Hesitation when accelerating.
 

So, I have a frustrating issue. After I have driven my '91 190E 2.6 for about half an hour or more, it hesitates pretty bad (sometimes) while accelerating from lower RPMs. I figured it was a crap fuel filter, replaced that, added some BG 44k fuel system cleaner, checked for vacuum leaks, etc. Still does it. It has great power most of the time, and the fuel filter seem to improve that. One of my technicians (I'm an Audi service advisor) suggested a new O2 sensor.
Note: I have a new set of spark plugs that are not installed yet...

Thoughts, people?

I would do the plugs, but they are probably not the issue. A hesitation like you describe can often be caused by the fuel mixture being off. I forget if too lean or too rich causes it. I had a car I was tuning, it was so bad I could stall it if I went WOT with the peddle too quick.

If you think the O2 is due, it would be a great place to start. You can usually get one for about $30 now days. A universal type.

I have a 1990 300SE with the M103 engine. I had hesitation and sputtering upon hard acceleration. I isolated the problem to the ignition coil. Once that was changed the problem was gone. You can read that thread by searching threads that I started.

With the air filter housing in place, put your multi-meter leads in the 2 and 3 pin of the X11 diagnostic socket and see if it is fluctuating rapidly. If so that is a decent indication that O2 sensor is ok.

You can also grab a signal directly from the O2 sensor connection if you really want to make sure. It should bounce from .2v to .9v and average .45v and you can check to see what happens when you lean the mixture by pulling a vacuum line or richen the mixture by pinching the fuel return line too.

If you have the duty cycle setting for % then is should be close to 50. If you can dial that in to average a 50% duty cycle and it fluctuates between like 45% and 55% then it would seem the computer is making good corrections and it needs a good O2/lambda sensor reading to do that.

The fuel mixture can be adjusted with help from this link:

BOSCH KE3-JETRONIC MIXTURE ADJUSTMENT

None of this works unless you have good components for ignition system like the coil, distributor cap, rotor, wires, plugs, sensors (temp and O2). But it should be responsive when the components are in good order and the fuel mixture is adjusted.

UberFisch,

This sounds like classic O2 sensor. My car (300E w/m103) did the same thing earlier this year. I'll bet your gas mileage has dropped, too. Do the oxygen sensor (Cheap and easy) and the plugs and you'll be amazed at the results. I'd go with a bosch original type sensor. They can be found for as low as $75.

I vote for distributor cap and rotor. If they haven't been done for a while start here and work back. At least have a look inside the cap.

Sounds like an ignition thing. A loose plug wire will act like that as well as a bad cap.

I had a hesitation issue on my 190. Only happens once when I start up and it's cold and then typically only ion summer when I have the A/C on. If I go WOT it will hesitate for a few seconds. If I take my foot of the gas, it will stall out and then restart with out issue. The hesitation never happens again for the day.

I had the MAF replaced as it was causing other issues. The hesitation issue went away. It's back now every now and then but not as regular as it was.

Cap and rotor replaced this week, as well as the Spark plug and alternator. The cap was FUBAR, so that seemed obvious. We shall see how it goes! Thanks!