Hi guys, this is my first post here in this forum, we have a 1984 w126 280se merc and it has some problems since we bought it back in 2014 but never worked on them as we first wanted to restore the exterior and interior.
So here goes the problem, regardless in which gear the car is, may it be neutral or any gear, the car cuts off power as soon as the rpm hits 3.5k rpm and above, and it starts jerking a LOT (like if some one is pushing gas and letting go again and again), so we visited mechanic and he suggested that we change the injectors asap, so yesterday we bought 6 original bosh k jetronic injectors along with their rubber seals and the mechanic installed them and tuned the car, but to our surprise, nothing changed, same slight missing and same jerking problem, then we removed spark plugs and cleaned them (as they are already new) changed one current wire that was short (the wire that comes from distributor cup to the spark plug), but same story, nothing changed.
Now this is where it gets tricky and the answer may lie, while the mechanic was trying different things to troubleshoot, he once hit the metal wires (the one that distributes fuel from fuel distributor to injectors) and the car started backfiring and literally clouds of purple (or whatever) smoke started coming from the cars exhaust and suddenly the engine started revving to the max rpm with absolute no trouble, test drove the car to 140 kph and it became as good as new and once we parked the car and started it again, exact same problem re appeared, now according to mechanics (there were 2) one is saying that the fuel pump/fuel filter is bad and the other is saying that there is no other problem except dirty fuel lines (or whatever that can be clogged) and we apparently think that the later one is right as there are moments when this jerking problem disappears totally and come back after some time. Now the thing is that we don't know how to proceed as the mechanics here are not so good and the parts are like gold expensive. Any guidance will be appreciated.

 

Haha lol what a turd...

 

Sounds to me like the arm and plate in your air intake manifold is sticking when open past a certain point, either that or incorrect Fuel : Air Mix (&/or timing)

 

Hi, thanks for the response, since my final post here following things are done:
Changed all spark plug wires (7) with new original set.
Changed the fuel pump and filter with a new Bosch one.
Change fuel filter (the one that inputs fuel to the fuel distributor.
Tuned the air/fuel mixture.
Same problem except missing is gone totally, car now knocks a lot, still has the jerking problem but now at ~4200 rpm instead of ~3k.

 

Have you adjusted the timing using a timing light yet?
They cost about $50 for a cheapish one from any auto store & are an invaluable tool in keeping your pre update (pre 1986) Benz running as it should.
The knock will be due to the adjustment in air : fuel ratio, engine "knocks" come from the aspirated fuel in the cylinder igniting too quickly so in effect exploding (at various points) rather than gently igniting starting at the electrode on the spark plug and radiating outward. It might be worth taking it to a mechanic who has the equipment to analyze the exhaust gasses as well as numerous other data points and readjust the air:fuel ratio so it's perfect....between the timing and this, I reckon your problem will disappear!

 

Once again thanks for the response, all of the work on our car is done by a very experienced mechanic, none is done personally, although these (there are 3 of them) mechanics are very experienced but I doubt they will have that said instrument, nonetheless I will ask them. But atm I am not worried about knocking but this jerking problem, it is a very serious condition that makes the car unsafe while overtaking (occurs at higher rpms) and potentially i think it can destroy the newly installed fuel pump prematurely.