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Hey guys 7.3 idi here, <4000 miles on the engine injectors, and injection pump, I'm running a facet lift pump. I never had it timed properly just by ear, not powerstroke clatter, but not quite like a gasser, just a good sounding diesel engine. My truck started running weird, I was idling for an extended period of time in construction when the truck randomly shot the rpm down and got quite. It's reading about 500 rpm now, and is not sounding like it did before. It won't rev past 2600 underload and rolls zero coal or any smoke when the throttle is smashed and very little smoke when cold. Before this issue he would roll coal easy and liked to smoke when cold.
Is the injection pump bad? I'm getting good flow from the lift pump and good pressure to the injection pump. The injection pump has already been replaced once since the first unit was faulty.
Fuel pressure would be the first thing I would look at. It's easy to connect a gauge to the filter housing and see what the lift pump is doing.
That is a damn slick idea right there! Not something needed for the average guy, but a simple way to see whats actually going on, I like it!
Sorry I don't have any help for the OP though..
Only thing to say is you may need to monitor the pressure going down the road to see how it changes under load.
When did it have 5psi? At idle, or going up a hill? You need to monitor the fuel pressure through the whole driving range, not just idling.
My old mechanical lift pump would be fine at idle... And randomly on the highway drop to less than 0 psi (sucking a vacuum) and then jump back up to 2.5psi. I replaced the fuel pump and it fixed the problem. Engine starts better too