Ok. First two codes tell us the ip (injection pump) is not happy with fuel situation inside, but when there is not proper fuel flow of good clean & air free fuel flowing through it- that can happen. So we set those aside for the moment because of the third code and more importantly your comment about what you saw in the clear is return line. Your description sounds like the LP is leaking back fuel to the tank and not supplying fuel right away.
I think you do not have adequate fuel supply to the ip or bad FSO. You said
“-tested fuel shutoff solenoid.
-tested lift pump (runs while cranking”
We need to know how this was done because some folks test by just listening for a sound on both.
The LP (Lift Pump) should be tested by a pressure gauge after the rubber hose ends at the ip inlet. It is done at this point to ensure not just the LP but the interior of rubber hoses in all locations are not having issues. MANY MANY ip have been replaced while good because the inside of the hose is deteriorated restricting flow and people replace both LP then ip then have same issue. The proper pressure is 8-14 psi. But as long as you have a couple pounds- it should be able to start and idle ok. When it goes into the negative it causes major damage to the ip & imo is the #1 cause of premature failure of ip. For this reason and speed of diagnosis my long term suggestion for owners is a metal T fitting attached to the ip inlet acter rubber hose ends. The 1/8” port on the T fitting is for a fuel pressure sensor and a permanent mounted gauge in the dash so you see it while driving.
At this point- run a jumper wire to the LP so it is on constant and read the pressure.
You are not getting fuel flowing through all the time like you should.
The FSO (Fuel Shut-off Solenoid) is in question at this point. It could be the electricity is not feeding it properly, or it is not operating properly, or fuel supply to the ip is inadequate.
Your FSO works like this: there is a plunger on the end inside the ip. Key off it sticks out and stops the fuel from leaving the ip. This restriction does not allow new fuel in and shuts off the engine. Pinching the 1/4” return line closed does the same thing. If you ever turn the key off and the engine won’t die- go pinch the 1/4” hose and the engine will cut off.
When the key is on, the plunger in the solenoid retracts allowing fuel flow through the ip and out the injector lines at roughly 4,500 psi- so never put your fingers infront of the metal injector lines.
Removing the FSO is done by unplugging the wire and unscrewing it like a bolt.
With it out & the key on, fuel will pump out that opening. If you unscrew it part way, a little can leak out, but will allow fuel flow through and start the engine-
Also by removing it- you can turn the key on and see it retract- but the LP should pump a bunch of fuel out the threaded hole when this occurs so unplugging the LP first is advisable- unless you want a messy 2 for 1 diagnosis.
You can remove the FSO place the body (not threads or plunger) on the negative battery post and add a small jumper wire from the positive post to the connector and see it activate. Those are the basic versions of testing it most folks do… but seeing it work in place is simply - turn on the key, jumper the LP and watch fuel flow through the clear 1/4” line & knowing you have proper inlet fuel pressure- tells you all that is working properly.
Anytime there is anything in question with the engine running right- step one from GM is clear line test and step 2 is fuel pressure at the ip. So I harp on gauge In dash & leaving the return line clear. Fuel-line.com sells clear line that is rated fuel line to withstand the ethanol in our diesel- it isn’t rated for things rubbing against it so just be intelligent enough when installing it to not let it rub anything. The stuff for the hardware store lasts a few years and needs replacing. Either one works.
Please describe your testing of these in more detail. You need to be 100% about these before going down the rabbit hole you are faced with next.
