Heat Leaking Into Cabin When Heat is Off

[philipguziec]

My shop seems to have debugged the autoheat on my 930, and appears to have it working. However, even when it is off, there is some heat leaking into the cabin. The amount varies based on load on the engine, and the airflow is proportional to vehicle speed.

Has anyone else fought this battle before? I'd imagine the flapper doors on the engine are not sealing completely, either through adjustment problems, or lack of a correct seal, but it would be good to hear from someone else who has dealt with this issue.

[steely]

I had a similar issue, but in mine the rear shift boot (950-424-294-06-OEM) at the rear of the tunnel was gone. I think it was sacrificed by the PO when they replaced the clutch.
In this case, the engine air was coming into the cabin (most noticeably) at the emergency brake handle where it meets the autoheat 'console'. Drive around and put your hand there to feel a breeze or remove the rear trans coupler tunnel cover and take a peak.

[philipguziec]

Thanks. This is definitely coming out the vents. Switching between windshield and lower vents switches the flow. No flow when stopped. Flow at speed.

[cabmandone]

One of my flapper boxes wasn't fully closed. You might want to check that.

As luck had it, the heat only came out the passenger side because that was the flapper that was open. My passengers suffered which is why it took me a while to fix the problem. If you have air coming from both sides, you might want to check both side flapper boxes and make sure they are completely closed.

[philipguziec]

This is definitely coming out of the heat vents. There is no flow when stopped the flow increases with speed and I can switch the flow between the windshield and the floor with the red control knob.

[cabmandone]

Check the flapper boxes. I'll bet one or both need adjusted. The air movement gets faster as you speed up because the engine fan is forcing it through the vent system.

[steely]

If it is the flapper boxes, I think you want them open to dump hot air. Closing them forces the hot air up front. I could be wrong.

on edit: oops, I am wrong, the internal butterfly valve closes to divert out the open 'vent'.

[frankc]

Is the heat coming out of both L & R sides? If you check the cable adjustments at the flapper boxes and they appear to be closing properly, then you may have the issue I did:

Heater Control Box Meltdown

[philipguziec]

Thanks. I'll suggest checking for melting.

On a related note, my car sat out in the rain last night. This morning, the windshield got foggy. When I put on the defroster, it fogged the bejezus out of the windshield, and I had to keep wiping it down until the car heated up.

Is this typical behavior? I haven't had the car get wet in years, and I can't recall this happening.

[r-mm]

Most certainly a flapper box issue. The cable (whether pulled by autoheat or the lever) pulls the door to the flapper open ( parallel to direction of airflow) and gives you heat. Turning heat "off" means providing slack on the cable which allows a (probably rusty if it even still exists) spring to try to pull the flapper door closed. The combination of the weakness of the spring, the deterioration of the little hi temp seals around the flapper door and any rust you have on the flapper box body means its unlikely to achieve a 100% tight seal. Pull um off and refinish them. If the labor hours are not being provided by you, for free, buy new ones and re install!

[philipguziec]

Perfect. Thank you. They were recently rebuilt when I had the autoheat overhauled. Must be poorly adjusted.

[fintstone]

Just look under the rear at the boxes When it is hot, they should be open and vent out the warm air, when cold, they should be closed to channel the warm air to the front. This time of year, they should be open. They may just be sticking a little. When they are not hot (so you don't burn yourself), just open them with your hand.

[tcar]

Originally Posted by philipguziec

and the airflow is proportional to vehicle speed.

You probably mean ENGINE speed, not vehicle speed.

Heater has nothing to do with vehicle speed.

It's your flapper boxes. They should both be all the way OPEN with the heater OFF.

CLOSED when heater is ON.