I have a 1986 300TE a few months ago I had a problem with the engine coughing & spluttering when driving but would do for for maybe a week then nothing for 4 weeks then for a day might only be for 5 Min's then would clear very random,so took plugs 1&2 out and they were wet with fuel,so changed plugs,cap,rotor arm and leads no joy,then it started stalling when you stopped after 2 or 3 Min's of cranking it would start but really stunk of fuel, then one day went out to start it after 10 Min's of turning over it would just catch but under throttle would just backfire in the throttle body but would not idle.
So after a compression test 1&2 was right down so thought inlet valve problem So took head off and they didn't look to clever so rebuilt the head put it back on compression back to 14 deep joy ,short lived tuned it over and still the same,changed ovp reg,inlet manifold,crank sensor and checked a load of other things but no joy,but did find out that if i placed a piece of card in the neck of the throttle body and just open the top flap by a hair it will idle and if I timed the opening the flap whilst operating the throttle by hand just right it will rev freely but that's the only way to get it to run,changed injection dist body but no joy again,so to say I'm pis**d off with it is an under statement,and haven't touched the car for a long time so if anyone has any ideas I'm listening HELP!!!!!

Have you checked the valve stem seals? Very cheap to fix ($30 iirc), just time consuming. They will go rock hard after time, not miles and leak profusely causing plugs to tarnish.

Just did it on my 124 88 300TE.

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Can you check system pressure and control pressure? What are you getting?

With the fuel pump relay jumpered so the pump runs constantly, does the sensor plate moving down easily when pressed, and come back up smoothly? It almost sounds like the fuel distributor is screwed up and the piston is sticking.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe