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Fuel delivery issue, or something else?

RangerX

Elite Ranger
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Elite Explorer
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City, State
Omao, Kauai
Year, Model & Trim Level
'93 Ranger XLT 4X4
Issue is with my 94 B4000 truck, 4.0 ohv, 5speed.
When I turn the key to start it, it will crank and crank and finally barely start running weakly until I bump the pedal to give it a little gas, then it roars to full running condition. Is this fuel pump, or something else?
When I’m driving, it drives relatively okay under a little throttle foot, but if I give it more throttle to accelerate or get up a slight hill, it will have hesitation, like no power for a split second, every couple seconds.
Overall, it seems to me like it doesn’t get enough gas, both when starting and when hesitating on the road.
I did replace the fuel filter.
Do fuel pumps get weak? I thought they either ran 100% or they didn’t run at all. Is this a sign of a slowly failing pump?
Or, possibly a vacuum issue? I do have a few of the smallest vacuum lines broken.
Trying to figure out which direction to look towards.
 



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fix the vacuum issues first
check fuel pressure at cold start
yes fuel pumps get weak FOR SURE

It sounds like weak pump....poor manifold vacuum doesn't help with the fuel pressure blips since the FPR runs on engine vacuum.
Long crank to start
Weak under load, popping/misfire through the intake is usually a lean condition
All signs point to weak / low fuel pressure / pump or plumbing problem w fuel pump/sending unit
Your pump may be supplying enough fuel to start and run but under any kind of demand/load it starts to lose pressure... at 36 psi the fuel rail operates perfectly at 32 is still operates okay until you mash the fuel feed, and at like 28 psi the injectors will start to drip instead of atomize.....make sense?
 






Yes, makes total sense, thanks Jamie. Wasn’t sure if pumps can get weak, now I know. Plus I did not know thr FPR is vacuum powered.
I’ll fix those small lines first, but I figure I’ll be pulling the bed to do the pump, too.
 






Pump relay can also get weak and not send full current. Really notice under a load. Took me a while to figure that one out. Swap in another, and see If it helps or not.
 






Before I drop the tank/pull the bed on these trucks I check for battery voltage at the pump power wires
if the voltage is less then the battery then you can work on the interia switch/fuse/relay to find the cause.
This has saved me from dropping quite a few tanks because the issue was power not the pump

Removing the bed on a ranger is toooooo easy!! However sometimes we just cut an access panel if you plan to keep the truck forever, have a ton of crap in your bed, like tool boxes, carpet kit, camping gear, camper shell, ladder rack, drop down dvd player, led bed lights, etc etc and you do not want to mess with all of that stuff.....just cut yourself a fuel pump access hole and make a new cover.
This makes fuel pump replacement back in the woods an easy chore instead of a nightmare
 






Pump relay can also get weak and not send full current. Really notice under a load. Took me a while to figure that one out. Swap in another, and see If it helps or not.
Not sure what that is or where it is. I don’t see it mentioned in my Haynes manual. Is it something on the fuel sending assembly? Or elsewhere? At the inertia switch? I see a fuel pump fuse on the under hood fuse block, but you probably don’t mean that...
 






Before I drop the tank/pull the bed on these trucks I check for battery voltage at the pump power wires
if the voltage is less then the battery then you can work on the interia switch/fuse/relay to find the cause.
This has saved me from dropping quite a few tanks because the issue was power not the pump
Where exactly would I test power on the pump power wires?
I do have several other electrical issues on the truck, so maybe it is power, not pump.
 






I'd say check for key on power at the inertia switch then work from there. It should have power for 2 seconds when you first turn the key to on.
 






Your vehicle manual, should tell you the fuel pump relay location in the power distribution box, under the hood. It's one of the larger relays, and you should be able to swap it for another in the box to test it.

The stock relay was causing the pump to starve under any type of a load, as it just wasn't sending full power out. I bought a $15 lifetime relay for it, and keep a spare in the glove box. If it goes again, I can make it to the store to exchange it.

The heat and age will deteriorate harness wiring under the box, and the relays themselves. Some stock relays are white underneath, and these make it easier to tell if heat, has been having it's way with them. The white will start to turn brown.
 






the fuel pump power wires are EASY to identify back at the tank they are the only two wires that are larger then the others
Usually yellow with white and black with white

I agree with these other dudes check the relay, fuse and intertia switch connections.. as these trucks get older the power dist boxes develop corrosion on the blades and the chances of a pin being pushed down into the box are greater. sometimes just pulling the fuse out and putting it back in 10 times will clean the blades enough to get power back
Also the fuel pump wiring connectors back at the tank, more then once I have seen a pin pushed in past the locking finger...making up intermittent contact
 






I just took the relay out of my Ranger, which is a known good and working relay, and put in in the Mazda, no change in the poor running. The Mazda relay looks in really good condition btw, better than the Ranger’s. But I still have not fixed the missing small vacuum hoses, so...
Another thing I did not mention, I cannot hear my pump prime when I turn the key on. I do hear a light electrical click coming from back there, but not the obvious whirr that I hear on my Ranger, and on my Escort.
So can the pump really not be priming and still be able to run and drive? If so, would that indicate clearly that it’s a failing pump, or could it still be caused by an elctrical issue of not enough power flowing to the pump?
I have not checked the power at the emerita switch or pump wiring, as I’m not smart enough to do that. :p
 






Also, @410Fortune when you say “Before I drop the tank/pull the bed on these trucks I check for battery voltage at the pump power wires” And I when asked where exactly to check them, it sounds like you’re saying to check them at the pump? That would involve dropping the tank/pulling the bed! Would those wires be the ones running from the enertia switch and they could be checked there, or somewhere more easily accessible than right at the tank and pump?
 






Sounds like a weak pump to me, but you need to make sure its not weak voltage causing the weak pump.
Easiest way to test his is at the tank wiring plug in itself, everything else the inertia switch, fuse and relay are upstream. So if there is a voltage issue it will show itself at the pump and you can work backwards from there.

There is a wiring plug just upstream from the pump, you know the 8 pin grey male and female plug where the pump plugs into the truck.

On an explorer it is just above and in front of the rear drivers tire.
I do not even have to remove a tire, just reach up there and unclip the wiring harness from its plastic keeper so you can unplug it and check power coming from the truck side of things.
On a Ranger its near the frame rail and front of the tank, you can access it without lifting the bed....

Then you can apply power to the pump side of things and see if the pump runs (jumper box)
Just a final test before actually pulling the bed/or dropping the tank.
Many times a truck that came in for a new fuel pump ends up just getting a fuse or relay..

I have a spare wire connector we use for this, I can plug it into the fuel pump and use my jumper box to run the pump. This is how we empty the fuel tank on trucks we plan to crush, without putting a hole in the tank like the junk yards do and without having to drop the tank. Also it usually gives me enough fuel to run my lawn mower, water pump and log splitter for the season :)

What you need to see is a 94 Ranger/Mazda fuel pump wiring diagram, it will make it more clear for you on what wires to test and where they are available....

Basically it goes something like this:
the Fuel pump fuse is powered by the power dist box. The fuse sends power to the power side of the fuel pump relay. The ground also comes from the power dist box.
The relay is triggered by the PCM fuel pump control wire, AFTER it goes through the inertia switch,. Then the relay acc wire turns on when the relay is triggered and runs back to the in tank fuel pump.
 






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