PeachParts Mercedes-Benz Forum - View Single Post - 92 300E idle surge

DAVID LEE · #2 06-29-2009, 09:14 AM

I spent quite a few hours this weekend thinking through this problem and doing some measurements with my DVM. The DVM has a frequency meter, but the measurements I've taken at the X11 connector are confusing me. At pins 2 & 3, the frequency measurement has a value of either 81.3, or 0 depending on how I adjust the idle mixture - very strange. I would think it should change fairly linearly as I adjust from lean to rich. This leads me to believe that possible the O2 sensor just isn't responding quickly enough. When I took VDC measurements at the O2 sensor, I am not getting a reading that is oscillating between .1 and .9 volts, but I'm not sure that my meter is capable of responding quickly enough. The "typical" reading on the VDC scale is about .75 volts, and when I disconnect a vacuum hose on the intake manifold and drive the misture lean, the reading drops down to around .15 volts, so I know the O2 sensor is responding, but perhaps it is responding too slowly.

What really has me baffled though is that when I disconnect the EHA connection and take the computer completely out of the loop as far as fuel/air mixture is concerned (please correct me if I'm wrong here), I still am getting the "surge". This morning I connected everything back up and drove to work (about 9 miles). There are several stops along the way, and I noticed that the closer I got to work, the fewer "surges" I encountered at stops.

Oh yes, I also did the "carburator & choke cleaner spray test" and could not find any vacuum leaks.

One thing I read in the support manuals was that there is an input to the closed loop system from a switch that tells the computer that the transmission has been placed in gear. Since my problem only occurs when the car is in gear could it be???? I do not know where this switch is, but more than likely it is located just below the shift lever cover. But once again, if the EHA is connector is disconnected then how can the fuel mixture be modified by the computer? There is some operating theory here I am missing.

My "logic" tells me that when I am putting the car in gear, that I am altering the fuel / air mixture to the point that "something" is trying to compensate. Putting the car in gear would also alter the vacuum as well, so....

Once more, I'm hoping someone out there has some ideas as to what is going on, or at least some tests that I can make.