I have a 92 with 124,000 original miles on it. The pump has been messed with some by the original owner but it is running pretty much stock as to settings. It has the diaphragm lift pump and I have a fuel pressure gauge tapped in where the bleeder screw was above the fuel/water separator. For the first year or so I ran it the fuel pressure would stay at a steady six pounds. Then it began to bounce, eventually going from 0 to 6 or 7 with every revolution of the engine. I pulled the lift pump and found a tiny piece of rubber in it, bled it out and no problem,,, for a short while. After a little driving the problem returned. Then one day the pressure dropped to zero so I nursed it slowly to the house and replaced the lift pump. Worked just fine with a rock steady six pounds for about 100 miles and then went to bouncing again. Since then I have replaced the filter in the tank, examined the fuel pick-up in the tank, replaced the line from tank to pump, replaced the banjo bolt on the return from the inj. pump that has the screen and the tiny hole in the side, tested with a different gauge, tried an Airtex E8153 electric pump but it would only maintain fuel pressure until I put the engine under a load, and hunted all over for some other check or regulator valve in the return lines. If I take things apart and then use the priming pump to bleed it out it will run steady pressure for a very short time but then go back to bouncing again. The engine runs just fine, starts the quickest of any diesel I have ever been around, and pulls well. Still, I know it isn't right and it is driving me to distraction.

On the Perkins 4-236 engines used in Chevy pickups in Brazil there was a spring-loaded check valve in the return line that would corrode and stick but I have not found anything like that in the Cummins system. Anybody know this system well enough to tell me where else to look?

 

Are you running a snubber?

 

I was thinking snubber too...
It will ease the spikes in fuel pressure.

 

I've read that not running one will also wear out the sending unit faster too if it's an electronic one.

 

Eventually they all wear out at least in My experience but a snubber can extend the life of the gauge, greatly.

 

If you mean by a snubber a tiny passage to the gauge, no, not more than what is on the gauge itself. To me that is like pulling the bulb out of the idiot light because you don't like looking at it.

A diaphragm pump should not cause a bounce in pressure if there is sufficient fuel there against the designed back pressure. Either the fuel is getting away some way, the cam or lift arm is worn causing a loss of pump action, or there is a lack of sufficient fuel flow to the pump.

According to the explanation in the Chrysler manual that pump should put out about two quarts per minute at cranking speed. It should build up back pressure causing the diaphragm to stay extended and as it comes back the lever works against it to supply more fuel. Since nothing has been done to increase the use of fuel on my engine the pressure should remain steady just like it did for a long time. Even now, if I take things apart and then prime it the pressure will stay rock steady for a time, maybe twenty miles at times, and then goes back to bouncing. Those spikes aren't just hard on the gauge, they are hard on more serious components of the system.

 

Well i don't know what to tell ya but even with my Raptor pump the gauge bounces if i open the snubber to much. And it's nothing like covering up and idiot light. It gives you a steady reading which in my case was about average or middle of where the needle was bouncing at.

 

I have had the same experience as 93-12Smoke, without a snubber with a stock lift pump on a P-pump 12V.
It smooths out the pulse so the gauge can read the average.

A little different that removing the idiot light, it is more like making the gauge usable.

 

Sounds to me like the replacment pump you put on is not a good quality pump and/or it is also starting to fail.

 

I would be inclined to agree with you but find it hard to believe on the third diaphragm pump in a row. I just purchased another pump and installed it, keeping the other as a backup. Same problem, if I disconnect anything in the system and then use the lever to pump out any air that may have entered the system the pressure will stay rock steady for a short time.

I did one more experiment to positively eliminate any possibility the problem was on the suction side by using a hose siphoning from a container to the inlet on the lift pump; gave the same fluctuation.

I then hooked the suction line from the tank back up and operated the lever a few times. When I started it the gauge went to a rock-steady six pounds and stayed there for several miles before pulsing once more.

If the fluctuation was normal it would do it all the time, there would not be a way to get that gauge to register a steady six pounds for a period of time. I don't have a lot of time to spend on looking but I will find it eventually.

 

I don't have a lot of time to spend on looking but I will find it eventually.[/QUOTE]

Have you tried a fuel pressure snubber yet?
Equalizing the fuel pressure spikes is what they are designed to do.
Or do you really not believe in them?
I would guess that will fix your problem, and it would be quick and easy...