The idle on my '87 420SEL is rough, but it smooths right out around 1,000 RPM and drives fine at speed. The engine seems to have enough power, but there is a slight hesitation from throttle tip-in to part throttle, and after that it starts to run strong.

I've replaced all the obvious stuff: cap, rotor, wires and plugs.

What else would you check if you were me?

Thanks a lot!

Is the car cold or already warmed up when this happens?

__________________
"It's normal for these things to empty your wallet and break your heart in the process."
2012 SLK 350
1987 420 SEL

Right, I forgot to mention that it does this both warm and cold.

I'll bet this problem has been covered under another thread on this board, but if so, I can't seem to find it. If such is the case, I would appreciate it very much if someone would refer me to it. Thank you.

Under the search type in "420 SEL rough idle", there are so many threads it looks like a sequel to War and Peace....good luck.

__________________
"It's normal for these things to empty your wallet and break your heart in the process."
2012 SLK 350
1987 420 SEL

Took it in for a smog pre-test yesterday and it failed. The smog technician pointed out to me that the miss doesn't really go away at higher RPMs; it's just not obvious when the engine spins up. He said to get it fixed before I foul the catalytic converter. So I am taking it in to Ernie in Orangevale in a couple of days.

Thanks for trying to help me, Mistress. I'll let you know what is causing the miss on this thread in a few days.

retro

A compression test will reveal if it's a valve related issue, a smoke test will reveal if it's a vacuum related issue, let us know what you find..

Hi, User. Well, I'm happy to say that the compression was excellent and the fuel pumps were delivering plenty of pressure. Ernie found that I had air leaks from the "rubber log" thingies that run alongside the fuel rails on both sides of the engine (I forget what those thingies are called). Replacing those cured most of the miss, but it still had a partial miss on #'s 5 and 8 that was caused by a defective fuel distributor.

Mercedes wants $2,400 list for a new fuel distributor. Rebuilt ones seem to sell for about $700 or so. However, ********Az has a CIS FlowTech rebuilt for $480, after a $300 core refund. I guess I'll see if Ernie thinks the AutohauaAz part is OK.

(Unfortunately, Fastlane offers only a new Bosch fuel distributor at $2,354.00. Although I'd like to patronize one of the site's sponsors, I can't afford that.)

You can also try e-bay for a used fuel distributor , I've seen them sell for about $100