420SEL (W126) hesitating, stumbling - help! - PeachParts Mercedes-Benz Forum

dieseldude · #1 07-24-2003, 06:05 PM

Hey guys, trying to help my Mom diagnose the issues with her new (to her) car. Here's her description of what's going on:
"It hesitates or jerks when I try to accelerate - like the power momentarily cuts out. Pretty random except on freeway driving-it doesn't do it at all (IF I haven't had to stop several times first). The slowing down and stopping a lot really seems to kick it in.

Idling is very rough - sometimes the whole car vibrates-you can see it shaking (if I idle very long it ALWAYS starts acting up). It does it especially when I accelerate - a whole bunch going up a hill."

I've been in it when it shakes at idle, but it always went away shortly thereafter - never been there for the worse performance described above. The car is an '89 with only has 45k mi on it (gorgeous!), so I started the 60k tune-up regimen early thinking perhaps under-use was the culprit. It now commutes at least 40 miles a day, so infrequent driving is not the issue.

Fuel filter's been replaced as have the plugs, cap, and rotor. Fuel additives seem to help for a short while, as have all the items mentioned above. I replaced the Cap and Rotor last week - car was totally fine until just a day or two ago....any ideas? hmmm reading the symptons again almost sounds like some debris occasionally clogging the fuel tank screen or something, but since that's normally a diesel-trait from algae, etc, I hadn't considered that...

Regards,
- Ryan

mhingram · #2 07-25-2003, 12:42 AM

ryan, this can be a big pain in the rear. WHen driving at speed is the car good? If you richen it up does it get better. Pull all your plugs and have a look see what they tell you. I had similar problems when i cleaned the engine off, I must have damaged my coil cause replaceing the coil fixed my problem but i spent the best part of 3 months messing around before I tried the coil. Rough idles on these engines are very tough. good luck
m

dieseldude · #3 07-25-2003, 05:59 PM

Thanks Martin, though not the answer I was hoping for!

unfortuantely I didn't change the plugs so can't attest to their condition. Also, how/where does one adjust the mixture on this M117 (is that the egine number for an '89 420SEL's powerplant?).

Anyone else have other ideas, or encountered other scenario's?

Thanks much!
- Ryan

mhingram · #4 07-25-2003, 09:24 PM

dieseldude,
To adjust the mixture a 3mm allen is needed in the hole in the center of the air filter. But trust me this is not the best way to approach the problem as you need to do any adjustment with an understanding of duty cycle and eha readings. The problems you have described are not indicative of a fuel problem yet. I would still pull your plugs whether you put them in or not they will tell a story. I believe you may actually have an ignition problem rather than a fuel problem. If it is only under load from a complete stop that you see it and the idle is rough like it has a dead miss, I am very suspisious of your ignition. First thing to do is to get your fuel mixture tested. You can do this by checking your duty cycle and eha readings. If all is well then get the emission gases checked if they are good then I would recommend checking your ignition next. None of this should cost a fortune if you can find a shop that does diagnostics rather than fixing by replacement.
m