I'm new to this forum, having recently bought a 2001 C270 CDI with 83K on it. Awsome car, amazing mpg, just one problem so far. I've searched the threads and can't find a mention of anyone getting an acrid smell through the heater. It doesn't smell like mould or stale water, or hot lubricating oil. It only happens when the engine is under load - like accelerating up a long hill. The smell doesn't get into the car if I set the heater to recirculate. I've been through the process of changing the pollen filter, and disinfecting the a/c system with one of those sprays (neither seemed a likely cause based on the type of smell)... I've had the engine covers off to look for leaks. There is a trace of oil around the black plastic cap (some sort of filter unit?) just in front of the oil filler cap, which might track down to the turbo. But that is all. The car is still under warranty for a few more days, but the dealers mechanics are a bit cluless. Has anyone any ideas please? Mike

 

The key here is in something you said. The car only does it under a "load" like uphill and on the gas, and it doesn't do it when the air is in cabin "recirc" mode. That tells me it's not mold on the evaporator, which is a common problem causing a bad smell w/ the AC, as you probably know. The recirc. thing tells me it's obviously not in the cabin, otherwise you would smell it then. So, the smell is coming from the engine into the engine compartment probably through the air intake where the pollen filter is. That's not an odor filter, it's just for dust and pollen, so the smell could easily be coming in there from the engine. Under load only tells me fumes are coming from somewhere, but sitting here I can't discern what type of a smell it is. That would help pinpoint it. There may be a leaky gasket somewhere that is only leaking under stress. If the mechanics are stumped, document your problem while the car is still under warranty and call MB USA 800 number, customer service and tell them your problem and that the car is under warranty and you want it FIXED. You have to be firm.
They will get it done.

 

Thanks for your sugestions Mr Coyote. Sadly, the car is long out of manufacturers warranty, so Mercedes simply isnt interested. The dealer who sold me the car has looked at it, but his mechanics dont have a clue. But I might have found the problem, after looking at the "exploded drawings" in the Mercs EPC system on-line. It seems the original battery was a sealed type, but with a vent pipe leading out of the compartment. The existing battery just vents through the top. The pollen filter and heater/air-con intake is straight over the battery. Those fumes could well be battery acid. A new battery was fitted when I bought the car. So, I'm going to get them to change it again....if I can.

 

My Mercedes started making a quite pungent burning/rubbery type smell coming out the vents. I believe mine is due to the monovalve (ac/heater valve) malfunctioning. Worth taking a look, if you haven't figured it out yet. I can smell mine quite strongly when I open the compartment on the right side of the vehicle next to the firewall (though yours could be on the left side).
Just a thought, if you were still trying to figure it out.

 

Thanks for that information. I'll follow it up as I am still trying to cure the smell. According to Mercedes EPC there should be a shield between the battery and the a/c intake and filter housing, but all that I have managed to get from Mercedes is the cover that hides the brake fluid reservoir (wrong side). Could you tell me if your car has a shield or cover between the battery and the a/c filter? Thanks. Mike