Smell from vents BMW 328i

I am wondering if anyone has suggestions for me. I have a 2011 BMW 328i and I get this sort of burnt chemical smell through the vents when the heat is on. It doesn’t happen until I have been driving for 10-15 minutes. After that you get the smell on some low level all the time, but it seems to be briefly exacerbated by making a hard turn (i.e. a U-turn). You get a stronger whiff coming out of the turn and then it returns to baseline. I also smell it stronger when setting off in low gear (1st or 2nd) like from a stop sign. It happens whether the car has the air re-circulation on or not. It has been in for service twice and they can’t find the problem. The car is a RWD manual transmission and only has 20k miles on it. I checked the fluid levels and as far as I can tell they don’t seem low. I looked in the service history and a coolant leak was fixed at one point, near the thermostat - yet the car still exhibits the smell. In addition to the smell, it kind of irritates the back of my throat after a while. Any help?

One possibility is a valve cover gasket leak that allows oil to drip onto something hot (usually part of the exhaust system). It doesn’t take much oil to make a noticeable smell, so you might not see the oil level dropping.

If a hard turn to the left makes it worse, here’s what I’d do. On a day that’s not too sunny, drive for fifteen minutes, then park sideways on a hill with the right side of the car downhill. Quickly open the hood and look for wisps of smoke, possibly using a bright flashlight to help you see. If you don’t see anything, try parking at different angles on the hill.

My wife’s previous BMW had a valve cover leak that we found this way, using our fairly steep driveway.

Hmmm…this sounds a little tricky. Usually any kind of smell from the vents is due to a Heater Core leak. The smell is or used to be a Sweet kind of smell and it would irritate your throat after time. I dont know what kind of new Anti Freeze they use in an 11’ BMW these days…but the days of everyone having the usual Green Anti Freeze are LONG GONE…who knows what this type is or what it smells like when atomized in the car. It could be anything. Its a bit confusing about the turning…it SUGGESTS a liquid moving within the heater box…something sloshing around.

I think you would know if it was an oil vapor as opposed to an Anti Freeze vapor…tho again its uncertain what these new fluids would smell like when atomized…this is a newer vehicle using the latest coolant and Motor oils.

One other thing that came to mind was the Blower motor speed regulator or resistor …in the days past…not that long ago really. The blower motor resistor was intentionally put into the air stream for cooling… The resistor would get fairly hot actually…so any liquid that could be in that box if it were to hit the resistor…would be turned into vapor or smoke. But I dont know if your system has this resistor in this fashion.

Im kinda stumped eventho I have repaired many of these but with more obvious symptoms. I guess I am stumped because I dont know the smells of the latest fluids used in vehicles these days…the number of Coolant types today is almost about 10 different types if not more believe it or not. Hmmm… I would focus on the coolant level to see if it is low…and also if a turn exacerbates the issue…this suggests a liquid sloshing around…and if that is happening…you will also get that same liquid coming out of the drain tube…the same tube that the AC system drains from.

In fact that gives me an idea…use the A/C in order to get some condensation going…its sort of a way to “Wash out” the heater A/C box in the vehicle. Notice where the condensation exits the vehicle…this is the same place your UNKNOWN fluid should be leaving the vehicle as well. Once you locate this…you may be able to place some cardboard under the car…it will usually be near the passenger side Glove box area under the vehicle. See what you catch on this cardboard over time. It may help you find the fluid…Dont forget to look to see if your coolant is low or getting lower. Bout all I can think of at this time.

Blackbird

I concur w/ @lion9car , some kind of fluid leak, most likely oil or transmission fluid. On a 2011, I also concur w/ lion, valve covers leaking would be the most likely. The only thing that is sort of inconsistent with this theory is that the odor seems to be the same independent of recirculation or fresh air mode. I’d think a valve cover leak would be most noticeable in fresh air mode. OP should re-do that experiment maybe, starting in recirculation mode. Perhaps when switching from fresh air to recirculation mode, the odor remains from the first mode.

