Hi friends. I need some tech advice from you. I have done some tests in my 1991-350SD, and I am concluding that the the crankshaft of the injection pump either is loose or broke. I removed the fuel pump and while cranking the engine the shaft doesn't move at all. The overhead cam is moving so I belive the timing chain is ok. Is diffficult to ask question about a 350SD because the answer is always, change the rods. The question is, is it difficult to remove the injection pump? Any special tool for that? How about a remanufactured pump? Thanks for reading.

Darrel Hillman
1991-350SD
102K

It is not too hard, but fairly involved. Read about it in the manual first. The only special tools that would be good to have is the IP lock tool and maybe the drip spout for timming. This would be a very odd and rare failure mode. I'd sure do some double checking before pulling the pump.

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Bob D.
Parrish, FL
1 SDL, 1 D, 1 TD, 1 Mog

Yes I may need to double check, but my last test was that I removed the fuel pump, press the rod on the pump, and the pump is pushing diesel thorugh the system. When I saw the cam in the IP that pushes the rod of the fuel pump, the cam doesn't move while the engine is turning. I am using a remote switch so I can see through the fuel pump access while cranking the engine. Does the crankshaft of the pump (if that is how it is called) should move all the time or does it has an engagement with the timming gear? The overhead cam is moving, so I believe the timing chain is ok, as previously stated. How much cost a rebuild pump? Is it better to repair this one? It is only 5 years old, 35k miles. It was replaced by the Mercedes Benz Dealer through the Encore Extended Warranty Program here in Puerto Rico. Thank you again Bob.

check it out but i believe the 91 3.5 engine has a seperate chain to drive the oil pump, wouldnt be working on the ip at this point. i had one that a 10cent washer got thru the pump screen and locked up the oil pump, if you are leaking some oil from the front of engine then the front cover gets taken out with the broken oil drive chain.
larry perkins lou ky 15027276903
64 old cars

Thank you for reading. I checked the service manual and as far as I can see in the diagram there is only one double chain that runs from the engine crankshaft to the overhead cam to the injection pump timing gear. The oil pump has a separate chain. Where is the best place to get the locking screw and the drip spout tools? Is it possible that the left hand thread screw becomes loose from the IP Cam shaft and that is why the pump is not turning?

Thank you again for your interest.

Darrel
350SD-1991
102K Mi

its more plausible that the injection pump drive thru the injection pump timer may be at fault.i cant see how the cam in the ip could ever break unless it had a faulty casting

I just re-tested the pump. I removed the valve cover and noticed that the timing chain is intact, it moves smoothly and it turns the IP timing gear when I crank the engine. I removed the fuel pump, crank the engine and I could see that the camshaft moves intermittently, then stopped even thought the engine was still turning. The shaft moved very slowly and jerky. Where can I get a diagram of the internal parts of this pump? What exactly is the timer of the injection pump. Does the cam that pushes the fuel pump rod is the same that connects to the timing gear? Thank you, please reply.

Darrel
1991-350SD
102K Mi

The shaft on the end of the IP has a fine spline that fits into the timer/drive gear(the gear has a weight and spring advance mechanism) I guess it could be possible that these splines have stripped. Is the pump bolted securely to the engine? when you remove the IP you unbolt it from the block and a support underneath and pull it up and back. the drive gear stays in place. Here is a link to an article that may help.

http://engine.articles.mbz.org/diesel/injtiming/

The other articles are good too.

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Bob D.
Parrish, FL
1 SDL, 1 D, 1 TD, 1 Mog