Another 300E rough idle cure
 

My 300TE has been plagued by a rather rough idle since I got it about four years ago. I've done all the "normal" fixes (new idle control valve hoses, etc) and set the mixture, but it was never really right.

It had a serious throttle lag problem, along with a stalling on hard stops problem, fixed by repairing the vac leaks and cleaning the idle valve.

Last week my mother complained that it was stalling again, and sure enough, it did, but only on stopping, or so I thought. Cleaned the idle valve again, checked for leaks, checked the mixture, etc. Worked fine for me, then stalled again for Mom the next day, but this time while driving off idle.

Pop, on goes the light bulb and I scurried off and bought a new fuel pump relay.

Wonder of wonders, the rough idle and most of the low speed vibration is gone, and the gas milage seems to have jumped up, too!

Who'd a thunk that a bad fuel pump relay would cause rough idle? I suppose the real problem was continuous on/off cycles on at least on of the fuel pumps, cuasing oscillating fuel pressure, which in turn caused the EHA to float around all over the place.

Sure does run nice now! All I have to do now is get the new evaporator in next week -- Mom is having a cataract removed on Wed, so I figure since I'm gonna take two days off, I could start the removal process Tuesday, get any parts I still need on Wed for Thurday delivery, and button it up on Thursday evening.

Always something....

Peter

Thanks for sharing the info...

It is what makes this site better than any repair manual...

Again thanks, and good luck.

Peter,

Thanks for the info.

I'm nearly in the same boat. I have to pull the 300E dash and replace the vac pods. It sure is hot here to have to mess with it.

Best of luck to your Mom for her surgery.

Have a great day,

Larry:

I'm gonna check out the vac system while I'm in there and replace anything iffy -- it's almost worth doing the heater core at the same time!

Peter

Re: Another 300E rough idle cure
 

That's a mighty ambitious timetable! Let us know how that turns out!!;)

I have similar problems in my 300E and have begun to seriously suspect the FPR. I have an aftermarket FPR in there, and wanted a spare anyway. Have one on order from the dealer (more, but not too bad). We'll see if same happens!

Nice wrench! Best wishes for your mom's operation.

Peter,
Thanks for the post. I have an 88 300SEL right now with a rough idle which is driving me bannanas. My 90 300SEL is as smooth as glass. I'll give that a shot. Right now I'm working on a theory to make the tachs work in the early diesels. I'll share when I'm confident of my results.

Don't worry about your mom. One of the good things about our time is modern medicine. My 81 year old mother had a brain operation a few years ago at UCSF medical center which was almost routine.

Take care,
Peter

A fresh set of plugs resolved the idle problem on my 300E 2.6. The old ones showed wear and the gaps had grown too large, causing an intermittent stumble when taking off from a stop.

My car is a 1990 and the fuel pump relay is part of the MAS relay unit. fairly expensive to replace ~ $300.

Peter, what year is your car? Is it pre MAS, with a separate OVP and relay? Thanks for the info!

Hi Larry,

I am thinking of replacing the dashboard ( too many cracks). Do you know how hard is it to take the dashboard off? A lot of people warned me, that is a big job. My friend's friend is MB mechanics, and he told me it takes 15 hrs . I thought it takes 15 hours to build a car...oops.

Any suggestions? Should I do it, or just keep looking at ithe cracked dash..

Thank You and have great day..

Sam:confused:

I would consider a professional repair of the vinyl first.

It's not that big a deal. About 4 to 5 hours. Just be careful not to break anything.

New dash will take about 12 hours, give or take a bit -- you have to remove the climate control and switch panel on the top of the console to get to the two nuts up there, the lower panels have to come off, the cluster has to come out (manual says the speedo cable has to come off the tranny, is this true???!!!), and the speaker cover has to come off both sides.

I'm to the point of taking the cluster out, but will have to wait until tomorrow afternoon (Mom's surgery is at 9:30, and I don't expect to be home til noon anyway). I worked on it about three hours this afternoon just taking stuff off, expect to have it done Thursday late since I'm going to have to wait on a vac servo from Benz (the regular supplier is out).

Only one of the servos holds vac, and that's only 5"! I'm replacing them all, I'd rather spend the cash than do this job twice.

The TE is an 88, separate OVP and FPR, the 300D is, of course, an 87, also needs a new OVP.

Peter

My new FPR came last night. Not a moment too soon, as I died 4 times on the way home (3 times while driving -- not just from idle)(sometimes would restart, sometimes not for several minutes). My car only does this in heat -- in the afternoon -- so it may be a bad connection or wire somewhere that is opening when hot.

Car Seems to run more smoothly now, we'll see if this FPR is a permanent fix.

Hi Peter,

First of all, Good luck with your mom's surgery. I hope everything goes well.

Second, thanks for the information.

Whenever you have the chance, can you please post the job number(s). I have the CD, and it hard to find things there? If you can give me the job number(s), I can do search and find them...hopefully :)

Thanks a lot

Sam