odd intermittent fuel pressure problems

[janderson4010]

searched but not really finding anything that answers my question.

I have a 86 Bronco II that I swapped a 4.0 in, great engine and a real step up from the 2.9. I've been running it for about 2 years now but I had a problem follow me from the old 2.9, when I run it for a while and especially when I let it sit idling for a minute it will start running really rough, not rev up and eventually die, wait about 30 minutes and it will run again for awhile.

so after replacing the filter, both fuel pumps, and various ignition components, I went an picked up a HF fuel pressure tester and went to town with it. What I am coming up with is 31psi at idle when running normally, jumps to 39 when the engine is quickly revved up, when I turn the engine off I get very quick leak down, much faster than it should be, but I don't think that is my problem.

After I got a baseline I let it idle for a few minutes to induce the problem, and sure enough it started idling poorly and wouldn't rev up, the fuel pressure had dropped to a steady 9-10 psi, and wouldn't come up at all, at first bk of the pressure leak down I suspected the fuel pressure regulator was just bypassing all my pressure through the return line and back to the tank, but when I blocked off the return line with vise grips it it didn't change, just stayed at 9-10. I would expect if the regulator was bad it would go up when I blocked off the return.

so then I decided to see if the pumps were on, with it idling at the 10 psi I listened and couldn't hear the main pump running, and then I shut it off and I could hear the main pump come on with the key but not the lift pump, and I had nothing over 10 psi on the gauge. So in summary I have intermittent fuel pump operation.

The fuel pump circuit is pretty simple, and I know I have power to them bk I have at least one running even when the problem is happening. So what is the problem? I went through the wiring and there is no obvious damage, but I must be getting some kind of wiring or ground problem. But since they share a common ground I would have expected them to both shit the bed when the problem is happening. Anybody have any ideas on this one? I appreciate the help in advance, I've been fighting this problem for a few years now, thank god I don't rely on this thing as a dd. anyway thanks in advance.

 

[janderson4010]

Oh I did forget to say I cycled the key like 40 times and it did build full pressure once but it started having the problem again a few seconds after I started it

 

[janderson4010]

no thoughts on this one huh?

 

[RonD]

Remove the gas cap and test it.
You could be building up negative air pressure in the tank, which limits the fuel pumps ability to pump fuel out of the tank.

Did the B2 have an EVAP system and did you hook it back up, thats the "vent" for the gas tank.
If hooked up then the EVAP solenoid could be stuck, they can be clean in most cases.

 

[alwaysFlOoReD]

I had a problem with the hose connecting the pump to the line in tank. The previous owner removed both pumps and installed a mustang pump in tank but failed to use fuel rated hose. It was cracked. It doesn't sound like you have that same problem bk you do have full fuel pressure most times.
Can you volume/pressure test the pumps outside of the vehicle?

 

[janderson4010]

Remove the gas cap and test it.
You could be building up negative air pressure in the tank, which limits the fuel pumps ability to pump fuel out of the tank.

Did the B2 have an EVAP system and did you hook it back up, thats the "vent" for the gas tank.
If hooked up then the EVAP solenoid could be stuck, they can be clean in most cases.

I did not hook the EVAP system up, I think it was missing some pieces when before the 4.0 swap, but the main pump just flat out quits running, I can't imagine why that would be caused by no evap. if anyting I would think that would make the in tank pump quit. I had replaced the fuel cap when I had the 2.9 in there, but it wouldn't be a big deal to throw a new one on.

I had a problem with the hose connecting the pump to the line in tank. The previous owner removed both pumps and installed a mustang pump in tank but failed to use fuel rated hose. It was cracked. It doesn't sound like you have that same problem bk you do have full fuel pressure most times.
Can you volume/pressure test the pumps outside of the vehicle?

I have seriously considered just running a single pump in a fuel cell but I don't have an extra couple hundred bones kicking around. I don't have a way to test the pumps off the vehicle, I think the next step will be rewire the whole fuel pump circuit.

thanks for the input fellas

 

[RonD]

The fuel pump relay is in the engine compartment, it is controlled by the computer(PCM)
The PCM grounds this relay to close it and send the fuel pumps(both) power.

The power from the relay runs to the inertia switch in the passenger foot well or kick panel, it would then split and run to both pumps.
Both pumps should also be grounded near the inertia switch, check those grounds.

If only one pump is working then it is either broken, or has no power or no ground.
The power and ground can be tested at the pumps location, if it has both then pump is bad.
I use sewing needles to penetrate wires and use my volt meter to test for voltage and ground while devices are connected.

 

[janderson4010]

The fuel pump relay is in the engine compartment, it is controlled by the computer(PCM)
The PCM grounds this relay to close it and send the fuel pumps(both) power.

