Possible faulty fuel pump relay. 4.0 1992 ranger

[Fast Doc]

Good evening gentlemen.

I would like to respectfully ask for some help fixing a problem.

I have a 1992 Ranger STX, 4.0, four-wheel-drive, 135,000 miles, all original, basically mint condition, daily driver. I love my little old truck.

I’m getting a check engine light lately. It comes on occasionally, usually after about 15 to 30 minutes of driving. The truck starts and drives and runs what seems to be perfectly. No change in drivability.

Once I shut the engine off and turn it back on again, the light goes out, but it may come on again next time I drive it under similar circumstances.

I ran the codes, and it is telling me that I have a lean cylinder bank, and that there is a fault with the fuel pump.

I ran the codes through ChatGPT and it seems to think that I have a faulty fuel pump relay. OK, good enough.

The problem is is that with this particular Ranger, it is not a separate relay, but it’s integrated into some other electrical goblin that can’t be readily replaced.

ChatGPT tells me that there is a bypass where I can bypass the relay, and somehow install another relay in its place and if this is a common problem with this era Ford and this is a common repair.

Is anybody familiar with this? I’d appreciate any input. It doesn’t look like a terribly difficult thing to do, but it looks like it would take at least several hours and before I invest the time I want to get your expert opinions.

Thank you in advance.

 

[lil_Blue_Ford]

AI is unreliable at best a lot of the time. You’re going to have to do more diagnostics than pulling codes and messaging AI.

I’ve never heard of the fuel pump relay in a 92 Ranger being integrated into something else. The relays are under the fuse block in the engine bay. Pretty easy. There’s two retaining tabs to remove the fuse block and two retaining tabs under that to release the relay block. I’m not sure your problem is relay related though.

You need to do a fuel pressure test. That will tell you the health of the fuel pump. Also pop the vacuum line off the fuel pressure regulator and see if there is fuel in the vacuum line.

Check the injectors. With the engine running, carefully use a long extension or screwdriver or something. Touch it to each injector and listen. You should hear a quick tick-tick-tick-tick as the injector cycles.

When was the last time the fuel filter was replaced? 20-30k is the recommended change interval.

When was the oxygen sensors replaced last? Generally they are considered to last 10 years/100k, whichever comes first.

The computers in these are highly susceptible to capacitor failure that causes all sorts of problems. I’d be inclined to pull it (passenger side kick panel under the dash), pop the cover, and take a peek.

There’s lots more I could suggest but we will start here…

 

[Fast Doc]

Thank you so much for your thoughtful reply. This is exactly the sort of thoughtful information that I was hoping to get here.

The fuel pump was changed about 40,000 miles ago, the air filter is clean, I replaced the fuel pump about seven years ago. It did not need to be replaced, but I needed to drop the tank to repair a brake line so I figured while all the work was done I put a new fuel pump in.

I did not check the fuel pressure. I do have a mechanics stethoscope so I could check the fuel injectors. I will also check the relays, now that you told me where they are. There is a rectangular box underneath the heavy duty fuses under the hood, and I actually suspect that that’s where the relays were, but AI steered me off and what appears to be the wrong direction.

 

[Fast Doc]

As far as the oxygen sensor goes, I have never replaced it. I’ve had the truck about 17 years, and I have no history on it before that.

 

[lil_Blue_Ford]

Unfortunately with parts quality being what it is these days, just because a part was recently replaced doesn’t necessarily mean that it’s good, so proper diagnostics requires verifying that things are works as they should.

 

[Curious Hound]

Sounds like Lil_Blue has you covered. Check fuel pressure while running. It could be the fuel pump relay. But usually if a relay fails, it just fails, meaning the fuel pump would not run and the engine would stop. A dirty fuel filter, bad fuel pressure regulator, bad injector(s) or failing fuel pump would most likely cause lean conditions.

If you replace oxygen sensors, you have 2. One in each leg of the Y pipe coming from the exhaust manifolds. Both same age Replace both. The Oxygen sensors are what give the lean/rich info to the computer.

You can take the vacuum line off the fuel pressure regulator and see if it smells like gas. Gas in that vacuum line would indicate a broken diaphragm in the regulator. Replace regulator. But it can fail other ways also.

 

[Fast Doc]

Thanks for your help guys. I’ll start by replacing the relay. I identified and removed it will see if local store has one. I’ll also pick up a fuel filter. If that does not solve it I’ll go after the sensors. If that does not solve it I’ll bring it in to somebody else. The issue is few shops here want to work on something 34 years old.

 

[Fast Doc]

A quick update, maybe it’ll help the next guy. I replaced the relay. I couldn’t find a 4 pin one so I used one with five pins. The fifth pin just inserts into an empty space on the holder so it doesn’t really matter. In any event, it seems to work fine, no check engine light. I’ll let you know if something changes, but hopefully this help the next guy.