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.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

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

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-Marty

1986 300E 220,000 miles+ transmission impossible
(Now waiting under a bridge in order to become one)

Reading your M103 duty cycle:
http://www.peachparts.com/shopforum/831799-post13.html
http://www.peachparts.com/shopforum/831807-post14.html

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...
Spark Plug Horror! Look!

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2009 Mercedes ML320 Bluetec

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.

__________________
-Marty

1986 300E 220,000 miles+ transmission impossible
(Now waiting under a bridge in order to become one)

Reading your M103 duty cycle:
http://www.peachparts.com/shopforum/831799-post13.html
http://www.peachparts.com/shopforum/831807-post14.html

Talk about dragging up ancient threads!

Just kidding. The good news is that someone is searching before starting a new thread. A good thing.

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

Annoy a Liberal, Read the Constitution

CJ have you checked the output from alternator?..I was reading ablout the same problem on the other forum.It may need a new alternator regulator. Check all earth points for bad contact clean and grease them up .Fuses as well at the same time , give them a clean.