1986 245 M46 here in sunny Oregon.
I've not been driving much lately so I sat on this for a few months: Over the winter I noticed (it was impossible not to) that after a few minutes of running the heater my vents would begin pumping white cloudy vapor out, and it could last the duration of my drive (10-30 minutes). I figured this had something to do with the rainwater leakage my car experiences, which is significant but also a very old problem that has never given me this kind of trouble before. It made defrosting the windows very difficult if not impossible (I don't have a working AC, either).

Since I am now aware of an unrevealed coolant leak on top of this, the case that the cause may lie in my heater core seems looks probable, as I've just scanned the BB forums and seen this diagnosis come from the same phenomena.

My only question is, before I tear the dash apart and everything, is if anyone has ever had this same problem of actual clouds of condensation coming out of their ventilation systems and NOT had it been the result of a coolant leak in their heater cores? Could rainwater leaking into the ventilation system also cause this type of phenomena?

As always, thanks a lot fellas

The carpets have been wet since I bought the car in 2009; it rains inside my car and I don't clean it often enough so those carpets are always just unfortunately wet until July. They smell like an old car so it's hard to tell if coolant has been getting onto them.

The steam depositing onto my windshield is possibly responsible for whatever impure film that feels to be left on there whenever I try and wipe it off, but I always assumed it to have something to do with the napkins or rags that I use when I am getting up to around 35 mph and suddenly can't see anything anymore.

It smells like coolant insofar as the entire car smells like an old car. At first I thought it definitely had its own type of smell but again I attributed this at the time to something that I thought might be disintegrating rubber that the water was leaking in through. I will take a whiff of my overfill tank tomorrow and compare it to that.

And yeah I have a mysterious coolant leak that I only discovered after the car started almost overheating. The amount of coolant that I've lost doesn't seem to register outside the car so that makes sense that it's primarily dumping inside. I'll do one good and final test run in order to get it leaking again and start looking inside once the vapor cloud has been blowing a while and the tank has subsisted.

Thanks for the suggestion, I will check Art's page on that tip.

If my page on heater core seems daunting, consider the chainsaw approach where the top of the box is cut to allow the core to be withdrawn up. Also remember the page I did was on a '91 with the hard line AC -- a considerably more difficult task than yours should be if one does not want to lose the R12 charge.

Sorry I cannot provide a link to the chainsaw heater core at this time.
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Art Benstein near Baltimore

If you lend someone $20 and never see that person again, it was probably worth it.

If your end up pulling your dash, and you plan on keeping the car for a long time, you might consider also doing the heater fan and heater valve since the dash is already out. Those two jobs are also notorious!

They have come along way with radiator stop leak products so I've heard. Might be worth looking into in your case.

sorry to say but you've got a leak in the heater core.

If you have any hope of it not being that. turn on the Heat and as the windows fog...wipe...greasy like? tough to clean off? If yes coolant.

Fog the coolant fog....I've put the fix off for 5 years by driving my other car in the winter.

Art at CleanFlametrap has a picture perfect detailed saga of what it takes.

I would suspect that there is a leak in your cooling system.

In order to investigate further, you may want to check the front carpet, under your floor mats, if you have them, and see if your carpet is moist or wet.

Also, do you smell the sweet smell that coolant usually gives off.

If so, you most likely have a coolant leak.

P.S. Have you checked your coolant level?? Is it low?

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If it needs to be maintained, repaired or replaced on a 1990 240, I've probably done it. '90 240DL, 355,705 miles, will she make it to 400K ?? >>You haven't really worked on a car until you draw blood<< :-}