300SDL Idle Surging ???

Ok I know that storage is really hard on a car but I really don't have any need for my Mercedes but I could convince myself to sell it, I have the 87 300SDL 124k

After I got it out from 3 months setting up, I noticed that after it warmed up about 10 min, I heard and felt in the steering wheel, a strange kind of like a humming, its like ummm, normal, ummm, normal, ummm, normal, pattern, when you put in in gear its almost unnoticeable, but its still their, and the rpm's doesn't move up and down, like my C230 did when my Mass air flow sensor went out, it just stays in the same spot that it always has.
I decided to adjust the idle with the little circle adjuster under the hood and when I moved the indicator from five to one, and two the surge was still their but it was a slower surge, and when I moved it to six , its a very fast surge pattern, like hummm, humm, humm, My engine was running perfect before and its a newly rebuilt engine so I'm thinking the problem is with the vacuum, or the idle control solenoid, My friend said that he was sure that it was the Engine mounts causing the problems, hes a diesel mechanic, I haven't had him look at it personally because he lives in another state but I described the problems, and he said engine mounts. What throws me off is that it only does it after the car warms up, and if I give it a little gas about 1000 rpm's it doesn't do it... If someone has had this problem please let me know before I take it in for repairs... One more thing the Car still runs and performs EXcellent since this problem has started, so it not to serious I hope. Thanks

[Capt Kirk]

What happens when you drive it for a few blocks and come back to an idle?

[psfred]

Set your idle dial at 4 or 5 -- you should then have 600 rpm or so idle speed. Factory setting is 4.

The surge is the electronic governor adjusting the idle speed with old fuel and an engine with dirty rings, etc. It should go away after you drive a tank or so of fuel through it.


You could have bad engine mounts, too -- check to see if they have gone flat (they are oil filled) or if one side is flatter than the other, and if the engine moves execessively on the mounts. If they don't look bad, you probably just have sloppy running at idle.

I had leaking pressure valve seals on mine, so that #4 wasn't firing at all at idle -- this seems to have crapped up that cylinder, so that now when I have good injection, I get the same problem you have. It is going away slowly -- idle speed is a little higher and I don't get the steady rumble I had just after the fix -- now it has a rythmic thrum that is getting less and less.

Change your oil and take it for a nice drive on the interstate -- it will idle much better after you get back.

Peter

To answer the question its ok as long as its cool, so a couple short trips around the neighborhood, dosen't allow it to warm up, so it dosen't start its humming. It only does it when its in park with the ac off, if I turn the a/c on its still their but barely noticeable and when in drive its gone .....Something else i remembered is a few times over the last 6-8 months ill be driving along se a light turn red let off the acclerator and the rpm dosent fall all the way down as it should instead it stops on 1000, but if you put it in park and mash the gas a couple times it goes back to 600 where its susposed to be...

After driving it a couple days I have noticed that when I start it, She really spits and sputters, but starts right up, I dont remember this being such a big issue before but maybe its because Iam used to my new MB I can never hear it starting or running because its so quite.

Also before I stored it I added some Red Line" that every one here swore by" into the tank and the last oil change I used Rotella instead of my usual Delo, and I got fuel at a Texco truck stop instead of my usual neighborhood chevron

I dont think this will help but this weekend I plan to change the oil, drain the rest of the tank out and put Chevron in, and change the engine mounts then after that ill give up and take her to the Dr.

[psfred]

Lubricate the throttle linkage, too, before going for help -- if it is sticking, you will get erratic idle and failure to return to idle when you let off! Take the bell cranks apart and clean, then lube with transmission fluid or antiseize. Don't clean with carb or brake cleaner spray, it will melt the plastic parts!

Peter

I have a 87 300SDL with 238,000 miles, there is no adjustment that will help you. You probably need
fuel filters and a fuel additive added to fuel tank.