I know that the IPs hardly ever go bad, but I was referring to the fuel pump on the side of the IP that the primer pump is screwed into. I don't have a clue how the fuel pumps actual function, but I assume they can go bad also. I went to the dealer, and that little unit (fuel pump) the primer pump is sitting on cost a little over $300.


When I visually inspect my IP, I cannot tell which pump I have, and I may have to go to the dealer to see which IP I have. Each IP (there are three different models) has a different Bosch designation fuel pump, and each varies with the engine model (617 or 616). 07.1-145 (entitled, checking fuel pump and overflow valve) of the manual shows the test method for each, and while the part is called a "fuel pump" in the manual, I am not sure it is actually a pump in the traditional sense. I say this because to test "the pump" you are measuring vacuum and delivery pressure in different places. Keep in mind this is all new to me, and even when I read the procedures in this part of the manual, I do not thoroughly understand them.


My primer pump looks original with the white nylon handle on top. ************** sells a primer pump made by Monarch that he calls the best pump he has seen. I will probably buy it to see if that fixes things. It may take a while to get the part and put it in (car is outside and the condo building officially states no working on cars; a bunch of busy bodies here.) Any thoughts after seeing this?

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1983 240D low miles 156000, and yes, MB still gives out badges

Well, as far as the Condo issue, my only advice would be tow the car to a friends or somewhere you can work on it. As far as fixing it, here is what I would do first. Look at what you know so far......it ran fine on diesel purge, but not when the cars fuel system was hooked up. That tells you the problem is most likely between the tank and the IP (since by running it off of Diesel purge you isolated the cars fuel system).

That said, I would definitely order and replace the old white handled primer pump you have. I would also change both fuel filters (be sure to pre-fill the spin on filter with fresh diesel or Diesel purge). Lastly, I would replace the leaking rubber return lines between the injectors.

When that is all done, I would crack all four injector lines at the injectors and crank (put some rags under the lines to catch the dripping fuel.) Crank for 10-15 seconds at a time then rest the starter for 30 seconds. You should see fuel coming out of the lines you cracked once the air is all bled out. Once the air is bled out, re-attache the lines and the car should start and run. Drive the car until the fuel tank is almost empty, then pull the fuel strainer out of the bottom of the fuel tank, clean and re-install. Fill with fresh fuel and you should be good to go from there.

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2012 Mercedes ML350 Bluetec 91K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 231K (mine)
1998 Volvo S70 T5 Turbo 196K (kids)
1994 Ford F150 4WD 249K (firewood hauler)
1983 Mercedes 300D 376K (diesel commuter)

If someone has not mentioned it already you can test for a clogged fuel tank screen by swapping the position of the Fuel Inlet Hose (the one that goes to the plastic filter) and the fuel return hose (the cigar hose) where the tubings are at the fender well.

You need to have at least 1/2 tank of fuel because doing that draws fuel from a higher positon in the fuel tank (note that it also means you would run out of fuel before the fuel gauge shows empty) so if you are not willing to keep the tank filled above 1/2 tank just swap the hoses for the test.

If power returns to normal then you fuel tank screen is restricting the fuel flow.

Another issue can be a plugged fuel tank vent. To test for that drive without the Fuel Fill Cap on. Doing that allows the tank to vent though the fuel fill. If your performance returns you need to clean the Fuel Tank Vent.
Note that the above can be a little messy if your fuel tank is full or near full.

If the above tests don't locate the problem you likely have an issue with the Lift Pump, Hand Primer and or the Fuel Pressure Relief Valve.
There is cheap rebuild kits for the Fuel Supply/Lift Pump. No one has mentioned it on Mercedes but on other Bosch Fuel Injection Pumps I have seen the Fuel Supply/Lift Pump Poston return Spring break. But, that has happend to no one I know of on this forum.

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84 300D, 82 Volvo 244Gl Diesel

Having driven new Mercedes from 1980 to the late nineties, every car I ever had one or two killer problems. Every three to four years I would lease a new car, and all but the first, was a diesel. Mercedes fixes their problems after they put the cars on the street, thus making changes after the sale that may not apply to all like models.


Everything on my car is like new from the tank to the fuel pump, see photo. The screen in the tank will be the last thing I check. The front of the car is to the left.


