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

Yes the pump is tightly secured to the engine block. I reviewed to the EPC and saw all the parts involved in the IP timer. Yes I agree that probably the weakest area may be the spline in the IP shaft. I guess that means to replace the pump. I also thought in the screws that attach the female spline to the timing gear. Anyway the only time I have to remove the pump is during the weekend. I will order the special tools. How can anything in that area break just like that?. Could it be that I changed to Mobil-1 oil 15W50 about a year ago? I know this engine is predicted to fail due to the bent rod, but I thought it was going to be slowly. Before this happened (3 weeks ago) it wasn't smoking and the idle was smooth. Compression test cold, (because I couldn't start it) was 500psi for cylinder #1, 460-480-390-460-480. Does that sound too bad? I will keep you posted of what I find when removing the IP, and inspect the IP timer. Thank you for all the advice.

Darrel
1991-350SD
102k Mi

Hmmm,
any one of your compression numbers alone isn't horrible, taken all together, if it was a valid test, isn't good. The big difference between 500 and 390 is the problem. The general rule of thumb is less than 10% difference from highest to lowest. A cold test is not really valid though. You may have had excess fuel or oil in cyl 1 that made that nuber so high. If you read the article in the link it mentions a collar between the timer and the pump. Maybe that craked or split.

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

You are right, once I figure out what is the problem from the IP, I will take the engine to running temperture and repeat the compression test. I read that if I need to replace the IP timer, I may need a pin puller for the pin that keep the chain in place around the IP timer. Do you know how to take this pin out? Do I need this special puller? I also saw that the pump timer only comes as a unit, instead of separate parts. Can I buy the unit and replace only what is broken, without removing the timer? Thank you again for the advice.

Darrel
1991-350SD
102K Mi

I couldn't wait until the weekend, so I removed the vaccum pump tonight and found the left hand screw that attaches the pump to the timer at the bottom of the IP Timer housing. There were 3 scored marks at the vaccum pump housing without any lost material, and a small piece of the casting from the housing of the IP Timer missing. I describe the missing piece of approx. 3 x 6 mm. Should I be concern for this small piece swiming in the oil? Also, should I use semi-permanent loc-tite for the left hand screw? I'll keep you inform with the progress.

Darrel
1991-350SD
102K mi