So I was under the impression until recently that the injection pump on these cars was the only means of suction to pull the fuel from the tank but now I read that there is an actual fuel pump prior. Is this where the primer pump is? SO I have had a couple incidents of clogged filters and was afraid it was stressing my IP, would this have been the case? Also I recently bought a liquid filled vac gauge to put between the filters and IP to check for filter clogging. It seems this would not be functional now that I know there is a pusher before the filters. So I guess just a pressure gauge in the same place may serve the purpose I need?

I had to hunt through prior posts to find your car, which seems to be an '81 300SD. That would indicate a 617A (turbo) engine, which has the lift (or boost) pump mounted on the side of the injector pump; you correctly noted that the primer pump is in the same housing.

The lift pump is a plunger driven off the regular IP cam, therefore it makes pulsed fuel (compared to, say, a centrifugal pump, which is smooth). That's why the "cigar" return hose from the filter housing back to the body pulses just like a heartbeat, and the bubbles in the pre-filter wiggle, too. All diesels have a lift pump to fill the IP housing with pressurized fuel, which ensures that each individual plunger is jammed full of fuel on each injection cycle. The excess fuel, about 9x% of what is pumped, is passed thru a pressure regulator and dumped back to the tank. Everything between the lift pump and the regulator is "medium" (maybe 30-100 PSI) pressure. The injectors and lines are "high" pressure (2xxx PSI on our old indirect injection engines). The other supply and return lines are "zero" pressure, basically feeling only the vapor pressure of the fuel and whatever differential is needed to pump the fuel.

Many rigs (eg, 6.5 Chevys) have an ELECTRIC lift pump, which fail and strand you. Our mechanical pumps are nearly maintenanceless, lasting for the life of the rig. Yeah, MB and Bosch!!!

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James Marriott
2003 Buick Regal
1983 300D (228k, frau Auto)
1996 Suburban K2500 (192k, 6.5 turbo diesel/4WD towmaster 10,000)
www.engineeringworks.biz
1987 300SDL junker 170k
1982 300SD junker, 265k

MB can be confusing at times, referring to the same item with different terminology depending on what section of what manual you're looking at.
The primer pump is a device to bring fuel to the Injection Pump in case of running out of fuel,or a filter change.
There is posted here the procedure to test the fuel pressure. Look up 07_1-145, that explains where to hook things up, but it can be difficult to intemperate.

Attached Files

07_1-235.pdf fuel pump.pdf (102.6 KB, 93 views)

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83 SD

84 CD

Sorry, I was going to put my year/make, I didn't think they varied that much, again, still learning. Thanks for the detailed explanation. So with this information, to answer the second part of my post, it would be good to get a 0-100psi gauge after the seconday and see what it reads when filters are new, then just keep an eye on it to see if it reduces in PSI.?

I wouldn't bother. For one thing, you'll need to put it on the clear/amber hard-plastic line going from the IP back to the return manifold; it seems like it would be tough to reliably mount. For another, unlike a gas engine, the car won't suddenly stall if you get a clogged filter. It's typically a "graceful" failure, where, even when floored, it will run fine but slowly (over a period of hundreds of miles) loose power: 110, 95, 87 . . . surging when floored will typically be your first clue.

__________________
James Marriott
2003 Buick Regal
1983 300D (228k, frau Auto)
1996 Suburban K2500 (192k, 6.5 turbo diesel/4WD towmaster 10,000)
www.engineeringworks.biz
1987 300SDL junker 170k
1982 300SD junker, 265k

Well I do mess around with veg a bit and in my earlier stages of getting it down I encountered a couple times when the clogging came on pretty quick. Since then I have taken a few measures where this will not happen again but still , that horrible feeling of cruising in morning traffic and your car is slowing down to 50-40-30 and then you are on the side of the road doesn't sit well and I would like to add a little extra insurance so that does not happen again.

Zero to thirty pound liquid filled gauge would meet your needs. Harbour freight etc probably 10.00 or less. You are correct in that it would give you early warning.

It is not good on these engines in my opinion to run with less than 10 pounds pressure present in the injection pump. They will run though. You cannot really monitor just the lift pump pressure because the relief valve on the injection pump is preset at whatever it is now with time and age.

