inside of the car to get at the core itself? Any other possiblilities?

Thanks,
Kev

--
Current: '95 945, 95k miles; '90 BMW 325i, 90k miles. Former Volvos, gone but not forgotten '85 245, 250k miles; '91 744T, 150k miles

Because the coolant is hot and the windows are cold, you would definitely have fogging on the windows if you had a heater core leak. (Think hot shower, cold day, bathroom window.) If you were using the heat and you didn't smell coolant, and you could still see out your windows, the odds are pretty good that you don't have any coolant leaking into the cabin.

-Eric
The voice of experience.
'89 765t, 179k, Spanking new heater core as of last summer.

Kev, I thought I had a minor heater core leak myself just a week ago. But all people I spoke with said you will have fog on the windows and a smell if that is the case.
It's not that difficult to look under the carpets, if you really want to, but most probably the leak is somewhere in the engine compartment.
The best method I think is to pressure test the system. It should be on a cold engine, I've found the leaks more easily spotted this way.
There are some places hard to look at where it may leak. Back of engine where the hard pipe from the water pump goes to a hose is one. Under inlet manifold is another. Don't rule out the cap for the reservoir. One possibility you don't want is a bad head gasket. Check for different coloration of the spark plugs.
In my case all clamps needed retightening after I changed all hoses.
When I put new clamps on new hoses I thought I tightened them real hard, but a couple of months later I had to retorque all clamps a number of turns.

HTH
Mikael Larsson