1991 4.0L auto fuel pressure drops extremely low and engine sometimes stalls

[RANGERGOBRRRRR]

Engine rpm will drop down to about 600 and idle really rough and smell like fuel normal idle is around 800. I unplugged maf and engine seems to run better until it idles 1000rpm and had a constant misfire almost like its running on 3 cylinders. I suspect ecm it looks ok with no bulging capacitors and no hot spots. Fuel pump looses prime every other day. I suspect the pump gets hot and starts to loose its pumping abilities. Iac almost kills the engine when unplugged. Tps does little to nothing when unplugged. Maf unplugged seems to make the truck run better but my experience with an 01 mustang an unplugged maf basically stalls the engine or does stall the engine. Those options are in my head or I have a wire beginning to open or short causing low voltage to the pump. While the truck runs like garbage at idle if you start driving it tends to run better like the pressure goes up but still seems low on power and will begin to run bad when idling again. Maf iac and tps and fuel regulator have been replaced a year ago. every once in a blue moon if I let the truck idle for a long time then drive it, it will throw a lean code any help would be a great way to go id rather not go full parts cannon and would like some ways to test these things or if anyone has had a similar issue with anymore ideas.
New engine, iac, tps, map, fuel pressure regulator, injectors, coolant temp sensors(both), plugs, wires, intake temp sensor, o2 sensor, intake manifold gaskets, pvc valve, checked for vacuum leaks recently <all done within the last year
Fuel filter, coil pack, transmission, tranmission vacuum regulator <Within the last 2 years

 

[franklin2]

Pull the codes. But first pull the battery cable off for a minute or two, then drive it for a little while, and then pull the codes. You probably have plenty of memory codes in it right now since you have been disconnecting sensors.

Talk to the ECM, it may have a clue that will lead you in the right direction with the codes.

 

[mikkelstuff]

Do you have a gauge installed to check your fuel pressure? That should be checked.

 

[Shran]

The pump losing its prime would by itself make me suspect that to be causing your issue. Fuel pressure isn't supposed to just disappear... it either has to leak past the injectors, out of the regulator and back into the tank, or backwards through the pump, or externally. Insufficient fuel pressure would also cause problems at the injectors which could result in your lean condition code.

I'd get a pressure number as mikkelstuff suggested and plan on replacing the pump if anything looks abnormal. You should be seeing about 35psi at idle and up to about 40psi under low vacuum - blip the throttle and watch the gauge. Anything way under that or way over will cause problems.

 

[RANGERGOBRRRRR]

The pump losing its prime would by itself make me suspect that to be causing your issue. Fuel pressure isn't supposed to just disappear... it either has to leak past the injectors, out of the regulator and back into the tank, or backwards through the pump, or externally. Insufficient fuel pressure would also cause problems at the injectors which could result in your lean condition code.

I'd get a pressure number as mikkelstuff suggested and plan on replacing the pump if anything looks abnormal. You should be seeing about 35psi at idle and up to about 40psi under low vacuum - blip the throttle and watch the gauge. Anything way under that or way over will cause problems.

I had a video I was going to link but is to big even cut down to 5 seconds of my pressure gauge it was running fine at idle 29 psi constant even wuth throttle blipped then the Guage dropped rather quickly like 5-10 seconds to 15 psi ran like garbage then 11 psi and ran even worse stayed there for a while then pressure just shot up to 35psi within 2 seconds ran good then settle at 30. This was all with regulator vacuum unhooked and plugged. That's the first time the truck ran fine by itself after running like that usually it will run bad like that until you start driving but I did have the eec iv scanner hooked up to it I wonder if that had something to do with it making it an ecm issue.

 

[superj]

the pressure dropping low is a problem. pump or regulator seems like something is wrong and i would start at the regulator. make sure its good, check the vacuum line attached to it and check the regulator diaphragm by pulling the vacuum line off the engine and sucking on it. if you get gas, the regulator is leaking and needs to be changed. if no gas, put your tongue on it and it should hold tight, like a straw does if you pinch the end and suck on it. if it doesn't hold tight, you have a bad hose or regulator.

 

[RANGERGOBRRRRR]

The pump losing its prime would by itself make me suspect that to be causing your issue. Fuel pressure isn't supposed to just disappear... it either has to leak past the injectors, out of the regulator and back into the tank, or backwards through the pump, or externally. Insufficient fuel pressure would also cause problems at the injectors which could result in your lean condition code.

I'd get a pressure number as mikkelstuff suggested and plan on replacing the pump if anything looks abnormal. You should be seeing about 35psi at idle and up to about 40psi under low vacuum - blip the throttle and watch the gauge. Anything way under that or way over will cause problems.

The truck has also lost prime sitting for the entire time I've owned it for the past 2+ it has just started doing this garbage 2 weeks after swapped in a rebuilt 4.0 after the engine swap it had alot more power and ran fenominal never missed a beat and it purrs really quiet the 4.0 auto even had the power to spin some aggressive 33s on stock 3.55s and do a standing burnout if I tried

 

[Shran]

It is totally possible that you are dealing with an ECM issue but its only purpose in the whole situation is to control when the pump turns on... if I understand correctly, it basically just triggers the fuel pump relay. Find your ECM test connector - there is a wire on there that you can ground out to make the pump run all the time. In my mind that will more or less rule out the computer entirely if you still have the same issue.

In any case if you are seeing LESS than 35psi, AND you're dealing with the pump losing its prime, my opinion would be that you have a bad pump. Low pressure by itself could be a wiring issue but losing its prime is not normal behavior. In theory the fuel system should maintain operating pressure indefinitely after you shut the truck off... months or more.

 

[RANGERGOBRRRRR]

It is totally possible that you are dealing with an ECM issue but its only purpose in the whole situation is to control when the pump turns on... if I understand correctly, it basically just triggers the fuel pump relay. Find your ECM test connector - there is a wire on there that you can ground out to make the pump run all the time. In my mind that will more or less rule out the computer entirely if you still have the same issue.

In any case if you are seeing LESS than 35psi, AND you're dealing with the pump losing its prime, my opinion would be that you have a bad pump. Low pressure by itself could be a wiring issue but losing its prime is not normal behavior. In theory the fuel system should maintain operating pressure indefinitely after you shut the truck off... months or more.

I don't know how old it is and I have a feeling I'm having an issue with the maf a tps sensors as well do to the truck running better with the maf unplugged then plugged in