Erratic engine after New Fuel Pump Installed

[eightynine4x4]

1989 Ranger 4x4 V6 2.9
I picked up a reputable replacement for the fuel pump since mine was seemingly on its way out, and swapped it in. It’s much quieter than the old one, when initially turning the key and letting sit for a few seconds. So quiet I thought it wasn’t working. But I think it is.
But after install, now there’s some random lumpy behavior and surging going on. Sounds like someone is lightly revving the pedal down and back up, very briefly. Some times it’s quite jarring, sometimes it’s very subtle. I can’t find a pattern in the behavior... doesn’t seem to coincide with much, like engine temp or speed. It’s a bit nullified at speeds like 40-50. Maybe that means the fuel pathway is flushed enough for this to not be much of an issue.
Is air maybe caught in the line before the injectors? I didn’t bleed anything after I did the pump swap since the Haynes doesn’t mention bleeding anything out afterwards.
It seems to me like either A I have air stuck in there, or B I caused something in my work, oC I suddenly have to also replace the lines, or D, my new ish fuel filter needs to now be replaced again. I replaced the fuel filter six months ago already.
At low speeds, like idle, sometimes it’s perfect. But sometimes it’s pumping and bumping pretty intensely. I’ve driven around 75 miles so far and this issue comes and goes.
I’m not 100% convinced the pump is working great, because it’s so quiet VS the old one. I can’t here it really. But, the engine fires up well, generally, so I guess it’s working.

 

[RonD]

"reputable replacement" is only Motorcraft pump, so if it ain't that then replace it under warranty

If next one has same symptom THEN you can look at other causes

Why did you replace the last fuel pump?
Was it just dead or were there other issues, and you just changed the pump to see if that was the issue?

 

[eightynine4x4]

I replaced it partially to eliminate it from possible causes of delivery issues, but also since I know the last pump was putting forth a lot of effort for who knows how long since the main line was severely pinched between the tank and the bed frame. I have had ongoing startup sluggishness so unbolted bed to take a look at fuel area and saw the badly pinched line. Pump has always been quite loud, under my ownership for the last six months. So, like a power steering pump under constant duress, I figured a replacement isn’t the worst idea. So I freed up the pinched line and installed the new pump.
It sure is quieter, and I’m not convinced it’s even working. It’s not Motorcraft, but it’s covered under warranty so can return if need be. But I assume that if it’s not working at all then I wouldn’t be able to even fire it up. I am, however, also interested in knowing if the truck would still run without the pump working at all. If so, maybe this one is a total dud and that explains my surging.

 

[RonD]

You could try a fuel pressure gauge, 30-35psi is expected when engine is running
If pressure drops to 20 and back up to 30 that will cause surging

Engine can not run under 10psi so will not run if pump is off

 

[eightynine4x4]

I think I have it diagnosed..
Took it on a trip for a couple hundred miles on farm roads so was able to get some experience with the issue. What I’m realizing is that there is in fact air getting in the fuel line, and that I can coax it out by carefully pressing the shrader valve until it bubbles for a while and stops bubbling, then it’s good to go and the engine runs great, BUT, that the spot where the truck frame had been pinching the line near pump for many years, and which I had recently freed up and massaged to be open, very likely has super small cuts or cracks from the abuse and so air will continue to enter the lines especially during higher speeds and the problem will need to be bled out all over again.
It’s the only thing that makes sense and it’s the pattern/theory that I found repeatable.
Also I’m pretty sure that the new fuel pump is working fine, but I’m going to pick up a fuel pressure test kit just to be sure. If not, and after this system is refreshed it doesn’t test well. I’ll put the old OEM back in.
So now I have to decide what fuel line(s) to replace. It’s an 89 4x4 2.9 short bed. Anybody know what I should expect to see when I start disconnecting? I’m guessing there will be one kind of line that’s meant to go from the pump, and one kind of like that’s meant to enter the filter, and some kind of juncture between? It would be great if there was just one long line.
Also, I think i need to replace the shrader valve piece. I was very careful but after touching the pin I was not able to get it to stay fully “closed” and fuel would slowly leak out the top no matter what. I bought some chrome tire valve caps in hopes they would fit and they do the job well. The valve doesn’t leak anywhere else, just the top, so I could roll with this cap for a while. Curious if anyone else has this same issue and requires the cap for it to not leak a little bit.

