Hello,
I am a newbie to the forum. I have enjoyed reading the posts and how everyone helps out each other.
Well, it's my turn to ask for help.
I have a 1993 300 CE. About two weeks ago I got a wild hair and decided to change the fuel filter. I had guy come over and replace it. Then He changed the plugs for me. Started right up and ran great.
The next day I went to pick up my son and about 7 miles into my trek I stopped at a light and the car ran really rough. I made it back home but the car was slow taking off but once it got going it ran good.
I have so far changed the injectors. I started the car and it ran great for a few seconds then it once again idled rough. Changed the plugs and the wires with no positive results. I'm getting ready to order three coil packs but I'm hesitant to do that. I feel that the problem is a simple one but I can't put my finger on it. I asked my local parts store guy about the gap for the plugs. he told me .028. is that correct? I checked the old plugs and they were at .038/
I need help!
I appreciate any and all help.
Thanks for your time

Welcome to the forum. Others more knowledgeable will chime in here, but one of the first things you must do is to replace most or all of the rubber bits in the vacuum system. I guarantee you there is at least one leak someplace. The ECU in the HFM-SFI system can compensate for some vac leaks up to a point, but it only goes so far. At this age, all those vacuum parts are as hard as a rock. Buy a couple feet of vacuum tubing and start replacing all the short pieces of tubing in the engine compartment. There are a couple down low, down near the trans. Then replace the two approx 1/2" diameter pre-formed hoses on the botttom of the intake manifold. You also need to replace the pre-formed elbows and tees in the system that connect to the EGR and the other sub-systems. Just start replacing the vacuum parts one by one in a logical manner.

You need the car to be placed on an ignition scope to verify if coils are bad (as far as I know).

How is the condition of your engine wiring harness? You do know that your car may be one of the ones with the disintegrating harness? DO a search on here and you will find out how to determine if your harness is going bye-bye. A degrading harness causes all manner of intermittent driveability problems.

New injectors?!?!? How dd you verify your old ones were no good?

Check the plug type (non-resistor only they say) and replace the three plug connectors directly under the coils. Common problem with this engine. Mark

Thanks for the replies. I did say I changed the injectors. I only changed the number 3 because like all people who know squat about cars like myself, I listened to a guy that said he checked them all and the number 3 was bad. Mind you I was not there when he made his diagnosis. So I didn't see him actually check anything. The one thing he did check twice was that the amount of the check I wrote him was correct. He was a friend of a friend.
OK, so I will start replacing hoses systematically.
In response to the other advice : I did change the three connectors under the coils. I also replaced the spark plugs with the Bosch Super Plus plugs gapped at .028.
I did notice a hose missing under and by the manifold. Hmmmmm. I will go check that now.
I will report in.

Right on. Do you want me to give you the part numbers for the vacuum parts? I am going through this very effort right now on my own '93 300E with M104 motor. I have the PN's for 95% of the hoses and fittings in the vacuum system. I used the parts diagram in the StarTek parts manual along with a knowledgeable MB parts guy to get the stuff. There are one or two I haven't tracked down yet though.

Yes Please. I refuse to take my car to the dealership and pay out the nose. So It would be very much appreciated to have part numbers and whatever else you can throw my way.
Oh , I was wrong about a hose by the manifold missing. I think the only thing missing here is my sanity in this matter. LOL

Aren't Bosch Super Plus resistor plugs?

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No, there are 2 variations of Super Plus. One is copper core w/o resistor and the other has yttrium.

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Allen Kroliczek
Oak Grove Autosport | Oak Grove Autosport
01 G500, 82 300TD, quite a few more.....

I will try the copper w/o the resistor then. I thank you for your advice. At this point I'm willing to try voodoo to get my car back on the road. The kicker is that my car went down the same day I got my plates. It was a kinda Charlie Brown sinking feeling with that squiggly line above his head.
My 300Ce is also a Sportline.

Ok. Good thing I kept the box of plugs. I used the Bosch Super Plus. It boasts a premium copper core PLUS Yttrium. Or are they the ones with the resistor?
So am I ok with these plugs gapped at .028?

I talked to a Mercedes guy here in vegas and he tells me that I may have wiring harness problems. He says its very common with my year .
My new question is will a wiring harness from a 300 SE work on my car. It is the same engine. I found one in a wrecked car.

ok. it's not the same engine. He has a 1989 300SE and a 3.0 engine. I don't think it is interchangeable. Is it?

Did you swap the correct plugs yet? In this economy I wouldn't give him a reason to charge you that money to do the wiring harness when it could be a $20 fix with the correct spark plugs gapped to the correct mm. You haven't covered the very basics yet. Start small and work your way up unless its blatantly obvious.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k