Question about OM617 IP pressure:
 

I saw reference in another thread about fuel pressure issues on an OM617. A few questions:

How is fuel pressure regulated on 617 IPs? How is it checked, (where do you mount the FP gauge), and what is it supposed to read? What part of the IP is the lift pump?

Sorry for all the *probably dumb* questions, but I still don't have a FSM for these cars and I know that there are people here who can explain it. I have one car that I suspect of having low delivery pressure but no idea how to measure and how to rectify if true. TIA as always! :)

The lift pump is on the side of the IP where the primer pump is mounted, along with the supply and discharge fuel lines. The lift pump overdelivers fuel to the IP, where input pressure is regulated by a relief valve. Excess fuel is returned to the tank.

The lift pump consists of a cam-driven piston (with spring return) and two check valves. The primer pump utilizes the same check valves as the primary pump.

Thanks! That's where I thought it was, (or rather what I thought that was).

Here's another dumb question: Is there a relationship between the hand pump and normal operation of lift pump? IOW, if you have a shot hand pump, will it effect FP in IP?

It's fairly common for the hand pump to leak air into the fuel lines. Aside from that, you could replace the hand pump with a plug and not affect the lift pump.

The hand pump uses the same valves as the engine-driven pump, so a bad valve will affect both pumps.

If you don't have the new style primer pump (black plastic plunger), then I suggest you replace it.

FYI
 

Here is the FSM free from MB.


Fuel injection pump starvation with a good lift pump
Fuel injection pump starvation with a good lift pump - PeachParts Mercedes ShopForum

Fuel pressure relief valve adjustment
Fuel pressure relief valve adjustment - PeachParts Mercedes ShopForum

lift pump test
1979 300td -- Can anybody walk me through a lift pump test? - PeachParts Mercedes ShopForum

fuel pressure
fuel pressure - PeachParts Mercedes ShopForum



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Mr. whunter, thanks a million! What a great board. :)

I just removed the check valve from my 1980 300TD, (617.912 engine), and took it apart. The spring only measures about 20mm and did not seem deformed or compressed. Are they all supposed to be ~27mm?

The other thread begins with a guy asking about his 616 motor, (240D), and then contains posts from people with 300D turbos and other diesel models.

Id be surprised if mine was that far off or compressed. My car runs pretty well but the idle is a little rough and sometimes power is off/lacking. TIA again!

Have you noticed any improvement in performance with a full fuel tank versus a near-empty tank?

Hmmm, not really. :cool:

Been reading a number of other threads on the subject of fuel pressure but still no answer to whether my IP relief spring is supposed to be 27mm. Are they all this spec?

Yes
 

All are a minimum of 27 MM.


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So they routinely compress to ~20mm with use? Mine, as well as others referenced in the other threads, measure about 20mm and do not appear to be compressed. This is strange.

Not to beat this to death, but has anyone ever measured a brand new one? :cool:

Answer
 

Yes.
I have the last two new/old stock springs.
Both are 27 - 28 MM long.

When I re-stretch the used spring, I generally go to 30 MM.


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FYI
 

I frequently find them at 13 MM.




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