Greetings folks.
Just picked up a Benz, just joined the list. Posted this to the 124 list, and have had no luck so far. (Hope I'm not voilating the no cross posts edict.)
As is, the car is almost undriveable.
I was a Benz mechanic through the ‘70s and early ‘80s, specializing in vintage restorations, 50’s through 70’s. Have kept semi-up to date on K and L Jetronics through the late 80s’ on other unmentionable Euro stuff.

Patient: 1990 300E, 118K, seemingly well maintained.
Recent repairs by P.O. New Oxygen sensor (2-wire), and new mixture adjustment tower on top of the fuel distributor. All to no avail, as far as I can tell.
Symptom(s):
1. Rough Idle
2. Mid-range hesitation/ Dead Zone, for about 1” of accelerator play, then the transmission kicks down, and it takes off like a scalded dog. Otherwise, going up a hill is a trial, the car slowing, and slowing.

I found that backing off the gas entirely, and pressing down again seems to reset the condition, and the car accelerates normally through the range that had just bogged down a minute ago, then hits a flat spot within a minute or two.
I also noticed that, in some cases, when I backed off the gas slightly, the engine picked up Rs, and the car speeded up, kinda like it was over-carburated, overjetted, and running too rich, and by backing off, it would lean out, and run faster.

3. Idle speed does not respond to pulling the dip-stick (Should it?)

Diagnostics to date: Possible vacuum leak. By isolating circuits, one at a time, and testing with a Mity-Vac, everything seems to be responding appropriately (As far as I can tell without any manuals, schematics, resources, etc. Isolating and blocking vacuum ports one at a time didn’t help.
With engine idling, I shot all of the injectors/seals w/WD-40 to check for vacuum leaks, as well as all of the usual suspect spots.. no avail..
Actuation of EGR valve makes idle rougher.
Pulling the plug on the Idle Stabilization valve caused the idle to drop to 450 or so, and after reconnecting, the car had to be shut down and restarted to reset the ISV.
Wires, Cap and Rotor recently replaced, and copper core single electrode Bosch plugs.
Any ideas would be appreciated, as would any suggested resources for technical info, manuals, etc.
Thanks in advance for your help.
I'm new to this list, so if my post is too long, or otherwise ill advised, I trust you'll let me know.
Thanks,
Mark

 

Well, sounds like its a fuel or ignition problem, most likely ignition. Why not with your backround try asking some mechs that you know what there feelings are?? Try also replacing some sensors like the MAF. Sounds like the previous owner knew of the problem and dumped the car on you.

 

I took it to a reputable shop in York, PA for a state inspection (currently failed on a hole in the rear muffler), and they wanted $300+ for the rear muffler + hangers installed, and told me to "budget $300 to start to diagnose the issue".
In the meantime, I've only had access to a few hours to start the diagnostics on my own. Still looking for recommendations on manuals, cds, etc.
The fact that letting up on the gas sometimes makes it surge, makes me think that its fuel / vacuum related.
I hope to get some quality time into it this weekend.
Thanks for the comments.
Mark
Oh yeah.. I purchased it from a family member.
I asked for first dibs years ago, then finally got it.
Just goes to show "be careful what you wish for"...

 

Pull the cover off the distributor cap and check the coil wire where it clips on to the case. Mine was burned at that point causing running problems. I used some MB - Loktite 5900 sealant on the wire. I need to replace it.

 

Beneath the air filter and under the linkages there is a throttle postion sensor. I'm having some strange throttle fluctuations as well, and I'm wondering if it could be this sensor. I'll post if I replace it and it cured the problem.

 

So... Several hundred dollars later.
The (new) O2 sensor checks out, no vacuum leaks, per the 'smoke test'.
Everything checks out OK so far, but it had codes for EGR and O2 sensor.
I pulled the EGR tonight, and the diaphragm is leaking, but the valve itself is tight and not leaking, i.e. not contributing to an internal vacuum leak.
The Benz shop has exhausted the usual suspects, and is looking for a bag 'o dollars to start the next (as yet undefined) stage in diagnostics.
Has anyone had trouble with the throttle position sensor, or whatever its called, under the throttle linkage, and does anyone have a diagnostic routine for it?
Best regards,
Mark

 

Does this model use the "flap" style airflow valve?

If the contacts are oxidized or worn out, this would cause the flat spot or sag. That style of air flow meter operated like a potentiometer and the varying resistance would tell the ECM how much throttle / airflow was taking place.

The TPS switches usually kick in enrichment when throttle is 70% or beyond and some systems also use this to tell when it closed for idle / decel conditions.

 

Thanks for the input. I plan on checking it tomorrow, if its not raining (too hard).
Mark