The FSM has a couple of lift pump tests that you could try. In one, you disconnect the output line and run it into a container. X seconds of cranking is supposed to produce a minimum of Y milliliters of fuel (sorry I don't remember the numbers).

The second test involves tee-ing the output line and inserting a pressure gauge, with some specified minimum pressure required for the pump to be good.

The pump could still be "good" even while it leaks fuel into the crankcase. Remember, the lift pump pumps far more fuel that the engine can possibly use; that's why there are return lines in our diesel fuel systems.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

The only other answer is the braided fuel bypass hose, from injector #1 to the fuel filter was stuck into the injector hole for weeks.

If the fuel tank vent valve is bad/plugged = This would cause an issue called breathing:
* Daily Heat expansion would flow diesel into cylinder #1.
* Nightly cooling would suck air back into the tank..
* A variation of 10° F (or more) between night and day is all that is needed, the wider the temperature difference = more fuel transfer...

Easy to test = remove the fuel cap and listen for pressure escaping..

That's why I don't think the FSM tests are going to help me.

In any case, I just used the mity vac to pressurize the outlet fitting while plugging the inlet. It held steady at 30 psi. Put in under water, no bubbles, shaft or otherwise. So the lift pump is not the problem.

I'll attempt to pressurize the tank with the injectors lines disconnected and see if they product a steady drip. If so, bonus = new way to prime the system without wearing out your starter and battery (603's have no hand pump)!

If the lines drip fuel, I think I'll just put new oil in and run it, checking the oil frequently to see if the level starts going up due to fuel in the crankcase, in which case, it must be either the IP or the injectors.

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1986 300 SDL, gray on black, #14 head, 180K miles
1982 300 CD, silver on palomino, 290K+ miles [sold]
1980 300 SD, white on palomino, 500K+ miles when retired [uncle's car - inspiration for the above]

whunter - I think you're onto something. I'll watch that line too. Yes, my tank breaths a sigh of relief when taking off the cap. Time to unplug the vent!

Fingers crossed for no damage due to extremely overfilled crankcase! RPM's were never above 2k, ran for only 3 min . . .

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1986 300 SDL, gray on black, #14 head, 180K miles
1982 300 CD, silver on palomino, 290K+ miles [sold]
1980 300 SD, white on palomino, 500K+ miles when retired [uncle's car - inspiration for the above]

I tend to agree as strange as it seems that the vent being obstructed on the fuel tank might have done it. Although a pretty well total obstruction of the vent would have been required.

Usually this will result in the car quitting after so many miles unless the drive is stopped prior to the vacumm building in the tank over comes the lift pump. Short local trips may not reveal this. This is very common.

One thing is to watch the oil level like a hawk for quite awhile.

No Way ANY amount of Fuel Tank Pressure will push Diesel THROUGH the I.P.
AND Injectors to the Cylinders.
(No Way it'll push Fuel through the I.P.'s NORMALLY PRESSURIZED SECTION,Period)

MAYBE,MAYBE,MAYBE with a BAD lift pump Fuel COULD be pushed through the
BAD LIFT PUMP into the I,P,'s LUBRICATION SUMP until it overflows into the Timing Chain Cavity.

1.Lift Pump
2.Lift Pump
3.Lift Pump

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'84 300SD sold
124.128

The injectors were pulled ,#1 .The lines were sitting above the injector holes #2 ,the rags put over the holes to prevent dust or liquid to enter were soaked #3 .Me thinkie #4 is the logical outcome .

But in post #13 he said:

"Just changed the oil and verified the level within 5 miles of driving. But didn't check it again immediately prior to the incident."

Timelline and sequence of events are important.

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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

x2

"Occam's razor" = "when you have two competing theories that make exactly the same predictions, the simpler one is the better."

__________________
Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John

/2 that should cancel it out.

__________________
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

Note also that I tested the lift pump with pressure from mity vac to the outlet fitting with the inlet plugged. The results were as follows, in order:

At 20 psi, it took about 15 seconds to leak down to 15 psi and held steady.
At 30 psi, it took about 30 second to leak down to 25 and held.
At 30 psi (again), it now held steady with no leak down.
Again at 30 psi, stuck it in a bucket of water and there were NO bubbles = no leaks.

That lift pump would have to be blasting fuel out of the shaft area into the crankcase for it fill it with 8-12 quarts of fuel in a matter of 3 hours total running time (200 miles) since changing/verifying oil level.

I noticed the lift pump shaft does not automatically return to the out-position (such as by spring action) when pressed in. Is it supposed to? I don't understand how it could operate unless the shaft, when pressed in by a cam (I assume) in the IP returns to the out position. Also, the shaft seemed stuck at about 0.5" out (very difficult to move) when I first removed the lift pump. I lubed it and it moves fine now.

__________________
1986 300 SDL, gray on black, #14 head, 180K miles
1982 300 CD, silver on palomino, 290K+ miles [sold]
1980 300 SD, white on palomino, 500K+ miles when retired [uncle's car - inspiration for the above]

funola - it is possible that I 'mis-remember' about checking the oil within 5 miles of driving. In either case, it sat for a few weeks after checking the oil level with the injectors out and oil level was not checked again until after the incident.

__________________
1986 300 SDL, gray on black, #14 head, 180K miles
1982 300 CD, silver on palomino, 290K+ miles [sold]
1980 300 SD, white on palomino, 500K+ miles when retired [uncle's car - inspiration for the above]

The shaft on the lift pump should be spring loaded. I think the way the lift pump should be tested is:

Plug outlet and pressurize inlet with pump shaft submerged in liquid while pressing and releasing the cam to simulate operation. No bubbles should be seen.

repeat above except plug inlet and pressurize outlet

__________________
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

Re" That lift pump would have to be blasting fuel out of the shaft area into the crankcase for it fill it with 8-12 quarts of fuel in a matter of 3 hours total running time (200 miles) since changing/verifying oil level."

In retrospect 8-12 quarts (2-3 gallons) is a lot less than my SDL will burn in 200 miles or 3 hours time. Just saying that I'd have to believe the pre IP delivery system has the flow capacity, thus could be the source. Your "shaft" concept may have some merit. I'll keep looking for ya Mr. Luke.

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1986 300SDL 440,xxx

The lift pump certainly has the capacity to push a lot of fuel. Most of the fuel delivered by the lift pump is returned to the tank, not consumed. A ratio of 45 to 1 is norm. The Mercedes lift pump is a mechanically driven positive displacemnt pump. At highway RPM, the volume through that pump is huge.

__________________
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