454 TBI fuel pressure drops

[Fxrtharley]

Hello, I have had intermittent power issues on my 1990 454 p30 chassis with a tbi. When I first got the truck in 2022 I dropped the tank and changed all of the fuel lines and the filter. The rig ran good but did start to lose power on occasion. Using the process of elimination I started with adding an inline pressure gage between filter and tbi . It seems when the fuel tank is full it runs better and the psi goes from 10 psi to 8 at acceleration.
This weekend was the worst, we were going over some steep and long hills and the rig would go down to 4-5 psi and 40 mph max. Also the tank was probably 1/3 full and the engine bucked a little and was constant 4 psi.
After I filled with fuel, (63 gallons) it ran better but still went from 10 to 8 psi at acceleration.
I did change the fuel cap also but did not help.
I know there are many pieces to the fuel delivery starting with the fuel pump and I certainly do not want to pull the tank again. If there are other things to look for please advise or if you know of an inline fuel pump setup please advise as well.
Also is there a way to to measure the health of the fuel pump without taking out the 80 gallon tank?
Any help would be appreciated. thanks

 

[Schurkey]

Best way to measure the health of the fuel pump involves testing it's inputs and it's outputs. Most folks won't have the equipment to test it's electrical inputs properly.

By comparison, the output is fairly easy--you've pretty-much done that already, and found it weak.

But AT MINIMUM you still need to measure voltage delivery to the pump, with the pump RUNNING. Take that measurement as close to the pump as practical, which is generally at the rearmost wiring connector before the harness goes up 'n' over the fuel tank--and hope there's no problems in the wiring harness after that. For the record, the in-tank wire harness is frequently degraded. You'd be looking for as close to battery voltage as possible--keeping in mind that GM often uses undersized wire so some voltage drop (VD) is inevitable.

You also need to take a voltage reading on the ground wire as close to the pump as practical, with the pump RUNNING. You want to see as little voltage as possible, keeping in mind that the pump is usually grounded to the frame, and steel is a crappy conductor. One volt or less should be OK.

If you have access to an oscilloscope, you'd use a low-amps probe on the supply wire, and you'd see the amperage draw of each individual bar of the fuel pump armature. If they all look about the same, and the amperage is reasonable, great. If one or more bars is significantly different from the others...the motor on the pump is defective and it's time for a new pump assembly.


It's also possible to calculate the RPM of the pump motor from the oscilloscope pattern, and if you have an idea of the proper RPM, you can verify that, too. But most of these specs won't be in the service manual. You're on-your-own.

 

[someotherguy]

Beyond pump health, sounds to me like the in-tank baffle is broken, or loose, so it's not doing the job. Vehicle on incline + low fuel level = pressure drops dramatically, there should be a baffle in the tank preventing fuel starvation. Of course not even a properly working baffle can help you if the fuel level is extremely low; simple gravity is your enemy here as the remaining fuel will be at one end of the tank, away from the pump pickup.

I've never looked inside an RV tank so your results may vary. In a stock pickup truck tank there's a plastic tray inside the tank that is attached to the floor with push-nut style fasteners, to studs on the tank floor. Sometimes water in the fuel rusts the fasteners and the baffle gets loose. (The later solution was GM eventually went to the bucket style pump/sender assemblies typically seen on most Vortec-engine trucks.)

Richard