E34 misting and running hot

[JOHNHOF]

1990 525ia
The coolant left dropped low once but the car is not driven much so I am not sure if this will happen again. Air is getting in the system. The car runs hot but the temp goes to normal or near normal when the car is in motion. Sometimes but not always you can see a mist or fog coming out of the defrosters.
My initial thought is the heater core is bad but I understand replacement is a big job and before I jump in to replace it I would like to make sure it is the core. Is there a test I can perform to confirm the core is bad? I also need the control valve (not sure of the correct name) for the heater as the fan only comes on at high. Thank you in advance for any help.

John Hoffner

[Ross]

Mist out of the defrosters is the core or maybe an o-ring, labor is about the same. 19 yrs old, I'd buy the core.
Check the rest of the cooling system also as this engine does not tolerate overheating well.

[DaveVoorhis]

I would recommend changing the core, pipes, and o-rings. Mist out the vents means it's at least one of these, and maybe all three. Replacement is tedious but not difficult.

If it idles hot but cools in motion, you've probably got a failed fan clutch.

[JOHNHOF]

I was looking for a little more solid way to identify if the core is bad before I start this 10+ hour job. So I thought about it last night and this may sound dumb but I am going to bypass the heater at the firewall in the engine compartment. And if the problems of air getting in the system and mist go away I will feel more confident the core is bad. I know this leaves room for error but it is the only test I can think of even if it's not comprehensive.

Thanks Dave and Ross for the responses.

Does anyone know af a good cheap place to buy a heatercore?

John

[pingu]

Have you tried Radweld or Stopleak? If you're getting mist then maybe your leak is way too bad for those but they've stopped my car from smelling of coolant when I put the heater on.

[Rus]

Have you tried Radweld or Stopleak? If you're getting mist then maybe your leak is way too bad for those but they've stopped my car from smelling of coolant when I put the heater on.

The quick remedies can do more harm than good. Considering how dependent these cars are on their cooling systems, it might not be a good idea to use something like Stopleak.

Bypassing the heater core is a sure way of testing whether it leaks or not.

HTH