intermittent burning oil smell from vents - Mercedes-Benz Forum
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#1 (permalink) Old 12-26-2011, 02:31 PM
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intermittent burning oil smell from vents

Hi all,

We own a 2001 C240 w203 base model, about 130Kmi. Recently there has been an intermittent "burning oil" smell from the vents when the recirculation is off (fresh air intake). It does not happen when the recirc is on (cabin air only). I assumed that a small amount of engine oil, like from the valve cover gaskets or something is leaking onto the exhaust and making its way past the hood-to-air intake seal and into the cabin. I tried taping over the intake port coming from the intake microfilter housing to the blower by the battery but still had the smell. There must be another way into the cabin air.

Any ideas on how to go about further diagnosing the problem? In the cold weather it is pretty bad because we don't want to open a window and the recirc clicks out every few minutes (standard feature).

Has anyone heard of this? Many thanks for any ideas.

Will
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#2 (permalink) Old 12-26-2011, 09:36 PM
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have you checked to see if you have the valve cover/vent gasket oil leak?
I had the burned oil smell to and oil residue on the heads untill I replaced the gaskets

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#3 (permalink) Old 12-26-2011, 09:43 PM
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Oil leaking from the valve covers and dripping onto the exhaust manifold is a common cause for this. At the mileage, it's probably time to replace the valve cover gaskets. You should also check the PCV valve and clean any sludge out of it - a clogged PCV can cause all sort of oil leaks. Our C240 has almost 100K miles, so I ordered gaskets and plugs and will likely be doing that job later this week when the parts arrive.

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#4 (permalink) Old 12-27-2011, 11:41 AM
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Rodney - it would be great if you could post some pics and/or notes on the procedure.

Will
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#5 (permalink) Old 12-27-2011, 06:27 PM
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Sure thing! I should have the parts tomorrow if UPS stays on schedule. I'll probably do the work Thursday.

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#6 (permalink) Old 12-28-2011, 09:56 PM
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We had a very similar problem with our '08 ML 350. We would smell burnt oil or rubber coming thru the vents. Nothing ever noticeable from the exterior of the vehicle as should have been with dripping oil.

Turned out it was serpentine belt idler pulley that would drag intermittently causing brief burning of the belt. Problem almost always appeared at low speeds (usually in 2nd-3rd gear) or when starting from a stopped position. We had it in the shop twice without them being able to duplicate the problem. Why is it the symptoms never show up after you go thru the trouble of getting it to the shop?

I kept the engine compartment very clean and never saw any signs of oil - either on the engine or in the garage. The "burnt rubber" smell eventually evolved into a squeaky pulley, which ultimately revealed the problem.

Pulley and belt replaced and odor is gone.

Last edited by swat253; 12-28-2011 at 10:03 PM.
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#7 (permalink) Old 12-29-2011, 08:19 PM
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OK, I finished the valve cover job. I'm sorry I wasn't able to take photos - it was too messy to stop and deal with a camera. I do have some advice though:

1. Be prepared to replace some rubber hoses! The breather cover vent lines on mine were so hardened that they could not be removed without breaking.

2. Replacing the breather hoses means removing the MAF. The plastic retainer on the MAF was so brittle, it broke!

3. I went ahead and replaced the spark plugs, too, and the left rear plug is a real b!tch. You WILL need a universal adapter for your socket wrench.

4. Get yourself a good small torque wrench. Somewhere, I have an "inch-pound" wrench with a dial indicator, but I couldn't find it, so I had to improvise with a 3/8 click-type that didn't go quite low enough.

I had ordered the gaskets and plugs (used the Bosch Iridium) from autohausaz. I had to run to the dealer to buy the hoses and MAF bracket. I replaced all four hoses - one on the right, two on the left, (the small one on the left is in two pieces), plus the Y connector in front of the MAF and the MAF retainer - all about $75 retail.

Had it not been for the hoses, I'd probably have done the job in about 3 hours, but dealing with the MAF added at least an hour, maybe more. I also found that the main vacuum intake connector for the power brake booster is broken. I suspect this is why the engine runs fast at idle and the brakes are hard at start-up. I put some sealant on it, but I suspect it will need replacing. It's $90 retail, but the dealer has to order it.

I haven't started the engine yet (waiting on the sealant to cure) but I'll keep you posted when I do in the morning.

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#8 (permalink) Old 12-30-2011, 07:02 AM
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you did reseal the vent caps did you????

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#9 (permalink) Old 12-30-2011, 06:43 PM
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Yes, I used Reinzosil. Oh, and I started the car this morning and all was not fine! It was spewing oil out of the back of the left side valve cover. Of course this is the most difficult one to remove - it's under the fuel line and has two breather hoses attached. Anyhow, I took it off and really couldn't find the problem! I expected that I'd find that the gasket didn't seat correctly, but I saw no signs of it being crimped. I carefully reinstalled everything and it was fine. The only thing I can think of is that even though I thought I was very careful cleaning everything, some piece of debris must have lodged itself between the gasket and the head. Maybe it was a piece of rubber from one of the rotten vent hoses? Anyhow, it's good now. The final bit of maintenance is to replace the check valve and vacuum line assembly for the brake booster. The check valve is cracked so it leaks until enough vacuum is built up to overcome it.

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#10 (permalink) Old 01-02-2012, 02:47 PM
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OK, I finished the valve cover job. I'm sorry I wasn't able to take photos - it was too messy to stop and deal with a camera. I do have some advice though:
Hi Rodney, where did you order gaskets and sealent for this job.
thanks
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