 

[eightynine4x4]

Replaced the pinched segment of fuel pump output line and securely spliced it into the OEM line. Also replaced the very worn out looking return line segment that connects from the long return line. Both are well secured.
Tested fuel pressure at schrader valve and it tests perfectly at 40 psi. Also ran the engine and it sits where it should.. about 35 psi, which is in the range spec’d in the Haynes chapter.
Drove it around and it works fairly well but there’s still air in there somewhere. The hope is that it’s not continuing to get in.
I stopped somewhere and bled some fuel at the schrader valve into a rag. With the KOEO, Air bubbles continued to come up for a few minutes at least. Then it stopped and only fuel came out. This was promising.
Then, the fuel stopped coming out and a steady stream of air fizzed for 30 seconds, then fuel for 10 seconds, then air fizz for 30 seconds again, then fuel for 10 seconds again, and it kept repeating indefinitely.
I drove it around again and once in a while I’d still get some lumps and surges.

Im replacing the schrader valve with a Motorcraft part today. Something tells me this isn’t going to be the fix, but maybe it could be. Maybe the schrader valve and pull IN air if leaking? If so, the this could be the final fix. I have been assuming the air would only ever come out there.
I’m also curious about doing all this with the regulator kept manually open by way of a vacuum pump hooked up to it.
Ready to replace the fuel filter as well, and will make an attempt to fill it and cap it and see if I can install without getting in.

 

[RonD]

Try testing with a FULL gas tank

The fuel line in the gas tank maybe the issue, cracked, so when uncovered its pulling in air as fuel flows passed the crack
Filling up the tank would cover it up so no air could come in

You tested fuel pressure, so does pressure hold above say 15psi after key off, it should hold pressure for MONTHS
If its dropping to 0psi that would indicate a leak in the pressure side, pump to FPR, and if you don't SMELL gasoline then it must be leaking inside the tank

 

[eightynine4x4]

Try testing with a FULL gas tank

The fuel line in the gas tank maybe the issue, cracked, so when uncovered its pulling in air as fuel flows passed the crack
Filling up the tank would cover it up so no air could come in

You tested fuel pressure, so does pressure hold above say 15psi after key off, it should hold pressure for MONTHS
If its dropping to 0psi that would indicate a leak in the pressure side, pump to FPR, and if you don't SMELL gasoline then it must be leaking inside the tank

Gotcha, I’ll hook up the gauge again and let it sit all day after the test.
The tank was around 7/8 full when testing. It’s around 3/4 now. The whole pump mechanism is brand new though, so that means all lines inside tank are new, correct?
In any case, if the pressure remains around 15, does that rule out the whole pressure side from having a leak? My concern is that none of this testing reflects wether or not air is stuck in there still. Pressure is pressure and will measure fine even with some air pockets, I’d think. So even if I’ve corrected leaks now, I could chase air for a while.

 

[RonD]

New pump could also be the problem, if this symptom wasn't there before its installation

Pumps can get/cause cavitation which generates air bubbles/vapor bubbles, in the pressure line

This type of fuel pump uses suction to pull fuel in thru a filter(the sock) at the bottom of the tank, if it can't pull the fuel in fast enough then cavitation occurs, so vapor bubbles in the pressure line
I assume the new pump assembly came with a new "sock" filter, and there was no issue installing the assembly, i.e. you didn't need to "make it fit" when putting the ring on, so didn't force pump down at bottom of tank restricting flow into its pickup

Discharge cavitation occurs when the pump used has a higher flow rate than the system can dissipate, "high flow" pumps are fine as long as the system can use or dissipate(return) un-needed flow to tank
Long shot on this one because flow would have to be less than 10% of pumps flow rate

And it could just be a bad "new" pump assembly, happens alot with 3rd party assemblies

Most Rangers used a 40 gallon an hour pump(155lph) which were good up to 350horse power

 

[eightynine4x4]

New pump could also be the problem, if this symptom wasn't there before its installation

Pumps can get/cause cavitation which generates air bubbles/vapor bubbles, in the pressure line

