First off I'd like to thank whunter for recently posting about the vac pump, and where the excess goes if there's a leak... defenitely gonna check that out... very cool.

Okay, now the problem:

1) I can barely start my '84 300TD. Foot to the floor cranking for like 20sec (more than that and I start to fear for my starter's life) and it finally goes just when I am convinced that I should stop now for the sake of my poor extremely hard to replace starter. Also, when I crack a line, fuel lethargically leaks out rather than spraying like it used to... but there is fuel.

2) Once it fires, but running cold, I really have to work the pedal to keep it going. If I let it down below about 1100rpm it runs really rough and below about 900 it just quits (this is with my foot in it a good deal mind you). When it is good and hot (after driving for over an hour), it is even harder to keep it running, necessitating almost 1/4-1/3 or more throttle to keep it running.

3) I installed a vacuum/pressure guage in the fuel inlet line just before the IP, was getting good pressure, but a really crazy oscillation of the needle (btw. 8 and 15 psi) in time with the engine. I.E.- 2000rpm = 2000 'wags' of the needle.

4) I eliminated the lift pump as a factor by removing it, taking the spring out, and putting it back on (to seal up the hole and keep my engine oil in) and I added an electronic solenoid fuel pump. I also modified the pressure valve on the IP return (using a .030 MIG tip no less... I'll tell you how if you really want to know). So with the restricted orifice on the IP return, there is about 10psi at the IP inlet, with fuel constantly flowing through. I know its not FSM spec but I know of quite a few who have done similar things with no adverse affects. DID NOT SOLVE THE PROBLEM-- oscillation of needle greatly diminished, but the car still won't run worth a *****. This work effectively eliminated the lift pump as a variable in the diagnosis.

5) Running down the road at normal throttle load and normal rpm (~2000 or more) it runs great, no smoke, no sputtering, good power, runs like a top.

6) Here's the kicker (and its a weird one): Running on veggie (yes I'm one of those guys) with a good system by any standards (140-180°F before IP, 190-220°F btw. IP and injectors w/ electric heaters) It RUNS GREAT AND IDLES FINE.... Also starts fine cold or hot

7) There is one way to make it idle on diesel... that is to crank the idle screw waaaay in... which defeats the governor so you better not touch the throttle or it runs away... a non-option fix if you ask me... plus when I switch to veggie it would run away.

Hypothesis: I think that my IP is not making enough pressure to push diesel through the injectors at low RPMs, but at higher RPMs, due to the higher speed of the injector plungers, it can sort of "bounce" the fuel through (sort of a hard mental image to communicate). With veggie, because it is slightly thicker than diesel (although only slightly at those temps), the IP can make more pressure at low RPMs, thereby popping the injectors, and thus allow it to idle. Am I even remotely accurate as to my knowledge of the injector pump?

Does this sound at all reasonable? Has anyone ever encountered an IP failure that had similar symptoms (minus the veggie use, probably)? Is there any way I can replace the plungers/sleeves (not really sure how these work) or any other part in the IP without removing it from the engine.. it looks like you could almost get to the guts of the actual individual pumps from the top through those 2 nuts on either side of the injector lines...? I am extremely confused here. I have a parts car sitting in the woods with a (formerly) running engine, so if all else fails I will swap that IP in this coming weekend.. but I really want to find out exactly what is wrong, and why, and if there is an easier/better, more reliable and more complete fix.

Anyone who has ever seen a dead IP, let me know what the symptoms were! Anyone who has ever seen anything like this let me know!

...in the mean time I will be looking up how to remove, install, and time the IP....