I’ve had this issue on a 1992 Ranger 4.0L OHV 4x4 I recently purchased from an auction site.

Main symptom besides the fuel pump continuously running with KOEO is that once I get it running by spraying carb cleaner in the air intake is that it runs super rich. Idles fine, under load it will sputter and die unless I feather the accelerator.

Of course I replaced the commensurate amount of engine sensors related to air/fuel mixture a person who doesn’t know what they are doing is required to do in order to find a solution, but to no avail.

After much research, even though I had a newish Mexican ECM rebuild from RockAuto I decided to have the ECM rebuilt by Keyworks in Richmond, VA (he has a YouTube channel). He replaced a few capacitors. The issue stopped for about a week. I emailed him and he said he stands behind his work and I could return it so he could verify his repairs, I said hold on as I wanted to dink around with it first.

while checking for shorts in the engine bay I decided to pull the fuse going to the fuel pump relay. The engine ran for another 10 seconds or so, then I put the fuse back in and the problem disappeared.

it’s been running fine all day, no engine codes, no problems accelerating. I expect the problem to return, but at least I know how to kick start it now.

Does anyone with electrical know-how think they know what the ultimate solution might be?

 

You've blended your issues together. Which one are you worried about? Running rich or the fuel pump? And you said "to no avail" but followed with "the issue stopped for about a week". Contradiction.

If it/they comes back and there are codes, report the codes. By number, not what the internet or some code reader says.

Might be a harness problem since messing around with fuses "fixed" it. Good luck.

 

The problem of running Rich and fuel pump continuously running appears to be one and the same as the richness is caused by unmetered fuel flowing to the engine.

this is a 35 year old truck and the OBD1 system is not as precise as the modern computers. It was not giving me any codes.

“To no avail,” not a contradiction, a shotgun replacement of new sensors did nothing to fix the problem. It was only after the ECM repair that the problem was resolved… for about a week, then it returned. It was by accident, after I pulled the fuel pump relay fuse… letting it run for about 10 more seconds, allowing the engine to die naturally… then replacing the fuel pump relay fuse did the issue “resolve” itself. I say “resolve” in that I expect it to return, but I at least know now how to kick start it again somehow by interrupting the power flow to the relay by pulling the fuse.

after this repair I used my OBD1 and ran it through the cycles. No codes appeared, nothing new, nothing stored.

so my question still remains, for those with some experience in this related issue, do you think you might know what is causing this?

 

The fuel pump just supplies volume and pressure to the fuel rail. The pressure is controlled by the fuel presuure regulator. The pump can run for hours/days with no fuel entering the intake manifold.

The air-fuel ratio is controlled (metered) by the computer using the signal from the mass air flow sensor and other sensors. The fuel pump is essentially independent of the computer. The computer just expects a constant supply of known fuel pressure. The only electrical component of the fuel pressure is if the pump runs or not. It runs at constant speed, constantly, when the engine is running.

How do you know that it is "running rich"? Your symptoms skew more toward lean than rich. Needing starting fluid to get it going. Are you smelling fuel? Lean misfires can allow unburned fuel to pass through the engine.

Might help to list the sensors that you replaced. A shorted coolant temperature sensor would cause the engine to run lean as the computer thinks it is very hot. There are diagnostics you can do before replacing parts. They'll help you zero in on the cause.

Although there is one thing that could cause a hard starting condition from the fuel pump running when it's not supposed to, before the engine starts. If there was a small leak in the fuel pressure regulator diaphragm, loading up the intake with raw unmetered fuel. That would be an odd combination of two defects though.

 

I see what you are saying. Gives me food for thought and reminds me of how much I don’t know… I had just made several assumptions… but one assumption that holds merit is that there still appears to be some correlation between the fuel pump relay and my issue as when the relay gets reset when I pull it’s fuse the fuel/O2 mixture also appears to change and the engine behavior changes. Rather than sputtering when I accelerate the engine begins to accelerate normally with no hesitation.

And it does run rich, not lean. The extra gas fouls the plugs with soot and the exhaust spits out soot and smells of unused fuel…

 

There is a timer inside the computer that is supposed to turn off the pump after a couple seconds. Have you checked the fuel pressure regulator to see if the diaphragm has ruptured? That will allow unmetered fuel to be drawn into the intake manifold through the vacuum hose.

The computer is constantly adjusting to maintain the correct air/fuel ratio. If you have MAF that is a more precise than Speed Density. No codes as in it won’t self test or are you getting the system pass code “111” for both KOEO and KOER tests?

There have been a few threads in the 1987-96 F-Series Forum where replacing the capacitors did not fix the problem because the fuel pump timer section was bad or there was something else wrong with the computer not on the circuit(s) with those capacitors.

 

The regulator and MAF are new. It’s an odd intermittent problem now.

I'm still going to check for shorts between the relay-fuel pump-ECU when it stops raining. Sometimes when I pull the fuse to the FP relay the FP stays running, sometimes it stops. Sometimes the engine starts normally as the FP isn’t stuck on, when it does this I start it by spraying carb cleaner into the air intake. After a few minutes I can feel the engine chirp and the CEL goes off and the FP flow normalizes.

truly odd and not consistent. The man who replaced the capacitors on the ECU suggested I change the relay, which I did, and the problem was solved for a few days. He also mentioned that there might be an issue with the ECU’s logic… not knowing computer-speak I assume that is what you were referring to in your comment. I don’t know how to verify that other than replacing the ECU with a known good one and going from there.

 

I don't know the circuitry on the fuel pump relay but I wonder if you're resetting the PCM when you pull the fuse. Going to open loop control, then when the PCM goes back to closed loop, using the O2 sensors, the problems reoccur. It might be that the two issues aren't actually related. I don't know how you'd determine that on 1992 PCM (ECM)/computer).

Have you pulled and examined the fuel pump relay itself? Do you know if the relay is PCM controlled?

I thnk that you need a 1992 Ranger wiring diagram and or service manual to understand how the engine control system works.