This type of fuel pump uses suction to pull fuel in thru a filter(the sock) at the bottom of the tank, if it can't pull the fuel in fast enough then cavitation occurs, so vapor bubbles in the pressure line
I assume the new pump assembly came with a new "sock" filter, and there was no issue installing the assembly, i.e. you didn't need to "make it fit" when putting the ring on, so didn't force pump down at bottom of tank restricting flow into its pickup

Discharge cavitation occurs when the pump used has a higher flow rate than the system can dissipate, "high flow" pumps are fine as long as the system can use or dissipate(return) un-needed flow to tank
Long shot on this one because flow would have to be less than 10% of pumps flow rate

And it could just be a bad "new" pump assembly, happens alot with 3rd party assemblies

Most Rangers used a 40 gallon an hour pump(155lph) which were good up to 350horse power

After almost two hours of sitting with key off, pressure gradually decreased and is now around 6psi. It was a very very slow decline and not visible to the eye.
So I have a small leak on the pressure side somewhere (unless the gauge is imperfect)
I did replace the schrader valve prior to this test, and this one doesn’t leak with KOEO like other one had, so that’s good.
When I installed the pump, it didn’t have height issues. I did make the mistake of installing it 90 degrees around, which due to tank shape inhibited the lever from going up so the tank read 1/2 when it was full and then read correctly from the 1/2 point and down. But i corrected the position and it reads and performs correctly now. But this has nothing to do with the sock. The sock is similar to the Motorcraft in size and in distance from the top of total mechanism.

When I installed a new fuel filter last fall, I had some struggles with the connection. The struggle was that j was never able to get the entry point disconnected once I connected it. I had wanted to detach it just to reattach it and see how it worked, but I could not get it out so I let it be. I’ve never smelled gas down by the filter though.
I have another new one on hand and may as well give it another go. Maybe I’ll improve the connection.
Also, when I turn to KOEO, the pump first reads 40 but then drops gradually. Should it hold at 40 forever if key is on?

 

[RonD]

No, if it was 6psi it should actually only go to 16-20psi, so thats odd

With key on computer boots up and turns on fuel pump for 2 seconds, and ONLY 2 seconds to Prime the system
It does this 2 second thing each time key is turned off and on again
This 2 seconds is usually about a 10psi increase

Computer will not turn on fuel pump full time until engine RPMs are above 400, cranking speed is 200rpms
This is a safety feature, if fuel line should break, in an accident or ??, the engine would stall(under 400rpms) and electric fuel pump would shut off preventing it from feeding a fire or starting one, lol

Anyway that jump to 40psi is a little odd
You don't have a jumper installed in fuel pump relay spot or in the OBD1 connector do you?

40psi is correct max pressure with engine off and cycling the key on and off a few times, fuel pressure regulator is spec'ed at 43psi

 

[eightynine4x4]

Just wanted to wrap this thread up since it’s somewhat solved..
In the end I believe it’s so complex that it wasn’t just one issue but a multitude of bugs and cans of worms opened up by consistently using an old vehicle for the first time in who knows how long.
I can’t remember the sequence of events, but ..
I replaced the fuel filter which had been new six months ago and that made a very noticable impact (so probably gunk had been quickly collected recently during work stages)
I tested the new pump output by plugging the output of fuel regulator and got a whopping surprise of +100 PSI (pinned the gauge) and a brief burst of fuel as it popped off the rubber cap I had used as the plug, concluding that the new pump is strong and that it’s possible the fuel regulator was faulty and staying open a smidge sometimes
I replaced the fuel regulator and the short return line that feeds main return
I redid the connection to fuel filter which was a little janky (although never leaked or smelled)
I also happened to be wanting to try Rotella 15w40 for genera oil health concerns, so did that change out including filter of course
I replaced all the spark plugs and noted that #4 and #5 may need more regular replacement than the others, as they have a light amount of oil on them where as the others are bone dry.

Now after a few test drives in areas of steep slow going hills, I don’t have any moments at a stop light or stop sign when the truck wants to stall and the oil meter stays put. Startup is consistently good or good enough to feel it’s reliable.

So I think this is all a symbiotic situation of dealing with an old vehicle.. fuel pressure, oil pressure, spark, all playing their part and all needing improvement.