Vibrating 1991 300E
 

Hey all;

I've got a 1991 300E with 192K on it. It runs wonderfully and I truely love it. One problem. It vibrates while stopped in gear... worse with the AC on. According to the tach, it's running at about 700rpm in gear with the AC on. If I put it in neutral it smooths right out, even with the AC on and it seems like the RPMs increase to approx. 800-900.
Any ideas? I once read that if the passenger seat vibrates while stopped and in gear, it's the motor mounts. Any truth to that?

Nick

Well,

There is a proceedure on in the FSM to check the mounts. Yes, it is the first place to check and it is quite possibly the cause.

FYI- replace a 3 if one is bad and for motor mounts I ussually only use factory parts. Not a big difference in price and they last. Also, it's pretty easy job to do.


M

i recently changed the passenger side on a 300ce...but the drivers side seems difficult to get to..the top bolt i mean..

i know i was able to do this job a few years ago with my existing tools...but i am wondering if you need a stubby ratchet to get to that top bolt to make it easier..

any thoughts?

Thanks for the responses and I'm sure I need mounts but would bad mounts cause the fluctuation in RPMs? The engine feels like it'll stall.

I have a 91 300E as well. 186K. Your Idle while in Neutral or park should be about 750 RPM. When in drive with AC on, about 550-600. If it feels like it is going to stall, could be ignition system. I replaced O2 Sensor...improved idle. Replaced coil, and rotor cap. Not much improvement. Replaced water temp sensor and improved a little more. Now, I have been having cold start issues lately where it seems like it is flooded. I have read that it could be the EHA Valve that pressurizes and warms the gas before going to engine. It starts up fine when warm outside or engine is still warm. However, I'm going to put new plug wires on first (76.00 from *********.com) because I heard and noticed "ticking" noises from the engine with hood open and this will cause lack of power when under load and idle problems while stopped at light. Plug wires are usually the last things people change. If I were you, I would replce the O2 Sensor first....should notice a big improvement because it sends the adjustment signal for air/fuel to computer based on exhaust output. Let me know if you need help. I bought the 95 Ford Mustang O2 Sensor (Bosch)and just spliced the wires in. Saved me $150.00. Same sensor as Mercedes Bosh replacment without the wire harness connector. Last thing after all that is adjusting the lambda tower (little hole on top of the Air Filter housing) but this could get you into a no start situation and then you're screwed. Takes tiny adjustment turns with 3mm allen wrench.

Steve

Thanks Steve

Does anyone know where the O2 sensor on a 1991 300E is?
I've read thay they are pretty easy to replace although Nobody shows where it is.

Nick

essentially underneath the passenger footwell. you have to spray wd 40 on it and let it sit overnight..... comes out with a wrench.... it plugs in under the passenger inside footwell..remove carpet and you should see it

Nick, typically there is a heat shield on the Oxygen sensor to protect the electrical connection from exhaust heat. You will need to remove it before unscrewing the Oxygen sensor.

Most auto parts stores sell the crow's foot tool for removing Oxygen sensors. The tool has a slot in it so it slips over the connecting wire. There should be a coat of Never-Seize on the new Oxygen sensor threads. If not, apply some before installing it.

Nick,
Just take the carpet cover off from the passenger side and follow the wires through the plug hole. Jack up that side of the car. I sprayed some penetrating oil and used a wrench underneath to coax it off (you do this from under the car). Wasn't very hard. Disconnect the wire loom from the housing so it can spin with the sensor and not tighten up as you wind it. Then, feed the new one in, splice the wires with the harness if you did it my cheap way (after you screw it in to the exhaust). Make sure to put the black plug ring around the wires before you splice so you can plug the hole back up where the wires go through the body. Fire it up and drive it for a awhile so the computer can get use to the new signals. You should notice a positive difference.

By the way, my recent problem seems to have been corrected by adjusting my Lambda Tower Screw. Going clockwise richens the air mixture and will cause gas and acceleration issues but might idle great! Anti-Clockwise will cause idle problems with eventual shutdown. if this happens turn screw back a quarter turn at a time and try to restsart. I had to make 1/6 Turns close the hood, drive the car, repeat, until I got it to accelerate with no hesitation. By the way, leave the air housing all intact while doing this. The idle went back to being just not so perfect but I think a defective EHA valve could be causing this this??? I put new plugs, new cap, rotor, and coil in. Any takers?

Wow
Thanks for all the great info!

Nick