Hi.
my 2013 E350 gasser idled perfectly for 3500 miles. By that I mean the tach
stayed at same exact rpm at idle, be it fast idle at startup or slow idle when warmed.
Now it sometimes varies slightly, you can see the tach needle waivering 50
or 75 rpm's.
Probably most variance (but still occasional) when restarted after an hour or two of sitting.
Mostly it is fine but I'd like to solve this .

Anyone else have this or have any ideas?
It happened to start doing this after my first and only fill with Holiday Blue
Planet gasoline. I believe it has different formulation than others.
Coincidence?

thanks for any ideas.
When it does this, it feels like an injector or plug is misfiring...
No errors codes come up on dash.

 

two words...direct injection

tell me if I'm wrong guys.

 

I noticed my C250 loaner with ~6K on it had the slightest tach needle flutter, moving maybe 50rpm if that. I didn't notice any perceptible idle variation and attributed it to an electronics issue, but who knows.

 

Carbon buildup after 3500 miles wouldn't be significant enough to cause an idle problem IMHO but it is one of the symptoms.

 

Water in gas? Matches your symptoms.

 

Maybe I'll try some SeaFoam in the gas. I'll read the instructions but I think it also absorbs water, besides cleaning fuel system.

From SeaFoam site:
"Moisture: Fuel can absorb moisture from the atmosphere and from condensation – even more so with the addition of ethanol. Sea Foam fully emulsifies the water in the fuel. Allowing the moisture to burn through the engine and be harmless."

 

Eiher water in the gas, or just a tank of bad gas altogether.

 

Before you try seafoam, first try a take or two of gas from a different station (preferably different chain). I've seen this happen on many cars where going to a different station fixed it. Use up as much of the gas in your tank as you can, then fill up with the highest octane at a different chain/station. Preferably one of the TopTier gas vendors. If a few tanks of that doesn't fix it, then try some seafoam although I doubt it'll do much good with only 3500 miles on it.

 

It is a brand new car. Seafoam is a product that was developed for carburetor engines in the 50's. Engine technology has come a long way and a quick internet search will point you at a zillion posts talking about problems caused by Seafoam.

If you think it is a problem then step away from the Seafoam and go to the dealer. Complain about a rough idle and see what they say.

Dubious self help methods seldom help during warranty and often lead to warranty claim denials down the road,

 

I also noticed that my 2012 with 10K miles idles funny. For exampel when I start it in the morning it idels real high. Also, when I drive very short distances the fan seems to stay on longer than I would think a normal car would. Almost like I drive the car to hot even though I don't. If it continues I will take it to the dealership and have them troubleshoot if not normal. What do you think?

 

Mine idles high too when it is cold but a few seconds later it returns back to normal level and stays there for the rest of the day
As for the fans running longer , I assume you leave your A/C on all the time , if this is the case , turn off the A/C before your turn off the car

 

I'm having the same problem, it seems like a cold start but it's summer. When I start it, it revs to over 1K Rpms at idle, but when I put it in reverse it goes back down to 500 Rpms.

 

You live in an area where the summer comes a little bit later than where I am. Is it possible that this problem started when your warm weather came? With 3500 miles on the clock you bought the car 2 - 3 months ago when your summer was not at best yet?

The rpm fluctuation could be caused by the AC compressor kicking in and out. To verify this let it idle with AC OFF and see if it fixes it.

 

My C300 and GLK both have a slight wiggle in the tach needle. No audible or tactile change matching the needle bounce.

 

I've been noticing a rough idle on my 10 350 when in D or R. I'm gonna take it in to the dealer for the 40k trans fluid service soon, and will ask about it. It doesn't make the needle bounce but I definitely notice a rougher feeling in the seat when idling at a stop.

I've been doing bp gas since I noticed it, but before sometimes got it from speedway and Kroger. Always get the highest premium. Also the premium prices are getting further away from reg. Like it's over $80 for a fill up now.

 

My car does it too. It hovers around 1000 for a minute or two and then it idles back to 500 rpms. If I put it into any gear sooner , then RPM quickly slows down. I generally wait till it slows down by itself if I'm not in a hurry. I'm not sure it is a problem though. It may be set up like that to reach normal operating temperatures or something like that.