300E misfire when hot.
 

1990 300E, 148K. Starts and runs great when cold. Runs great on highway. When running around town shows no symptoms. Only after a highway run (I commute 40 miles) it idles rough and takes some babying to accelerate from a stop but smooths out at higher rpm. Attempting to accelerate harder makes it worse. By rough idle I don't mean continuous, rather it misses hard and often but not continuously and not at any particular interval. (Make sense?) After driving around town a while (and not shutting down or cooling off) the miss improves to an almost undiscernable level. Even after a very short rest the symptoms go away and don't reappear until a highway run. The tuneup items are relatively current (the cap, rotor, wires and plugs have 10K on them and all items are Bosch oem spec) and the car performs very well in all areas. This just started last week. No weather changes to speak of and no driving style change. I ran a (strong) tank of Chevron with Techron through with no change. I'm going to start with the plugs but would love to hear input from the forum.

Do a search - check the coolant temp sensor.

I'll throw in my usual "try to pull the O2 sensor while while running rough" to see if it cures...

Thanks Mercoleza. Searched coolant temp sensor but found nothing related to this. If the CTS was faulty, wouldn't the symptoms also show up after extended in-town driving with engine at operating temp? It only happens after a highway run and doesn't seem to be linked to temp reading. Does disconnecting the CTS default to hot or cold perceived temp?

Is it using any oil?
Look at the plugs when you pull them.... look what we found with a similar hot only misfire...
http://www.peachparts.com/shopforum/tech-help/168126-spark-plug-horror-look.html

Merkey your plugs look hideous! I remember seeing that thread before.
No oil consumption to speak of. 1 qt in 4000 miles maybe. The plugs looked great and the new ones I put in last night didn't help a bit. Also I have failed to mention that the fuel economy is unchanged and I haven't had a check engine light. And when driving down the road, one would never detect the problem - the car runs great. Don't know if that helps. When I look into the O2 sensor, do I just unplug it below the passenger floor when the problem is apparent and if it improves the sensor is bad?
Thanks to all.

Yes. :)

Thanks Marty. I'll check it out as soon as practical. Right now in lovely NW Oklahoma the wind chill is 15 deg F and we are expecting anywhere from 7-11 inches of snow tonight. Hopefully there'll be some freezing rain thrown in for good measure. So, not a good environment to work on a car. Just for informational purposes the symptoms seem to be getting slightly worse. Does that tell anyone anything?
Chuck.

Thats the problem with intermitent stuff, it's never around to work on.
 

Look at the worsening as a good thing that will make the problem easier to find ;)

The O2 sensor signal, if it's out of range, can either over enrichen and cause bad gas mileage and bad emmissions, or over lean and cause stuttering and stalling. This would only happen if the coolant temp was up over 60+some C.
If you dont have a passenger often... or even if you do ...you can pull out the sensor wire from under the passenger seat carpet for easy access when the symptoms manifest themselves. Or just pull it now and see if they don't come back. Hopefully it's something that easy.

Well, I couldn't stand the suspense any more so I put on the coveralls and messed with it last night. The O2 sensor had absolutely no effect on the problem. I plugged and unplugged the wire several times with no change in the rough idle. I then decided to attempt a seat-of-the-pants duty cycle adjustment thinking that I could always reverse anything I did in that area. It took a bit to get a feel for the adjustment screw (I had a thread from this forum to reference) and I actually killed the engine a couple of times before getting the screw to index properly. About a quarter turn clockwise (richer?) smoothed it out dramatically. A couple of questions: Was this "the" problem or did my adjustment possibly mask the true problem? Is the screw typically difficult to turn? I haven't driven the car any length since the move but preliminary indications are positive. When I have more time I may get a meter so as to set the duty cycle properly. In the meantime I'll drive and enjoy and wait for the next crisis. Sure like the car but wish it gave me less trouble.
Thanks to all, especially Marty and have a great day.
Chuck.

I think the hex screw is spring loaded so it needs pushed down to engage.

It's real hard to guess without a duty cycle reading... once you get a meter you can use the links in my sig. to see the factory instructions for reading at X11 (the diagnostic port)... actually even with a reading it's no sure thing but it will tell you what the lambda system is doing. Could be a failing EHA (back of the fuel distributor) of a vacuum leak getting worse somewhere, a faulty fuel pump relay... ...

Just another idea - you might want to crack open the EHA and turn the adjustment screw clockwise about 1/2 turn to see if things improve. This enriches the low-rpm mixture a minute amount and can help offset accumulated wear and/or vacuum leaks in an older engine.

Sounds great, thanks! I recall reading a thread along those lines with regard to the EHA and normal wear and age. I actually thought that's what I was doing with the adjustment screw through the breather. Guess my memory left out some details from the other thread. I'll give that a shot if my next long drive winds up with a misfiring engine at the end of it. So far only short drives with 20 deg weather so the results from last nights efforts are inconclusive. Did I do any harm with my adjustment?
Thanks again to all.

Chuck.

If you adjusted the duty cycle, it's a crap shoot as to whether or not any damage was done. You have no reference point in terms of what the duty cycle reading is/was both before and after your adjustment. Not a very scientific way to do things, really, but what's done is done. I would get yourself a duty cycle-capable VOM from Sears. I picked mine up on sale over the summer for half price, something like $30. You will pay many times that if you pay someone to diagnose, so I think it's a good deal.

Click HERE and check out post #50. Might be helpful...