Werner1111

I was driving my rig yesterday and noticed that the fuel pressure had dropped to an indicated 1.5psi, which made me worry, however it eventually went back up the normal 5psi, per my ISSPRO gauge. I have a stock lift pump, but it looks like it may have been replaced at some point. The PO seemed to maintain the truck well. A little more color on the day:

1. I started the morning in the driveway at 5psi.
2. I drove about 60 miles to a meeting, from Los Angeles to Littlerock (high desert).
3. After the meeting, I stopped and filled up with fuel.
4. I honestly did not pay attention to the fuel pressure since I left in the morning.
5. After filling up, I decided to take the scenic way from Littlerock to Los Angeles and headed through Angeles Forest. As I started to climb up the mountains, I thought to look at the EGTs and noticed that the fuel pressure was at 1.5-2 psi.
6. I pulled over and checked for leaks in the engine bay, but saw none.
7. I was already out of cell phone range, so pressed on. There were no operating symptoms (e.g. loss of power).
8. Over the next 10 minutes, the pressures rose back to about 3 psi, while I was still climbing. On the way down the hill, the psi stayed at about 3psi and remained between 3-4 for the rest of the uneventful trip to my office.
9. Later that evening, at about 6PM, I started my rig and the pressure was at 5 psi. It was at 5psi this morning.

The two things that stick out in my mind are 1) I filled up my truck with fuel just before I noticed this happening and 2) I probably got to >4,000 ft elevation. But the indicated low fuel pressure continued when I got back to sea level.

I've been keeping the THD HVLP fuel pump in the back of my mind. Maybe it's time to order, just in case.

I'd appreciate any thoughts on what may have caused this and anything I should be looking for. Thanks all.

oliver foster

This has to be Andrew from LA.
I just sent you some interior headliner trim parts for that great looking Candy Cane colored '93 of yours..right?
Welcome to DTR!

When was the fuel filter last changed?

That PSI is a bit low, but the motor should not be harmed as long as there is a positive head pressure at the pump.

I just installed a couple of the THD HVLP lift pumps on my trucks. They hold a stead 10 PSI pressure at idle. I will need a few weeks of nice weather to wash away the salt off the roads to really road test them.

Werner1111

Yep, Oliver, it's me. Thanks for the info. I'll change the fuel filter b/c it has not been changed since I've owned it (about 2,000 miles).

Thanks again for the trim parts.

oliver foster

I am glad I could help you dial in that great looking truck of yours a bit.

Another possibility and a problem I just had on Poncho, my '91.0, is the fuel PSI was occasionally reading low. It even stalled a few times on me.
As a test I tried running the truck of a different tank of diesel with a new line. The pressure held steady at 10 PSI on the new tank with a fresh length of fuel line.

This made it apparent that I had a small air leak somewhere in the old fuel line or the pick up from the tank. After testing the line and the fuel pick unit for leaks. I replaced the two rubber hoses on the ends on the metal fuel line from the tank and to the LP. The problem is now solved.