Why does my car steam up in the rain?

[burgundyben]

When I drive my car in the rain, or when I use the heating for the first ten minutes when its been parked in the rain it steams up.

I dont mean its a bit misty inside, I mean its like a full on sauna.

A proper full on sauna (Except no naked fat Scandinavian people are inside).

I had a 911 some years ago, I know they are not great at demisting but this is something wrong, very wrong.

It condenses on the windscreen and is really not driveable, I have to pull over and keep wiping away until its burnt off.

Somehow, water is getting into the heating ducts somewhere and as soon as I use the heating its turning into steam and blowing into the car.

Car is a 78 SC, its a 993 motor, 993 HE's, a mixture of black neoprene and red silicon 60mm ducting from the shroud down to the HE's, and up into the chassis.

I have got the sill covers missing and the jacking point plugs are not fitted. Tonight I have run the car until its all clear.

The screw holes and bungs on the sills that were open to the weather I have bunged and covered. I've covered the ends of the jacking points.

There are no rust holes in the car.

Where else could water be getting in?

[attamz]

Targa or coupe?

[Hugh R]

If you have A/C run it. You need to knock out the moisture in the air, otherwise run the heat to raise the dew point.

[burgundyben]

Its a coupe.

I dont have air con.

Next time I fiddle with the car I'll do some before and after shots.

[ossiblue]

This could be within the realm of "normal," especially if what you describe is happening relatively soon after you are running the engine. Remember, the air coming from the exchangers has not been dried like from an AC and still contains moisture from the rainy atmosphere. Until the heated, moist air from the ducts can also heat the interior glass above the dew point, you will get condensation. Now, if you are actually seeing steam emitting from the ducts, that is another story and could mean actual standing water is in your exchangers. Otherwise, I wouldn't think there is actual water in them given your description of their condition.

[James Brown]

This happens to me all the time, pisses me off. Water, from rain overnight, leaks into the heat tubes somehow. does not show up when cold but as soon as it's warm, BAMM no windshield! Not the light stuff you see when turning on the A/C of a modern car but the full-on rain that does not wipe away. no other choice but to pull over and wait for it to go away. Pain, yes.

[mickey356]

Just happened to me tonight. We had our first 60* day today and when I left fo rwork tonight I got a steamy windshield for about 10 mins until the car warmed up.

I did actually get steam coming out of the vents a few weeks back. Sadly, my car sat out in the snow for a week or so. When I finally dug it out and started it all was good but as soon as I got the revs up (heater back date) I literally got steam shooting out of the vents. Scared the crap out of me for a second. It went away pretty quick.

[gtc]

Happens to me also... I just leave the heater off for the first few minutes after I start the car. This dries out the system so to speak, venting the steam under the car instead of into the cabin. After a couple minutes it is usually safe to open the heater valves and let heat in.

[stlrj]

Rain on heat exchangers is the same as water on hot rocks...steam.

[yelcab1]

And this is why I don't drive or store my 911 in the rain.

[88911coupe]

same here, happened on my cab as well. Trick as someone mentioned is running a/c, although I think the OP said doesn't have a/c. Also let the system get good and hot first which I think maybe helps dry things out a bit and makes the a/c's job a little easier.
Very frustrating and dangerous.

[88911coupe]

same here, happened on my cab as well. Trick as someone mentioned is running a/c, although I think the OP said doesn't have a/c. Also let the system get good and hot first which I think maybe helps dry things out a bit and makes the a/c's job a little easier.
Very frustrating and dangerous.

[RWebb]

Originally Posted by stlrj

Rain on heat exchangers is the same as water on hot rocks...steam.

I suspect it is usually not the rain, per se. More likely that driving thru puddles of water, splashes it up where it can then get into the heat exchangers. That is what happened to my '75 all the time - even after it stopped raining - and the car was in a garage until then.

[stlrj]

Originally Posted by RWebb

I suspect it is usually not the rain, per se. More likely that driving thru puddles of water, splashes it up where it can then get into the heat exchangers. That is what happened to my '75 all the time - even after it stopped raining - and the car was in a garage until then.

I see you got the idea I was trying to convey.

[kent olsen]

Many years ago while in the Navy in Southern Calif, I remember driving in a heavy rain in my 62 Corvair. Calif has no place for the water to go so at each intersection there was this great big puddle. Each time I went thru it, about one foot deep I was instantlly fogged up!

[midlife]

I experience the same problem. I have not tried it, but I wonder if it would help to leave the heat off until the exchangers heat up enough to dry out.

[RWebb]

that's an idea - heat OFF really means the dump valves are allowing air to flow down and out of the system -- so that would work if the pavement is dry but the car still "steams" up*

of course, if you are driving thru a puddle, the dump valves will let water splash up INTO the system

guess you can just try it each way in each condition


* it's water vapor, not steam

[wwest]

A couple of inexpensive 12 volt (computer "slot") centrifical blowers to FORCE cabin air out of the exhauster port above the rear window will do wonders.

[rusnak]

Or unzipping the headliner can help as well.

Running the AC is the quickest way to demist the windows if you're in the rain.

If the roadway is not wet, then turn the heat to full, and direct the heater flapper to push air onto the windshield, and rotate the side vents to direct air to the wing windows, so you can see out the side mirrors.

[burgundyben]

Thanks for all the comments.

To update, I have found a way of preventing it from happening, a very successful way.

Last night I placed a sheet of polythene over the motor under the ducktail.

Its been raining all day.

I opened the rear lid, whipped out the poly sheet, shut the lid, fired up, put the heating on straight away and no mist at all. Nothing. Dry as a bone.

Just need to find a way of making that the case without having to open the lid to use the car.