198 420 SEL, 127k miles, second owner, dealer serviced up to 123k miles.
Car had a misfire when we bought it - 4 new plug wire connectors fixed that. Transmission was a bit clunky then, but not so bad that it made my wife suspicious of the car (she's had Merc's since driving age).
Later on 'check engine' light comes on, turns out to be code 12 (EGR temp sensorfault ?). Re-set CIS box and no more engine check indications since then (thanks, AJD).
Now, engine runs rough at idle - shorting No. 4 out makes no difference to idle. Shorting any of other's out produces markedly rougher idle. Plugs on Nos. 5 - 8 all look like they should, as do 2 & 3 but 1 looks a shade darker and 4 darker still. Wires and plugs all ok (as in new). Distributor cap has large crack running circumferentially and down to lower edge of cap from just under No. 6's contact post (about 1 1/2" long). It still accelerates well and runs on the road ok though. Ideas anyone ?

Transmission - clunks and jerks through some up-changes like a big clunky jerky thing clunking and jerking, and drops out of drive with a vengenance when coming to a stop. As in Not Smooth. Checking the fluid as per handbook - hot (80C) engine at idle - level appears to be well over top line, which I find odd for a car serviced at a local & reputable dealer. Comments anyone ? Do I need fear the Large Bill For Transmission Repairs Nightmare ........... ?

I would change the Distributor Cap for sure. Ohm out the Spark wires while bending and pulling.

What is your engine's idle speed?

__________________
1983 500SL Euro

The clunking and jerking could be engine relayed, the engine miss, mounts. . Too much oil in the transmission causes shifting problems. The high level will get into the gear train, the drums and planetary will aerate the oil, causing oil pump cavitations. Not good for a hydraulic system. Neutralizing at a stop could be a plugged Tran’s filter or a broken, leaking B2 piston.

Fix the engine problem first, then do a transmission service. Removing the pan will show the condition of the transmission.

Wires are new - same cylinder is off with new wires as was with original's (new ones are Bosch). Idle is around 800rpm in N, 600rpm in D.

Check the vacuum line to the transmission vacuum modulator. The 420s seem to have them fall off quite regularly. It makes one of the back cylinders misfire at idle and makes harsh no vacuum shifts all the time.

The line can be seen (somewhat), by looking straight down the middle of the rear of the motor. The line come from the rear of the lower intake plenum. If one follows the big brake booster line down behind the motor it ends within a inch or two from the tube the mod hose hooks to. Unfortunately the actual tube is under something (maybe the booster banjo) and its hard to see. Look for the hard vacuum line that goes to the trans and move it to see if it is attached.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Steve - huge round of thanks there to you. I looked down the back, and there it was - off. One new piece of connector hose later - plus several minutes of fiddling, removing the banjo to give me just that little bit more room, reassemblng and all's perfect.
I guess from the state of the pipe I removed that there's so much heat from the block and the exhaust pipe that runs by, plus maybe oil / fuel mist in the hose (?) it just rots and falls away.

Note to MB - fit pipe better suited to hostile environments.

I doubt I'd have ever got to finding this without this forum and members such as yourself - Thanks very much.

You may find that MB did that precaution. The original junction rubber hose is a special silicon hose I think. It is sitting just inches from the exhaust crossover pipe and it did go 15 years.

Standard rubber hosing might not last long but I have used it often.

Took me a while to find this misfire the first time but the clincher is the way the trans operates.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Just checked out the '84 380SE we have - it's got a METAL pipe from the banjo all the way to the Transmission. Call it cost saving, design for manufacture or whatever, it's cutting corners to me.
The hose I took off appears to be 'standard' rubber. It's completely fine at the end which slipped over the nylon pipe, but quite rotted away at the end which slipped over the banjo fitting - and I mean just the exact length that was in contact with the banjo. Engine / intake manifold heat maybe - or the banjo's plating / finish & rubber's chemical interaction ?