I would like this to be a learning experience, and while I might be slow on input, I hope you will tag along.


"A" is a clear plastic tube that I intend to replace. The manual states that you should loosen the hollow nut on the large fuel filter this tube goes to, and start pumping. Close the hollow nut when no more bubbles are visible. This nut only needs to be loosened a little as fuel spurts everywhere with each pump. I believe the part barely visible under nut "A" is the Fuel Pump check valve. I say this because it is threaded on both ends and the "A" nut needs threads. Does this check valve have two little ball bearings and a spring in it? One of the two valves (I am guessing only two valves in the pump) has the bearings and spring. If you don't know or know for sure, please respond. It is shown in Pelican parts as # 000-090-01-10-MBZ.


"B" is my lovely original primer pump that I now see is leaking as we speak, I intend to replace it with one manufactured by Monarch in Germany (same company that does injection nozzles). While it may be the only problem, it may not, and I want to learn from this if possible.


Step two in the manual states that both of the two IP pumps that apply to my 616 have either (1) a bleeder screw on top to loosen (it is to the right of the four fuel lines to injectors) or (2) there should be a similar one way valve (perhaps in its place) that should hiss or bleed the air out of the IP. If there is a bleeder up there, you continue to pump until no more air. Then again, the car may have neither. It is a Mercedes after all, so refer to my first paragraph.


"C" is the feed from the plastic main fuel filter, which assuredly has a brass nipple on top. I am guessing that under the nipple, the nut looking part is another check valve. Can any one confirm this?


Who knows what part "D" is? If I remember, Mercedes only stocks the fuel pump, the primer pump and only one of the valves and gaskets. No springs and no ball bearings, and I don't remember seeing this on the parts diagrams at the dealer.


One other thing that I would mention is that in testing the fuel pump, the manual says you are measuring vacuum. Given that, a weak or faulty vacuum pump on the front of the engine may also be involved. On the other hand, since it is German, they perhaps meant to say suction, although this makes no sense as the measurement is in "bar," .6-.8 bar.


I also saw one posting on the forum mentioning clogged up valves in the pump. Perhaps after 34 years, and sometimes sitting idle for periods, perhaps the fuel pump should also be replaced. My screen in the fuel tank looked like dirt, so why not where check valves are located. Doesn't hurt to take it apart first, however. Anyone in attendance do this? I assume that none of the fuel pump extends into the IP. Anyone?

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1983 240D low miles 156000, and yes, MB still gives out badges

I now note you mentioned the venting in the fuel tank. When I pulled the tank 4-5 years ago, it was like new, inside and out, and clean as a whistle (after I power washed the inside with a cleaner), so I am OK there.

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1983 240D low miles 156000, and yes, MB still gives out badges

Did you try and do the bleeding procedure and do the simple thing I said first before throwing parts at it???

These cars don't self prime like an om606. If it starts up for a second when the fuel lines are connected up like normal and stalls, it's cause there's air in the system!

It has taken me 2 hours to bleed an om616 before after replacing 3 different fuel components. I had to recharge a battery from cranking it!

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k

It should not take 2 hours to bleed a 616 to start if there are no air leaks or restrictions in the fuel system.

1. fill the secondary fuel filter with fuel

2. pump the primer pump till restriction is felt and you hear the rattling of the ove-flow valve

3. crack one injector line nut (ONLY ONE!) 1/4 turn and put a rag around it

4. glow the glow plugs then crank the engine till it starts, which should take no more than 5 to 15 seconds of cranking.

6. The idle will be rough since the injector with the cracked line nut is not firing, it was used to bleed the air out of the other 3 injector lines and injectors. Wear goggles while tightening the cracked injector line nut and the idle will smooth out as the remaining air in the system bleeds off to the tank.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

What you call the fuel pump check valve (aka over-flow valve) is on the side of IP facing the engine block. It has 2 hexes which takes a 17 mm and 14 mm wrench. Do not touch "D" unless you have the lift pump off for a rebuild. If the engine ran well while doing a diesel purge, your problems are either air leaks in the supply line that you disconnected all the way back to the tank or a clogged tank strainer. The metal line often develop pin hole leaks under the clamps.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

your right it shouldn't take that long. this car was an exception, it had been sitting with air in the lines for quite sometime. read a thread from a member "goldeneagle" with a 300cd that had a lot of air in the system after removing the IP and some other work done to it. it took him a couple hours to get any fuel at the injectors. it can happen!

