88' 300E runs rough at low speed all of a sudden
 

Hello all, hopefully you can help me solve this issue in short order as I'm trying to sell this car ASAP. Here's the story: car was running fine save for a dying battery. Alternator tested fine so I got a good strong used MB battery from Johnhef, installed with no issues, after a short fairly high speed highway run from the junkyard to his house later in the day the car started running poorly at lower speeds (especially idle "put put put put") still runs fairly smooth at highway speeds and such. I replaced the cap and rotor to no avail and the plugs and wires are both fairly new and in good condition. I suspect a possible 02 sensor problem but I'm wondering if this could cause this sort of problem. it was replaced about 2 years shortly after I bought the car and I thought it should last longer than that. I did unplug it while the car was running to see if it ran worse and I didn't detect any changes besides the CEL coming on (how do I clear this by the way).

It sounds like the OVP...
 

Hopefully, it is just the fuse. Look behind the battery compartment or do a search uncer OVP. Fairly commmon problem. specially, if you have been playing with the electrical system.

I will check that again when I get home from work, I'm almost certain the fuse is ok since I checked it recently (before this problem started though) and the only time any warning lights come on is if I hit a HARD bump.

Check OVP socket
 

My '86 300e had some idle problems and the antilock (ABS) warning light would come on, especially on a rough set of bumps. The problem was poor or loose contacts where the OVP plugs in. The OVP was free to loosen in the socket from road and/or engine vibration.

I understand that later models had some sort of a hold-down to prevent the OVP from shaking loose but my '86 did not have this. I don't know what your '88 has. I cleaned the OVP contacts with OxyKlean and then secured the OVP tightly to its socket with a zip tie. The problem has not returned for several months now so I think it is fixed. :D

Can it be that your cars engine management computer has failed and you are running it in "Limp home" mode (mechanical backup). Sounds exactly like what happend to a friends 300E some years back. He drove it for months with ok performance at speed but crappy at idle and low speeds until someone figured out what it was.

- Peter.

It is possible, I unplugged the unit and it had no effect on the idle. Also my car is supposed to have a hold down I believe but the current OVP doesn't have the mount.

OVP has two fuses, both are fine, connectors look good unit looks fairly new. I'll have to get another OVP and an ECU if possible to try it out.