Idealy it should be set at 19 pounds. In your case your earliest warning might be the pressure sagging off at speed as the filters start to load up and cannot pass the increased volume of fuel. Instead acting as a major obstruction to fuel flow and dropping the pressure in the base of the injection pump.

If you go for it it will meet your needs in my opinion. I still think the addition of a gauge to the base of the injection pump is a good ideal for any of these old diesels for the same reasons as yours. Low injection pump base pump pressures causes issues like poorer milage. I would also plumb the gauge back into a place like the glove compartment..Or perhaps out in the open if a neat arrangement can be done.

That addition not only gives you advance notice a fuel filter change is coming up. It allows you some room to do it at your convienience before the engine quits. Also running any fuel it will aid in troubleshooting down the road perhaps. Even indicating other deteriorating conditions. I have always though it is worth the effort and minimal cost in todays world to have one installed.

As suggested above I would hold off on that. Your filters will usually give you warning when they need changing. going up a hill you will notice the slightest "hesitation" now and then, thats "when" to change them. You will probably be able to drive for miles on the "flat" but change them when you get home and if you want to avoid needless cranking do it while the engines warm, fill the filter with clean fuel and crank it up. A Cummins does not like to be starved for fuel but our MBs aren't so particular. I keep rubber gloves, wrench for the filter bolt, and filters in the trunk in case I'm on a long trip and need to change them.

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

This is pretty much the practice I am employing now, just like the added safety. Not to mention any excuse to stick another cool doo dad on the car.

If you do not invest in a gauge to monitor that pressure. The fuel filters should be changed at the regular mercedes recommended interval. Perhaps not the primary one as it can be checked and cleaned usually if required.

. Below a certain injection pump base pressure there may be some negative aspects of running with the four and five cylinder engines. There again is a possibility that the damage is major in nature long term especially on the four cylinder 616.

The one solid item is unrestricted filters seem to deliver higher milage. I first noticed this when people were saying they had changed their fuel filter and noticed milage got noticeably better. I originally had to think that one over for awhile but am a believer now.

I did not doubt them just though the inverse should be true. I just eventually decided with low injection pump feed pressure the power balance across the engine might be less equal. Running an unbalanced engine powerwise is going to consume more fuel. Some cylinders will be working harder than others and even the sequential timing might retard on some of the underfed injection pump elements.

To even make this worse once the feed pressure drops below the relief valve opening pressure the cooling ability of the fuel is also lost. Normally a percentage of it absorbs excess heat and flows out the return back to the tank.

Normally most would wonder about that claim as well. So leaving the filters in till noticable effects occur is really false economy. Far before that effects are occuring.

Also eventually trouble of one sort or another will occur in the fuel supply circuit. The gauge should indicate the onset of it instead of us just waiting for it to happen. As a troubleshooting aid it can really shine. I have not totally thought it through but a minumin of ten pounds at cruising speed should probably be the lowest pressure allowed by the cars owner. Really again you want the pressure that is evident when the relief valve is open. Never less except perhaps at very low rpm.

Technically the injection pump is building heat if the actual pressure is below the relief valves output pressure point. The work of compressing fluid does build a lot of heat.

It is just an opinion that anyone with a gauge installed over a reasonable time and milage period. If putting their experience up on the site will make believers of people that this approach was well worth the effort. It will take time but should occur.

For example just clamping your return line off every six months verifies the lift pump has not aged if the pressure goes to thirty pounds unless something is wrong.

One breakdown on the highway is going to always cost far more than installing a monitoring gauge. Can any reader presently state the condition of their lift pump currently? A lot of them are twenty five years old now and the valves have gotten leaky. What shape is your lift pump in, your tank filter, your primary and secondary filters,your relief valve. Most have no ideal other than the car is running.

Barry.. You make a good case for changing my ways, I didn't believe in MB coolant at first either

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

If you are messing with WVO & you dont have a proper system for it, you will have problems. Even with a proper system you are headed for motor trouble. Have a look at

http://www.ncat.org/special/oilseeds_innovations4.php

If these guys cant get a commercial system to run satisfactorily long term, what hope has any one else!!!

Good luck!!

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Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving