Greetings folks.
Just picked up a Benz, just joined the list. 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.
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.
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

 

A search for rough idle or whatever the problem as there will be plenty of posts to this regard. My 91 300e is heading in for a checkup tomorrow evening for a hard start issue. I just got mine Saturday w/ 110.000 miles and maintenance records a mile long. Nice cars the 124 are! I dont need to tell you per your intro though. Good luck w/ the new benz! tg

 

Had a trans problem also. Don't know if this will help. Trans felt like it had a serious gap between 1st and 2nd gear. It would "slam" into 2nd after much hesitation (at slow speeds). When i "stepped on it", I wouldn't feel the hesitation. Tech replaced trans pan gasket and electrical plug connector o-rings. Haven't had the problem since. have parts #'s if you like. lol. I also have rough idle. they replaced the crank vibration damper, alligned the wheels, replaced the rotors and the entire steering column (all for other issues) this did not help. Please post a reply if you find a solution to the rough idle.

 

There are folks on this board who sell factory repair manulas on CD's for $10. That is a good start, but they are very different and in my opinion, not as useful as a Haynes manual. If you used to work on benz's then you are probably more comfortable with the factory repair manual format. The best overall book I have found is Stu Ritter's W124 Owners Bible. This is a great mix of technical and historical info, as well as trouble shooting, preventative maint. and problem areas. If I had to buy one book it would be that one.

 

i have the same problem and may I schedule an appointment with anyone of you please ... how much would it be ? and I stay in Los Angeles where are you ?

 

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

 

Is not you throttle position sensor and 2 wire O2 Sensor??? I used a 3 wire one from a 1990 mustang 5.0 and my car passed the CA smog check! Anyways check that your E.H.A valve is in working order and sometimes most people don't look at their temp switch can not be working and telling your computer that your car is cold and making it run rich. If you feel like the transmission is not shifting properly adjust the cable but it may take a couple of days and tries to make it shift smooth.