P38 Cold air in Cab

[kjg48359]

Hi,

There seems to be cold air coming out of the dash in that small gap under the steering wheel at the top of the knee board/panel (right where you can raise/lower the steering column).

I think the blend doors are working ok, at least the major ones seem ok (air is getting to the feet and window and as long as they're working I can get by until warmer weather before I have to rip the dash apart).

A quick look in the service manual the other day mentions "seals" but no idea of what and where to look for them. The actual ports on the vehicle do in fact put out warmed air (foot and windshield are all fine).

I'm wondering if there might be a leak in a "intake" hose or maybe something might not be working on the firewall. I recently drove for a long weekend in cold weather (about 12F) and I had to put a coat over my left leg because the draft was so strong. But it seems to come and go.

When I first picked it up at 102K, I replaced the cabin filters right away as it looks like they had never been replaced (they were awful, almost full of black tar). And I've replaced those a couple times since then.

Would anyone have any ideas what I could look for that maybe has seen something like this?

(It's a 2000 P38 w/ Air & about 140K miles now).

Thanks.

 

[discostew]

The blower motors are down in that area. One on each side of the dash. The recirc motors are in those housings and not part of the three blend door motor kit you get when you do blend door motors. Pull that knee bolster out and maybe you can get some idea whats going on. If you pull the pollen filters out you should be able to see the blower motors and that should help you understand whats in that area.

Since you say it comes and goes while driving a long way I would suspect that possible the duct that comes out of that blower motor housing could be loose from the heater box. I think that because if you put these trucks on recirc they will open the recirc door every so often to let fresh air into the cabin. But that air should go thru the heater core instead of out on your leg like that.

 

[jhmover]

There's a hose that goes from the left top vent on the driver's side to the bottom vent in the middle. It's possible it came loose.

 

[kjg48359]

thanks jhmover: I actually checked that last week (now I'm suspicious there might actually be a cracked hose even).

I'm thinking maybe I'll take the panel off and drive it while feeling around under the panel to see if I can help pinpoint a more general location...

 

[discostew]

There's a hose that goes from the left top vent on the driver's side to the bottom vent in the middle. It's possible it came loose.

Yep after that air goes thru the heater core and out that top left vent then thru that hose behind the dash its way colder than if it was still hooked up to that vent. No way that cold air is coming from outside, like thru the blower housing. I don't know what I was thinking.

 

[kjg48359]

Thanks Discostew. What a PIA.

So according to the service manual, I took off the knee panel and a small black plastic panel that is down by the feet.

The foam that looks like it's supposed to help seal the duct that runs between the left blower housing and the central core unit I think is essentially shot. There were some big gaps there.



So if you look at the diagram, I ended up just wrapping duct tape in the areas where the red arrows are. It appears to be a little better. I think as it warms up there's still some sort of leak, but after about 10 minutes it subsides.

I have RSW Solutions software that can read some of the codes in the vehicle (kudo's to the author, he's made a lot of improvements over the last year!) and it can now read HVAC faults. I cleared them out (some problems include left blower motor issues which piss me off as I replaced it within the first month of ownership). But there's one that comes back quickly and it's "Distribution Door motor circuit fault." I'll have to do more looking into this, but item #7 in the diagram I think is the intake where there's a recirculating mechanism (I could feel the 'mesh' of it's opening when I was feeling around in there with the panels off). I'm getting suspicious that it's failing. I'm wondering that it should be either fully open to the outside or fully closed for recirculation? Mayby it's sticking allowing cold air to come into the cab at that point?

If I do have to replace that, it's going to be really difficult, there's almost no room up under there (although I have to study a little more, so far I've spent only a few minutes here and there to dig into this a little).

After reading the service manual, I was surprised to learn that the system has it's own self diagnostics. When new it goes through a self calibration check after a series of starts to the system and then as it ages, it spreads out the diagnostics to only occur once in 500 starts. Maybe RSW's software might be able to allow me to force that diagnostic and self-calibration routine to see what I can do.

It's getting cold here in MI next week, that will be my first test of cold weather (it's going to get down to about 10 degrees F).

[In the meantime, my windshield has cracked from just under where the right passenger wiper rests on the widow to all the way almost to the A pillar on the left side - so I'm getting that replaced hopefully Monday morning: thank goodness for good auto insurance to cover windshield cracks.].