Opinions please, Rain fixes Idle and hesitation!
 

Hi All, here's the story:



Rough Idle and hesitation, as well as hot start long cranking were my problems. Diaged with a bad fuel distributor by the dealer in Seattle. My E was not running $1k bad, so I declined the repair and kept driving it. I bought the parts to do it DYI and ended up returning them, because, when it finally started raining two weeks ago, the car began running and idling fine and I mean running very nicely! Perplexing. Did the dealer do a false diag or could moisture temp fix the fuel distributor? Do I have a vacuum system issue temporarily solved by moisture? Do I have some other seal somewhere temporarily sealed by moisture? We'll see what happens when the weather dries out again. I can't really do simple spray bottle tests because it is running well now! (Hot start is still there, so I now think that is the fuel pump check valve and/or accumulator.)

What do you all think?

Bad ground connections are often "fixed" by wet weather and notoriously tough to diagnose otherwise. I'd look over the ground connections; clean metal...snug 'em up. FWIW

Seattle, huh........
 

Nothing works in rainy Seattle unless it's wet, does it??:D :D . For what it's worth, mine used to run best(not stall and miss) when the temp was below 35 degrees F. Go figure!!

Ok might be a long shot but this one sparked a memory of a problem car that went through 2 dealers and 4 indi's before me . It was a 300e though (same setup) . When it rained the car would do fine but dry it would act up. After 3 hrs with the car I found that the cover plug for the CISe control module was not sealed properly and it had corrosion on terminals when it got damp it would run fine. This control unit is on pass side behind battery, first one is ABS and one in Rear is CISe. Ck it .After I fixed that car the customer gave me a 100.00 gift certificate, LOL he was happy he only paid 3.5 hrs as to he had already sunk 4k in it at dealers and indi's earlier that year. Good luck.:D

Sounds like one of our tech guru's needs to jump in on this one. I'm not familiar with Mercedes, but it sounds like an air density issue. GM uses air intake temp sensor, mass airflow, and manifold absolute pressure sensor to calculate this. Mercedes, most likely, does something similar. If it has one , my bet is an air intake temp sensor sending a false reading within specs.

I see your point, but this autumn 1st we had two weeks of cold and dry weather -- car ran rough still. Then we got the cold but warmer and wet weather -- car runs sweet. So cold temp alone did not make an improvement.

I remember a professor telling me cars run best in cool damp weather. The water in the air expands during the firing phase, giving the car an extra boost of energy. This is the premise used with water injection systems.