A fundamental disagreement has ocurred which a search of this forum did not provide the definitive answer for. Maybe someone here can settle it.

On Mercedes diesel engines is the fuel pump (not the Injection pump) a constant voume or constant pressure design?Or were there two differnt plunger type fuel pump used?

One of our (VO conversions) fourm members seems to believe that the fuel pumps on MBs use a cam to push a constant volume of fuel to the IP..the other that the cam merely provides for make up fuel and the internal spring provides a constant pressure to the IP.

Here are the actual description of the individual with the "constant volume" opinion.

And heres the link to the discussion itself.

If anyone can settle this definatively it would be of immense help.
Thanks.

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Dana Linscott
Vegoil converted truck...vegoil converted 1987 190DT, 300 series next.

http://vegoilconversions.netfirms.com/

I wouldn't trust anyone who can't spell syringe.

Danny

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1984 300SD Turbo Diesel 150,000 miles

OBK member #23

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Does anyone have any evidence on this one?
Perhaps an exploded parts list view?
Perhaps the fuel pump tests from the online manual http://skinnerbox.steaky.org/Service/W123/Main.html(Section 07-145) gives the answer.

The Fuel Delivery Pressure (with the return line open) specifies a pressure of 0.6 - 0.8 Bar at idle and 0.8 Bar @ 3K RPM. This is adjusted by lengthening the pressure relief valve spring in the IP return line banjo bolt.
The Fuel pressure End Test requires that the return line is pinched (closed off), then the maximum pressure attainable should be 1.1 Bar at idle or 1.3 Bar at 3K RPM.

This indicates that, with an unloaded OM616/617 the fuel consumption would be minimal, the pressure is limited by the fuel pump, not the return line pressure relief valve (which is blocked off, along with the air bleed from the filter banjo bolt.


I would post the text, but it is a pdf file scanned from the paper document.

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Tony from West Oz.
Fatmobile 3 84 300D 295kkm Silver grey/Blue int. 2 tank WVO - Recipient of TurboDesel engine.
Josephine '82 300D 390kkm White/Palamino int.
Elizabeth '81 280E, sporting a '79 300D engine.
Lucille '87 W124 300D non-turbo 6 cylinder OM603, Pearl Grey with light grey interior


Various parts cars including 280E, 230C & 300D in various states of disassembly.

I don't have a reference, but as evidence of the constant pressure side, I can say with some conviction, based on readings from my fuel pressure gauge I have between the lift and Injection pumps on my '84 300D, that the pressure stays at all engine speeds between 10 and 15 psi. This range appears unaltered whether I have the pedal fully depressed or am at idle. It seems logical to me that if it were a constant volume supply that the pressure would be greatest at idle when the demand from the IP was lowest.

Probably not definitive, but it's logical within my limited scope of knowledge.

Ben

I think also this has a lot to do with the spillover/pressure regulator line on the back side of the IP. (its the bajo bolt with the extra nut thing).

I think the pump puts out extra and this unit regulates a constant pressure inside the housing my letting extra fuel past at a certain pressure.

Just my thoughts

It is not the excentric that moves the plunger that makes the pressure, it is the preload on the return spring that sets the line pressure, so that no matter how fast the cam excentric turns, the spring controls the pressure.
Where it the otherway round, then line pressure would increase as a function of speed.

FYI. Maximum volume is at zero pressure. Maximum pressure is at zero flow.

P.S. Only positive displacement pumps have rising pressure after chamber fill.
IE : The rack chambers in the IP..
.

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"A liberal is someone who feels they owe a great debt to their fellow man, which debt he proposes to pay off with your money."

I have observed the opposite of this to be true on mine. I have a pressure gauge right in front of the IP on my SDL, and when I hammer on the accelerator, you can see the pressure drop quite a ways. At idle I see around 10 psi (although it does fluctuate back and forth between 9 and 12 or so), then with no load on the engine I can bring the rpm up and watch the pressure gauge come up to 15 psi and level off there.

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Keep everything as simple as possible-but no simpler--Albert Einstein

I didn't even know that our cars have a fuel pump, I thought there was just the injection pump. Where is the fuel pump located?

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL