My 1991 300SE has 125,000 miles on it, and uses about 2 quarts of oil
per 1,000 miles. When I get stuck in traffic and have extended idle
time (10 minutes will do it), the engine develops a hard miss that
will not go away, even at highway speeds.

When the car sits overnight, it is usually fine the next day. Letting
it sit for 3 or 4 hours doesn't help.

I supposedly had the valve guides replaced at 90,000, but considering
the current oil consumption, it may have been done on paper only. Can
worn valve guides cause this type of problem? Any other guesses?

Thanks,

Jim in Norfolk

Two quarts of oil per 1K miles on an engine that's gone only 125K miles
is awful! There's something wrong, probably a large leak.

The symptom of valve guide seal wear is a puff of blue smoke after a few
minutes of idling.

I believe your engine has a heat related electrical failure. Something
related to one cylinder - a spark plug, an ignition wire, distributor
cap, possibly a fuel injector.

Hope this helps you solve it and get that oil use fixed.

Check the spark plugs to see which one fouled out.