How do I go about checking/cleaning/replacing my delivery valves? Is there a way I can check them with the IP in the car? What do the delivery valves do? Would stuck/worn delivery valves contribute to a decrease in injection pressure at low rpm?

Its a little hard to do the screwdriver method, not impossible with another person, but I would have to run the engine at 1200rpm or so... would I still be able to hear it do you think? What am I listening for? What could this possibly tell me? The car runs smooth enough at higher speeds that I think my injectors are okay... I could have them tested and matched for kicks but I would like to make sure they are bad (and it doesnt seem they are) before I go spending money on them, if there is something else I could do instead.

As I said before, I eliminated the lift pump and the fuel supply system to the IP as a culprit by installing electric fuel pumps and modifying the return pressure valve on the IP. This modification is a proven one and did not effect the problem whatsoever... its still there. This was effectively the same as running from a bottle because I put diesel fuel in my vegetable oil tank (completely separate from diesel system (different lines, filter) and tried to run it to no avail... There is one more thing I can check, and that is my switchover valves. I will do that as soon as possible. Possibly an air leak in one of the valves could cause this problem, but I doubt it because they are under PRESSURE 100% of the time, never vacuum, verified by the pressure guage I installed 4 inches before the IP inlet (the solenoid valves are before (upstream of) the gauge).

I seriously think that the problem is something in the IP, because I have eliminated the rest of the fuel system (except for the solenoids, which I will check shortly). Also, another forum member was having a similar problem, up and replaced his IP, and the problem went away. I do not want to take this route, because I actually want to figure out what, specifically, is wrong. If at all possible, I want to avoid removing, installing and timing an IP as well Whether it is the fuel switchover valves, delivery valves, the pumping elements, or what I don't know but I would sure like to find out for sure, and I am wondering how to go about that.

I am slowly but surely narrowing it down to individual components. If anyone knows what the delivery valves look like, how they work, where they are, and what happens if one fails, please let me know. Thanks for all the support.