The power from the relay runs to the inertia switch in the passenger foot well or kick panel, it would then split and run to both pumps.
Both pumps should also be grounded near the inertia switch, check those grounds.

If only one pump is working then it is either broken, or has no power or no ground.
The power and ground can be tested at the pumps location, if it has both then pump is bad.
I use sewing needles to penetrate wires and use my volt meter to test for voltage and ground while devices are connected.

that is good information, I had wondered why the fuel pump ground ran into the cab, and where it terminated. Seems odd that one would crap out while the other would run. One other thing and I may have mentioned this, when I cut a hole in the floor and checked the power to the in tank pump I noticed that the plug was freaking gooped in dialectic grease, and not on the outside of the connector but on the pins themselves, what do you think of the possibility of this causing an intermittent connectivity problem? I cleaned the connecter off as best I could and then applied dialectic grease to the outside of the connector to keep water out. I know a thin layer of grease usually won't effect conductivity but the guy must have used a whole tube on that one connector.

 

[janderson4010]

btw i'm not saying you can't buy a bad pump(i've seen it), but I did recently replace both fuel pumps.

 

[RonD]

I have heard of after market fuel pumps failing 30-90days after install, not common but not unheard of either.

Grounds are always the problem, even when it's "for sure not the ground"....it's the ground.
All I know about automotive wiring I learned from wiring my first trailer, it's always the ground, lol.
You can chase 12v 'til the cows come home, and then it's the ground

 

[janderson4010]

:icon_confused:so what you're telling me here is that it might be the ground? lol

 

[4x4junkie]

I have seriously considered just running a single pump in a fuel cell but I don't have an extra couple hundred bones kicking around. I don't have a way to test the pumps off the vehicle, I think the next step will be rewire the whole fuel pump circuit.

thanks for the input fellas

I would definitely go the single-pump route. If I'm not mistaken, the '89-'90 high-pressure in-tank pump & sender will fit your '86 tank. I seem to recall them being around $130 or so new.

 

[Surrey]

I would definitely go the single-pump route. If I'm not mistaken, the '89-'90 high-pressure in-tank pump & sender will fit your '86 tank. I seem to recall them being around $130 or so new.

double check that its the same size hole, for some reason I wanna say its not.

But if it is, definitely do the single pump route, eliminates half your testing at least.

 

[janderson4010]

i've got a sheet of 16 gauge sitting in the shop, I should just build a fuel cell and pick up an aftermarket pump. I drove the rig today and it crapped out on my while I was driving down the road, which is not something it normally does, normally it craps out at low rpm like when I let it idle or i'm crawling. This time I was looking around under it while it was dying and saw that the feed line into the main pump was almost flat and the high pressure pump was actually running, but not as loud as it normally does. I don't think the in tank pump was working at all. I have to assume now that the in tank pump is no good or at least is not getting power intermittently. I really need to just eliminate this entire fuel system, wire up a whole new one myself with new lines, tank and pump. If I had just done this in the first place it would probably run just peachy. Thanks for the input guys, even if I gave you all incomplete info it is nice to have some people to bat ideas around with.

 

[janderson4010]

ordered a bosch in tank pump for it, looked at my previous order and it was a cheapo airtex that I put in it last time. I am gonna completely rewire this system. Does anyone know why in the wiring diagram it shows a resistor on the power wire running to the in tank pump? I was just gonna wire them both to the same power wire and ground them right next to the pumps. Don't want to send to much juice to the one in the back if there is a reason it has that resistor.

 

[Gilbee01]

Are you using a 2.9 pump or the one for a 3.0.. If you went with the stock for the 2.9 you will have nothing but trouble. I had similar issue and ended up buying new tank and pumps ( some kind of crap caked the inside if my tank) go with premium pumps no cheap knock offs. And flush the fuel lines before install. Idle could be the TBI sensor . Replaced that twice. And are u using the 2.9 ecc (pcu)? I reccomend getting one for the 3.0 and relocate to a cooler area not on firewall

 

[janderson4010]

Are you using a 2.9 pump or the one for a 3.0.. If you went with the stock for the 2.9 you will have nothing but trouble. I had similar issue and ended up buying new tank and pumps ( some kind of crap caked the inside if my tank) go with premium pumps no cheap knock offs. And flush the fuel lines before install. Idle could be the TBI sensor . Replaced that twice. And are u using the 2.9 ecc (pcu)? I reccomend getting one for the 3.0 and relocate to a cooler area not on firewall

I swapped in a 4.0 out of an explorer, the tank was clean inside when I replaced the pump the last time, admittedly it was a cheap one, which is why I think it crapped out this time. Other than that it runs really well, I don't really have an idle problem, besides when I only have 10psi of fuel pressure, pretty understandable that it runs poorly then.