If the condition does NOT change with the Recirc On or Off strongly suggests the smell is coming from INSIDE the Heater A/C Box. Also the turning of the vehicle making the smell change in strength also suggests a fluid Inside the same box…behind the glove box. If this were a valve cover oil leak or a tranny leak or any under hood emanating smell…it WOULD BE affected by turning On or Off the Recirc

Blackbird.

Thanks for the suggestions. I’ll try some of them and report back. Subjectively I don’t seem to smell it with the AC on (just the heat) but I need to test more. It hasn’t really been hot enough for me to use the AC much. Is there an objective way to test for coolant in the air? Like test strips? A meter?

Coolant has a sweet smell, bordering on a pleasant, fragrant odor. Burned oil or xmission fluid has an acrid unpleasant odor.

1 Like

At least the old coolant most of us are familiar with smells sweetish/pleasant. I dont know what type of coolant is used in that BMW but I can guarantee that it IS NOT the old faithful Green coolant we have all come to know and love. This new stuff could smell like Sweaty Clowns for all I know…

Blackbird

1 Like

That would be sweaty German clowns for sure…

I think it would still smell somewhat like maple syrup, unless it were the embittered variety that is mandated by some states.

The smell is kind of a sweet/spicy smell that sometimes takes on a burnt nature. Usually this is upon turning hard or sometimes getting in/out of 1st or 2nd gear.

I smelled the BMW coolant and it isn’t an exact match, but maybe if it gets heated up it smells a bit different. The current BMW coolant is “blue” FWIW.

The only other thing I can think of is the clutch or power steering fluid. But it doesn’t smell exactly like either of those either…

Thats probably the heater core being the issue…and the coolant as the Sweetish smell… Perhaps all coolants smell this way when heated and partially vaporized. The turning exacerbating the issue is another clue we are talking about a fluid…within the Heater/AC box inside the car…and what is in that Heater Box? The heater core…with coolant flowing thru it… Since time immemorial we have been literally building cars n truck AROUND the heater core…its a nasty B of a Job just because it is quite literally the first thing installed into the interior of the vehicle. There is a way to verify this for certain and it involves stopping all possible coolant flow into the heater core and then also rinsing out the Heater box in the car and then seeing if the issue persists. But there isnt much reason for all this…Your issue is basically a Textbook description of a Heater Core leaking.

Blackbird

+1 @Honda Blackbird

OK guys thanks for the advice. I guess its back to the dealer. :neutral:

Hey asus389 were you able to find out the issue for the burning smell when turning for the bmw. I have the same problem. Thanks

It turned out to be a leaking AC evaporator. The evaporator oil was burning when the heat was on. Also when the AC was on there was a chemical smell. This is a common problem with this model of BMW.

The situation was confused because the dealer initially put some fragrance spray in there which masked it and made it smell sweet when they should have just diagnosed and fixed the problem to begin with.

You can have them check this by removing the AC refrigerant and weighing it and then comparing it to the weight of a full charge (it will be lower if there is a leak). Also, they can add UV dye to it and the condensate will glow when it drips out.

They have to pull the dashboard out to replace the evaporator.

2 Likes

asus389! you’re awesome! Thanks so much for getting back to me on an old post. This I can almost say is 99 % of the problem because this smell and the smoking only happens when I turn on my a/c. I’ve been keeping the a/c off for the past couple days and the smell/ smoke hasn’t been there just because I had a suspicion it was something linked to that. But I may have never came to this conclusion without your help thanks much appreciated!!

Thank you as well. 2011 328i and having the same issue. Any idea on the cost for repairs?

This is a big job I would say around or above $2,700 USD

I just had my car towed in to be checked on. Smoke came out of the AC vents this morning really bad. Will update with findings once received.

UA common problem on BMWs that also causes a burning smell is a leaking valve cover gasket. Oil drips on the exhaust causing a burning oil smell. I’d think you would smell it in the engine bay also and see the leak, whereas with a smell originating in the heater/Ac you wouldn’t.