Author Topic: W108 250 SE Injection Pump adjustment- how to begin  (Read 2395 times)


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W108 250 SE Injection Pump adjustment- how to begin
« on: August 17, 2021, 20:46:48 »
I know, I know. Leave the FI pump alone. Don't even touch it as a last resort. Well, here's where I am.

I have a 1971 280SE that I bought last year in barely running condition. It has a 250 motor currently in it. The motor will remain until the motor fairy delivers a rebuilt 2.8 in the medium distant future, distant enough that I want to get the current motor running decently, but not so distant that I'm willing to lay out a lot of money to get there.

Once started, the engine restarts fairly easily and runs fairly smoothly. However as it warms, it loses power until eventually it will not start and will not run. Several times I have had to leave the car to cool down before I could get it home.

The motor is very rich at idle, so much so that I can rev the motor vigorously just by opening the throttle butterfly while the linkage is disconnected from the FI pump.

What I've done engine-wise to get to where we are
- replaced plugs, cap, rotor, plug wires, fluids, and filters
- adjusted and rechecked ignition timing
- adjusted throttle linkage by the process shared on-site
- cleaned injectors such that they all open around 220, with some slight dripping at a sustained ~215 pressure. More pressure makes them sing and spray.
- checked injection pump output for each cylinder at both closed throttle and full throttle using ziplock bags and a centigram scale. The amount of fuel put out varies some from cylinder to cylinder, but there's no fuel from number 2.
- Replaced the fuel delivery pump when I found the original full of varnish (10 years sitting!). I used the DBdepot substitute as I could not find the parts I needed to rebuild the original delivery pump.
- Professionally cleaned fuel tank

I don't know what would cause the power diminishment and subsequent refusal to run, beyond a wildly rich mixture. So have I reached the point where the laying on of hands (on the FI pump adjustment screws) is appropriate? What approach do you recommend? I was imagining starting with leaning the main rack adjustment to get into the ballpark.

Thoughts? Ideas? Wisdom?

Many thanks,

Dru Martin
'71 280SE 
'90 250D 5-speed
'95 E220T Sportline
'80 Targa
1971 280SE "Beverly"
1990 250D 5-speed "Opa"
1995 E220T Sportline "Sport Kombi"
1980 911SC Targa "Petrol"


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Re: W108 250 SE Injection Pump adjustment- how to begin
« Reply #1 on: August 19, 2021, 20:44:03 »
Hook-up a  fuel pressure gauge and monitor fuel pressure while the shut-down is occurring. if fuel pressure begins to fall, then you still have a fuel delivery problem.
Joe Alexander
Blacklick, Ohio
1969 Dark Olive 280SL
2002 ML55 AMG (tow vehicle)
2002 SLK32 AMG (350 hp)
1982 300TD Wagon turbo 4spd.
1963 404 Mercedes Unimog (Swedish Army)
1989 flu419 Mercedes Unimog (US Army)
1998 E430
1974 450SLC Rally
1965 220SE Finback