93 300e M104 Start/Idle Problem Details
 

So...its been about 3 weeks since I put in the new plugs and some system cleanser and I decided I'd take them out to see if I could get help diagnosing the problem.

All 6 plugs are black and sooty on the spark ends. This time, there was no oil on them. Would someone have fixed a valve leak etc and just put the same older plugs back in?

The ignition wires that are tucked in behind the head and connect to the coils and the other 3 plugs were heavily taped and the insulation is splitting on a few of the wires. Under the black plate that covers the injection electronics I found very poor wiring conditions with insulation missing on almost every wire for at least a few inches here and there.

With the plugs in this condition...I was guessing there's a fuel problem...injector related...I could be wrong. I'm begining to wonder if its the wiring issues that are causing my problems. The car starts and runs fantastically when cold (sitting over night or allowed to cool in the shade for a few hours). It has great off the line acceleration and a normal idle.

Once the vehicle reaches operating temp (80 and above) or has been sitting in the sun for sometime...it becomes very difficult to start and requires some throttle to keep it going at first. The idle then immediately drops to just over 500 and it surges no matter what position the shifter is in. It's not a terrible surge (only 100 or so RPM) but when it surges a harsh vibration is sent through the car. Once beyond 1500 the car runs pretty well, with good responsiveness. I believe all of this could also be causing my transmission flare on 2-3 3-4 shifts. They tend to become harder the heavier I am on the throttle.

Would a new main harness replace these worn sections, or do I need to purchase a couple of individual harnesses for the ignition and injection systems?

Sorry for the length and possible redundancy, some new info was brought to my attention and I believe it may not be the O2 sensor and such. All help is appreciated. Thanks

PS: Sorry if the pics are bad-they're from my phone since the Mrs has the camera right now.

At minimum, you need to replace your engine wiring harness, coil wires, coil supressor boots, and spark plugs.

Only use the plain, non-resistor, Bosch copper plugs.

I would do a thorough search on posts by Arthur Dalton. He is surely the #1 authority on diagnosing problems with this engine / chassis.

Only plugs I'll ever use in this thing...that's what I put in there 3 weeks ago...suprisingly reasonable at the dealer...but even they can't gouge you on a spark plug I suppose...

I would definitely replace the engine wiring harness. They were made from a biodegradable material to be more eco-friendly. Although the Germans' hearts were in the right place, the idea was not meant to be. The harness is $$$, and a dealer-only item. Last I heard it was about $650 (at least in CT).

Jer

Yeah...figured the harness would be shot at some point...I knew before purchasing about the harness issue and all the other common probs...but for the price I got the car I don't mind replacing it or repairing it for the time being with some new wiring and a solder gun/heatshrink tubing...we'll see though...and if that doesn't fix it...on to the injectors...Thanks...

After replacing the wiring harness, I think your next step ought to be to check codes out of the DM to determine what is causing codes to pop up.

You might also need a throttle actuator, mass air meter, or might even have a simple vacum leak somewhere causing the idle fluctuation.

An injector problem would be the exception, not the rule.

I figured the idle was only a semi-related problem...figured it was a mixture issue caused by a leak or malfunctioning MAF...I'm trying to go in order of driveability...so I figured if the harness doesn't fix the hard start...it might be a leaky injector or an accumulator problem (I've read about that being a possible cause)...I was going to check them anyway but at a later date...so I'll check them now and I'll busy myself with the MAF and check the O2 sensor while I'm at it...I planned on building the code reader this weekend to check things out...from there I'll tackle the idle...I haven't even gotten to the throttle actuator yet...that's for tomorrow I'll check its wiring and functioning...fun weekend...its not supposed to rain...but we'll see about that...thanks again

hate to say it but you REALLY need a new wiring harness. Although I have no personal experience with the problems caused by a bad wiring harness do a search on this and other MB forums and you'll find that they've been the culprit of many, many different engine related problems. Its obvious from your pictures that the insulation is flaking away in the spot that you can see, so I don't understand why you'd try to fix other things when you have an obvious fault staring you in the face. Start with the harness..then work from there.

just my .02

No worries, I was on the same page as you. I did mention that I would look at other causes "if the harness doesn't fix the hard start"...Have the harness already and was going to do that this weekend and then start reading the codes etc...I definately wouldn't bother trying to read the codes or throw parts at any of the problems since God knows what the harness could be causing. First things first the harness goes on in the morning then we'll see what she has to tell me. When I said repair, that was in reference to the harness. I know a few people that fixed minor insulation problems for a short term stop gap...though when I cut back some of the harness wrapping I knew that was a pointless endeavour thanks to serious degradation. Dealer had one in stock oddly enough. Gotta love those treehuggers from the early 90s. Thanks.

And MB USA claims to have "never heard of this problem" HA!! I'm sure they have to replace these a few times a year with as many 93 - 95 MB's on the road.