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Kind of, but opposite actually. It holds back fuel until boost pressure comes up to prevent black smoke out the exhaust. It is just a diaphragm against spring pressure, and a lever down to the main fuel rack. Boost pressure in the top reference line pushes on the diaphragm and against the...

Another good path. Anything is possible. Easy to check and rule out.

Symptoms are indicative of fuel delivery as one potential issue. Reverse could mask such low power situation due to its gear ratio being more than double that of first gear.

Here is a pic of the 8.3L boost reference line. Very short, goes from the intake manifold to the injection pump. Allows for increased fueling as the boost pressure comes up.

Sight glass works amazing! They are listed for sale in the classifieds. Can get return side, suction side, or both to better identify a tank pickup problem. Yours ships out tomorrow, Monday.

I’ll look at my boost reference line later today and post pics of it has such. It should I imagine.

Yes. But the RPM drop for the converter locking is much smaller than the RPM drop during the shift. Also, the TC does not lock as soon as it shift into second, or is later in the shift. You may also be able to just put the shifter in 2 and hold there to see if it locks, I have never tried it.

Good point on gears. Once into the upper RPM’s of second, you should feel it do half a shift. This is the torque converter locking up, as a function check of that as well. Here is the prototype clear return line, simply threads inline into the return fitting at the tank. I’ll test fit it...

I’m making the first proto tomorrow, can ship out Monday. Will install at fuel tank return line connection to cover entire system, and work for all M939’s.

I forgot if the 8.3L MW pump is boost referenced, it should be. Make sure this pressure line is clear, connected correctly, and could check the diaphragm, spring, and internal movements associated with it.

Maybe I’ll make 100 of these and sell them. Buddy works at a hydro shop very close to my house. Yeah, I’m doing it. If you want to wait until Monday, I’ll have the first one down. Test it and you can have it for free.

Could, but I would just have a hydraulic hose place make one up to add to or replace an existing one. There is zero pressure on it, so just clear tube with the correct fittings on each end. Or short piece with fittings to add into an existing connection.

That seems good. I’d next still check for air bubbles in the return line by using a piece of clear tube along any part of it. Pain in the butt, but excellent diagnostic tool. Aerated fuel from a suction leak will do exactly as you are describing.

If little to no pressure, pinch the return line closed which exits near the front of the injection pump. If pressure increases, the lift pump is maybe OK and the overflow valve in the injection pump is then suspect, which is just a simple spring loaded check ball integrated into the banjo...

Yep. Sounds like you’ve done decent homework, thanks. You could probably even drill out the center of the transducer you are removing and tap it for pipe thread then add gauge and hose? I have never had one in my hand to see, but that is what I would try.

You could try a small outboard motor fuel tank to help diagnose. Connect that to lift pump, if runs good then problem is before lift pump.

No androids. Sounds like air in the fuel, or weak fuel lift pump. Replace the filters, make sure all seated correctly, replace soft lines on the suction side, then test fuel pressure after the lift pump. This will also tell you if the overflow valve in the injection pump is OK. Install a...