lift pump OK on start but not run

[PFahlberg]

My truck is a 92 GMC 6.2 "J" with OEM frame rail mounted pump.

Changed the fuel filter. Jumped pump fuse on firewall passenger side to prime system, plenty of flow. Hooked test light into pump fuse circuit, light illuminates when cranking, goes out when engine starts key switch released from crank position. With engine running, opened fuel bleed valve at front of engine, engine stalled. Dash has oil pressure gage which functions, shows normal oil pressure. Had helper listen to lift pump during cranking, pump works. Had helper listen to lift pump with engine running, pump did not work, jumpered pump circuit with engine running, pump works.

Tested fuel pump relay by removing CRNK fuse, applied 12V to purple wire on relay harness, relay clicks, lift pump operates.

My Chilton manual has diagrams which help me understand how the lift pump circuit functions when cranking, but the running circuit is not shown.

Anyone know what to do here?
Thanks in advance.

[HammerWerf]

PFahlberg,

First off, Welcome to the Diesel Page. Lots of good info in here, use the search function. Might want to expand the range of posts to look through by going to the bottom of the page and expanding the time criteria.

From your description, It sounds like your Oil Pressure switch (OPS) has failed. The OPS does two functions: 1) send engine oil pressure reading to the gauge. 2) it has a relay type function that powers the frame mounted fuel pump. Especially on the earlier 90's models, this switch has a high failure rate as full current is carried by the contacts. The switch is located on the back, top of the engine. Replacing the OPS is the best thing right now.

HammerWerf

[PFahlberg]

Hammer thanx for the reply. My next question is how did you find that out?

[HammerWerf]

PFahlberg,

There are a bunch of posts on this very topic in here. I've read them, and have been paying attention on my 99 Sub. My OPS is still operating OK since my fuel pump is running with the motor running. There are descriptions of how to test (what you did in describing your diagnostics) that help.

HammerWerf

[PFahlberg]

Hammer thanx for the help. I put a new OPS in, lift pump works.