I see this repeated all the time regarding W123 tanks and it is simply FALSE! The return line to the tank is not half way up in the tank. It will not run out of fuell if the tank is less than 1/2 full if you swap the return and supply lines. The return line is less than 1 " off the bottom of the tank. I know because I have cut open a W123 tank and studied its construction.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

jake12tech, right now, the engine turns, but shows no hope of starting. Further, the primer pump is original and is dripping at the bottom at about 1 drop per second. It would seem this could introduce air into the system. I have ordered the Monarch primer pump. I haven't had this much trouble before.


funola, I would like to clarify terminology with you. The Mercedes manual refers to the whole unit that the hand pump is attached to as a "fuel pump." It includes the hand pump and the iron body it is attached to, and is sold as a unit, even today. Mercedes does not sell the hand pump alone. You have used two terms, "primer pump" and "lift pump." When you refer to primer pump, that is obvious to me, but when you shift to the words "have the lift pump off for a rebuild," are you referring to removing the cast iron body the primer pump is mounted to and rebuilding it?


Other than clearing air from the system, does the "fuel pump," as defined by Mercedes (above) do anything else, other than prime? Is the IP the only thing pulling fuel from the tank, or does the "fuel pump" assist in doing that?


funola, I also checked the back side of the IP, and in my case, there are no hex nuts back there (engine side of the IP), so no over-flow valve in that location. So, I guess, if there is an "over-flow valve," it must be elsewhere. Maybe, I do not have one.

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1983 240D low miles 156000, and yes, MB still gives out badges

lift pump aka fuel pump aka cast iron body pump and is engine driven. It "lifts" the fuel from the tank and provides pressurized fuel to the IP.

Primer pump aka hand pump. Pump it by hand to prime the fuel system when there is air in it in order to start the engine. How else? If you have a drip at the base of the primer pump. You need to fix it! There should be no wetness from fuel anywhere! It should be totally dry!

I'd suggest replacing the cigar hose with a temporary clear PVC line in place of the cigar hose (highest point of the fuel system, returns fuel to the tank) to look for air. It should have solid fuel in it at all times, running or shut down overnight. Should not have any bubbles in the clear line.

The overflow valve is a special Banjo bolt where the clear line on the left side of this pic connects to on the IP.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

The acting President of the condo board told me that some people complained about me revving my engine and working on the car (diesel purge at that time). I can hardly wait to get out of this place, buy another home and have my own garage.


My car came from Florida 5 years ago, and the engine compartment was as clean as your or better. Now, after 5 years in the Chicago area, that nasty salt is corroding my hard lines. Oh, well. I assume you are in a salt free state.


Thank you for your input. I obviously have to replace the primer pump, and if that doesn't work, I guess I have to look at the fuel pump. Everything from the tank to the fuel pump and large fuel filter is fine.


When I was doing the diesel purge, I drove the car, and the brakes seemed a little hard to push. It maybe that I had that feeling since the car has sat there for the last few months, and have been driving our Nissan Versa which is easy braking. It makes me ask the question; is the fuel pump fully mechanical or does vacuum come into play in any way?

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1983 240D low miles 156000, and yes, MB still gives out badges

I like to share something that has worked quite well.


When I bought the car about five years ago, one of its many issues was that the rubber breather hose coming off the valve cover was melting away from the heat.


I replaced the breather tube, and I also ordered a spark plug protector from Amazon to enshroud the breather tube and keep the heat away.


After five years, it has done its job nicely, and looks pretty much like the day I put it on the car. if you like the idea, please let me know.

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1983 240D low miles 156000, and yes, MB still gives out badges

Have you gotten it running yet? Have you stopped the return lines from leaking? I've found that NAPA has rolls of 1/8" fuel line that is diesel compatible for ~ $1.20/ft. Buy 3' or 6 and store some in the trunk with fuel filters, tools to change them and a set of used but still serviceable belts.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.