I have a 1999 GMC 3500 Diesel that is having a fuel issue and won’t start/rather won’t stay running. About 65,000 original miles.
At first I determined it was the oil pressure sensor/switch was the issue, even sure of it. I took it off after buying a 37 dollar new switch and 35 dollar special socket. I took it off and the thing was broken, almost in half, and I just knew I fixed it. That is Until I turned the key and it just cranked as usual with no run.

I determined the secondary fuel pump or lift was not getting power. I then got the vehicle started running well by running a hot wire to the positive wire at the fuel pump. It completed the circuit the motor kicked on and then the vehicle started and ran beautifully. I took the hot wire off and it ran for a few moments then died again.

Definitely the problem is no power to the fuel lift under the seat. But how do I fix from here? I’m not good with electrical issues.

Here is what I’ve tried/know:
1. I looked at all the fuses and relays I could find. Can’t really tell if the relay is bad—-it clicks. The fuses look OK.
2. I noticed a mouse nest under the upper air intake. I vacuumed out what I could see and can’t really see any gnawed wires but I can’t see much either.
3. I put on both new batteries which were definitely weak (and I recharged them twice in a few days, another 200 dollars) and made sure the cables were good.

Any suggestions? Is it worth ripping the intake manifold off and looking for gnawed wires? Are even any wires there?

if someone tells me to start looking for a short I’m probably going to need a guide and how to...again not an electrician.

My plan B is to wire the hot wire to a toggle switch and limp it to Jacks Auto Repair shop, I’m retired so my income is somewhat limited.

thanks.....just for laughs here is a pic of the oil pressure switch...still can’t believe that wasn’t it681845F7-00B1-40BC-9781-F64227224220.jpg