Noticed yellowish-sludge-like stuff on the inside of the filler cap of '99 E320.
Cleaned it off, drove 30 miles and noticed that it seemed to be returning. Any ideas? Coolant level is fine and dipstick residue seemed normal. Last oil change was five months and around 2,000 miles ago.

 

i recommend you search. covered easily 50 times.

its normal

 

I would not take this issue too lightly. Experienced the same problem last year though I initially had more of a white-ish substance in the cooling reservoir - the yellow gunk only appeared later on the oil filler cap. I did search this forum and others and there are opinions that this is normal for the V6 engine, especially in colder areas and where vehicles do short distances. On a long distance trip I "lost" a substantial amount of oil - it ended up in my cooling system. Responses I got was that it was the cylinder head gaskets that went. Typical symptoms would be an ngine that is misfiring and some smoke from the exhaust. I had none of those symptoms but this is what I found in the cooling system:
https://mbworld.org/forums/e-class-w...stem-woes.html

I removed the cylinder head as a result, but did not see traces of any cylinder head gasket failure on the old ones but did find an O-ring between the cylinders on the right hand bank as per previous posting. New cylinder head gaskets were fitted and evrything was fine for about 2 weeks doing short local distance only. Cooling water then became milky again and signs (yellow goo albeit very little) appeared on the oil filler cap again. Further investigation lead to a Benzworld member pointing out that behind the cam chain housing there are 2 seals between the engine block and the cam housing in the cooling system channel back to the water pump.

I have since removed the cam housing (2 weeks ago) and there were no seals in place. I bought 2 new ones but progress stopped as I could not get new water pump gaskets at the time. I also left Johannesburg to be in the beautiful Cape Town for other business.

I am not saying you have the same problem, I am just reflecting on my (similar) problem and experience. Best of luck

 

ive had the gunk under my oil cap every winter for years. car runs perfect, no significant oil loss or coolant problems. just condensation

 

I did notice from many replies on postings that it seems to be a common feature (only using this word because of a lack for a better word ) on the V6 engines. Then again, in South Africa we don't experience the same cold winters as abroad. I could have had an isolated problem and as such I merely shared my experience. My penny's worth of advice is that should the condition gets worse, explore all possible causes bacause in most cases the cylinder head gaskets would be blamed. In my case I had a severe problem with oil going into the cooling system and the logical conclusion was a failed cylinder head gasket even though other symptoms like misfiring or smoke from the exhaust were not there. I had nothing else to go on. Closer inspection of the gaskets after the heads were removed did not show signs that they failed either, but too late and had to be replaced. I used my Benz for about 5 months - distances of not more that 120 km at a time - with the goo signs on the oil filler cap and milky cooling fluid which I noticed again 2 weeks after I replaced the cylinder head gaskets. I did not consider this to be normal and did not have faith in my car to travel long distance and thats why I decided to do further checks.

Should you decide the problem is not major and keep on using your vehicle, please just check fluid levels and condition regularly. Happy

 

If you are in Bismark ND, I'd say it's probably normal condensation/emulsion. If you are in Equador that would be a different story. Background information matters.
As a word of advice, if it is normal condensation you should check your PCV and clean it too.
I have a BMW and it blew a Valve Cover Gasket last year. Clean up was a ***** in sub zero conditions. Cause was frozen condensation/oil emulsion in the valve creating pressure build up in the engine. Don't know if the Mercedes has the same design feature though.