Burnt popcorn smell and smoke from defrost vent

[GregoryF]

I searched and found one thread regarding a nasty burnt popcorn smell and smoke coming from the defrost vent of a stock 83 911SC Targa. I spent all day tearing apart the HVAC system up front to see into the ducting and everything is clean. I did see in the surrounding areas some very minor evidence of maple tree seeds pods that were most likely deposited by a rodent. But since I have smoke billowing from the defrost vent, I know I have high heat causing something to burn. That kind of heat come from the heat exchangers if I'm thinking correctly. I was able to look inside the driver's side heat exchanger and it was really clean. I could not take off the passenger side because of the oil line in the way. Incidentally, the smoke is coming primarily from the passenger side defrost vent. Any thoughts on where the smoke causing debris might be? Is it possible for critters to get inside the heat exchangers?

[GregoryF]

Also, I should note that the post the search turned up did not have a resolution, thus my question to the Pelican community. Thanks in advance for any guidance.

[TibetanT]

I am certainly not an expert, but you might want to check your Heater Control boxes at the rear.

You may have allowed rodents to get inside if the hoses connecting the heat exchangers to the tunnels are 1) destroyed, 2) old and completely gone, or 3) the heater control box is left open.

Mice love to make nesting sites on top of the engine under the shroud. Most engine tear downs that I have done for others entailed finding nests on the top of the heads and cylinders.

I would guess that the other spot where mice can chew and get in would be in the trunk area since those are paper hoses for the ventilation.

Dryer sheets should deter those pests from coming near those hoses during the Winter months when we, Northern States, park our P-cars.

As far as the burning smell…hmm? Not too sure why that is happening, but you may be correct that something is inside and getting burned/heated/torched.

Hope this helps.

[TRE Cup]

A Long time ago, we met a couple at a Porsche Parade. Their SC was entered in the concours and we got to chatting about their car, how they decided to go to full concours competition.
Well.... they had a funny smell coming out of the fresh air vents , and some small debris of some sort with the fan on. Turns out some small field mice migrated up through the drain tube into the system ! I know, that hole is pretty darn small and the route somewhat circuitous.

Remove the cardboard cover in the trunk, disconnect the cable retainers on the center fan assembly, then unfasten the hard ware / remove the fan. Look inside and see if some critters set up house there. While you have the fan assembly out, check out the fan motor to see if it runs freely and is not the culprit.

[GregoryF]

@TibetanT and TRE Cup, Thank you for the replies. I tore apart most of the hoses on the ventilation system up front and everything that I could see was really clean. Although I'm not sure how the mice got in (assuming mice), I do believe they made they way back to the heat exchanger area because that's the only place it gets hot enough to burn dry debris from nuts, etc. I tried to take it to the shop after I did all I could do, but the smoke was really thick coming from the passenger side defrost vent. That's the heat exchanger I could not properly assess without removing an oil line. So I flat bedded it to the shop where they have more experience with this kind thing. I appreciate the replies and I'll update after I speak with the shop tomorrow.

[GregoryF]

Update: As I suspected, the debris that was burning was in the passenger side heat exchanger - tbe one I could not access. It turns out it was NOT a mouse, but the seed pods from a maple tree, the ones that act like little helicopters when they fall from the trees. Somehow they got sucked up inside and were burning. When the tech dismantled the exchanger they found actual embers still smoldering!! I haven't talked with them about the HOW the debris got in yet. But this is crazy and scary as hell....

[30westrob]

Mice brought those seeds into your right heat exchanger as a food store. Seen this many times. Rob

[GregoryF]

@30westrob This is what I figured as well. Just glad I didn't have a rodent nest in there too. I'll be building a 5 gallon bucket mouse trap this week to take care of any critters that may still be hanging around.

[tonythetarga]

If I recall correctly, the flapper valve at the exchanger will dump heat outside when you don't need heat. That also means the open flapper is a nice hiding place or a storage vault for mice. When you store the car, clearly you need to close the flapper in the position that would call for heat in the cabin. That shuts the door for external entry.

[em78911]

yep.. MICE! mine smelled like Chestnuts roasting on a open fire.. found a large stash of Acorns....

[GregoryF]

@Tony So you’re saying to leave the two red handled heater controls pulled up when stored to keep the valves closed? Versus leaving them down in the non-heat position?

[tonythetarga]

Yes. Turning on the heat, so to speak, closes the flapper.

[GregoryF]

@Tony Got it! Thank you. I’m doing it right now��