OK, this is an odd one - interior odor only when braking downhill

2009 BMW 128i. Over the last few months I've noticed an odor that I can best describe as similar to melting crayons that appears only when I brake on a surface that is significantly downhill. It's definitely noticeable but not overwhelming, and dissipates very quickly after resuming forward motion. At first I though it might be something from the engine bay, but I can't find anything there that looks suspect. Next I figured it might be related to the open blower motor wiring recall, but that's since been done (long harness and regulator replaced) and the odor continues to appear.

At this point I'm running out of ideas. If it's electrical, it's not a normal electrical smell - not acrid or smoky. If it's something else, I can't figure out what. Aside from some light weeping in the PS plumbing, everything is dry. All maintenance is up to date. I'm thinking of pulling the panels under the steering column and looking at the brake-related wiring, but if it's that why is it only producing the smell on downhill grades? And why isn't a fuse popping? I have a lot of questions and no answers. Thoughts?

Look for something loose.  Loose enough that grade + braking moves it to a hot spot.

How downhill is downhill? Is there a grade rating? Speed related at all?

Something in the firewall/HVAC system is opening to the outside and you're smelling brakes?

Can you go around with a heat gun and blast stuff and then smell it?

Weird, the crayon smell is common and there's canisters sold to bomb it out of your car. Can't explain the braking part though 

Is it when you come to a complete stop, or just keeping speed in check?

Can you turn off stability control to see if it is causing a significant shift in brake bias due to the angle?

Is it worse after it rains or when the AC is running? Could be fluid sloshing around in the hvac.

Thanks for the thoughts. I'll try to address these points.

- It seems more prevalent when coming to a stop; I don't know if that's meaningful, or if it's just a byproduct of the lack of airflow allowing the odor to concentrate and linger.

- Speeds aren't high - I often notice it coming down a hill to a stoplight on my way home from work where it's like 30-35mph to a stop. The brakes are fairly new all around, and I don't notice the smell in heavy braking; it doesn't smell like brakes to me.

- I've actually had the DSC off the last couple of days because a wheel speed sensor failed, and the smell is unchanged. Doesn't seem to matter if it's raining or not.

I need to spend a little time in there looking for loose items that might be moving around and contacting something hot. The downhill attitude seems to suggest it might be something dangling, but as of now I haven't found it.

In reply to 02Pilot :

Regarding your first sentence in your last post, on a very cold day, before my crew cab pickup is fully warm inside, I can round a corner and feel a wave of cold air rushing up around me from the side. When I brake hard, the cold air rushes up to the front from the rear area.

Makes me wonder if the crayon smell I see reported coming from the rear area and trunk on BMWs is just wafting toward you as your car slows down. 

ebonyandivory said:

In reply to 02Pilot :

Regarding your first sentence in your last post, on a very cold day, before my crew cab pickup is fully warm inside, I can round a corner and feel a wave of cold air rushing up around me from the side. When I brake hard, the cold air rushes up to the front from the rear area.

Makes me wonder if the crayon smell I see reported coming from the rear area and trunk on BMWs is just wafting toward you as your car slows down.

 It's an interesting thought. The trunk is pretty well isolated from the cabin, but it's worth a look, especially considering that the battery is back there. Beyond that, however, I don't carry much in there normally beyond a few basic tools and a backpack. Nonetheless, it's easy enough to have a look.

I had a similar issue on an rx7.  It was actually water and leaves in the evaporator.  The smell was oddly similar to crayons.  

I unclogged the drain and it went away.

Does your car leak water when the a.c. is running?  If not, that could be it!

"If you own or rode in a BMW, Mercedes, Volvo, SAAB, or Volkswagen and smelled what you think is a 64-count box of Crayola crayons, you aren’t, in fact, imagining things and there’s a very specific reason it smells like first grade. It’s called “Thermoplastic Adhesive” and it’s used to seal, protect, and provide sound dampening for all different areas of a car, most notably under interior carpeting, behind speakers, under the rear decklid, in the trunk, and anywhere were two different materials need to bond (think carpeting to bare metal.)"

I don't know that the thermoplastic adhesive would start off-gassing after 11 years, but I suppose if it's coming into contact with something hot it could be the source.

As far as water in the evaporator box goes, I'll check the drain when I have the car up to deal with the unrelated wheel speed sensor issue later this week.

How is your brake fluid level? Might it be brake fluid leaking into the booster, and then getting sucked into the intake? When that gets really bad, it can cause a lot of smoke in the exhaust.

Pull the coolant reservoir cap and stick your nose in it.  Smell similar?

My guess is a leaking heater core.  Not enough to show up on the carpet, but enough that a couple drops slosh forward in the evap drip tray.

I've dealt with the brake fluid in the master on other cars, but that's not the problem here. Fluid was just changed last fall, and the level is holding fine.

I don think it's coolant either. I had a leaking heater core in my old E28, and that's a different and more persistent smell than what I'm getting. Level is also holding full and steady.

Right now plugged evaporator drains is my leading contender. I won't have time to get under there for a couple days, but when I do I'll see what turns up. If it's below freezing I may have to run the car for a bit to get any water that might be in there into a liquid state.

I was under the car today and I found nothing touching the exhaust, nor any clear sign of fluid on the pipes. I suppose washer fluid would evaporate and not leave much of a trace, but I also didn't see any indication of a leak.

I found the evaporator drain tube, but it's got a rigid plastic trap that forms a 180deg turn at the end to keep debris out, so there's no way to get a probe in there. I thought about blowing it out with compressed air, but decided I should see what the correct procedure is before I screw something up. Anyone know how best to clear that drain from the bottom?

I would try feeding a zip tie into the drain hose from the bottom end. 

In reply to Danny Shields :

Way too long a run for a zip tie to work (it comes from above the trans down to one side - probably an 18" tube, and where it comes through the floor is totally inaccessible), and the trap at the bottom end is a real obstacle. Thanks, BMW....

If you can access the bottom end of the drain hose, you could introduce a little air pressure to back flush the hose. If you don't have a blow gun that fits up to the drain, a tire valve makes a handy adapter.  Protect your floors from any condensate that might be blown back into the heater ductwork, if the drain was indeed plugged up.

I blew the evap tray drain out with compressed air. No indication of blockage or trapped water, so I think we can scratch that off the list. The only thing I can find that could be contributing is that the washer reservoir is very full; the fill tube is right over the exhaust manifold. If it's spilling out it could conceivably be what's causing it. I sprayed some on the manifold and there's some similarity, though it's not quite the same smell. I guess the charcoal filter could be affecting how it smells inside the cabin.

I'm just hoping that it's not some wiring melting somewhere. I have seen absolutely zero indication of this, but the possibility still freaks me out a little. I hate chasing weird problems.

Over time, all strange sounds and smells in a car will either go away, stay the same, or become easier to identify. 

More research. I am able to induce the smell with hard straight-line braking, but there's a caveat. Every time it's occurred, the braking has been followed by a right-hand turn, usually a fairly sharp one. I need to do some testing to see if I can repeat this with a left turn or no turn at all. Regardless, the combination that seems to produce the smell is a compressed front suspension, possibly coinciding with a sharp-angle turn. I plan to pull the undertrays and see if I can find any sign of anything leaking out of the struts (they're Bilsteins, so I hope not) or the steering rack when deflected.

In reply to 02Pilot :

If the smell is originating from the right side, that would make sense why you'd smell it in right turns. 

Are the tires contacting an inner fender liner? 

You said you recently did the brakes.  Did it possibly coincide with the onset of this smell?