4.0 OHV idles great but runs horrible

[rhauf]

so my '91 has developed a problem.. and i can't quite put my finger on it.

it's always taken a bit of cranking to get it to start after it's been sitting for a while. so that may or may not be related.

the problem is, it stumbles a bit when i hit the gas, and if i give it more than about 1/4 throttle it starts surging and stuff (almost like a carb'd car running out of fuel or with a clogged fuel filter) and i'm baffled at what the problem is.

so far i've replaced the fuel filter, TPS, and MAF, and nothing has changed. the check engine light is not on.

another thing i noticed, was last time i went wheelin' with it, i was on a fairly high trail (about 6,000 feet elevation) and it was a hot day (100 degrees) and after i climbed up a bit of the trail i shut it off to help my friend tip his truck back onto its wheels... when i got back in the explorer it didn't want to start, seemed to be vaperlocked (i was getting what appeared to be just fuel vapor out of the fuel rails shrader valve, along with some spurts of fuel) i let it sit for about a half hour and it fired up. this was before i started replacing parts.

 

[MrShorty]

1st thing I'd probably do would be to put a fuel pressure gauge on it and see what is going on with the fuel system. There's a "mini-diary" thread in the Under the Hood forum showing how to do a fuel pressure test.

 

[rhauf]

I figured fuel pressure would be my next test. i havn't been able to locate a fuel pressure gauge in town here.. so i'll have to go out lookin' for one.. maybe tomorrow being Saturday.

hopefully it's just low fuel pressure and i'll replace the fuel pump and that fixes it, but i have a feeling the fuel pressure is going to be fine.

 

[rhauf]

Well i got a fuel pressure gauge, hooked it up.. when i turn the key the gauge doesn't even move for several seconds, the slowly starts climbing upward in psi.. by the time it reaches 10psi or so the engine starts to fire (after probably 10 - 15 seconds of cranking) then it starts.. and the fuel pressure slowly climbs to about 28psi. if i rev the engine, the fuel pressure drops.

so does this sound like a bad fuel pump or fuel pressure regulator?

 

[MrShorty]

Not enough to tell for sure, yet, but this

if i rev the engine, the fuel pressure drops.

is definitely incorrect. If anything, the fuel pressure should temporarily increase (from the decrease in engine vacuum) when you rev the engine.

If you ground the fuel pump test lead in the self-test connector, how long does it take for the pressure to come up?

Is there any fuel getting past the FPR? A couple of ways to test this. If you find a "soft" section of the return line and clamp it off, does the pressure increase? In some cases, maybe using a stethoscope, you can hear the fuel returning to the tank. When I had FPR issues, I simply took the return line off of the FPR, ran the pump briefly with the jumper wire, and see if gasoline comes out of the FPR before the pressure comes up.

Basically, if there is never enough pressure to open the regulator, then something is wrong upstream of the regulator. You've already replaced the filter, so we'll assume it's clean. Maybe a kink/clog in one of the fuel lines, or maybe a weak pump. If the regulator is opening prematurely, then the regulator should be replaced.

 

[rhauf]

Not enough to tell for sure, yet, but this is definitely incorrect. If anything, the fuel pressure should temporarily increase (from the decrease in engine vacuum) when you rev the engine.

If you ground the fuel pump test lead in the self-test connector, how long does it take for the pressure to come up?

Is there any fuel getting past the FPR? A couple of ways to test this. If you find a "soft" section of the return line and clamp it off, does the pressure increase? In some cases, maybe using a stethoscope, you can hear the fuel returning to the tank. When I had FPR issues, I simply took the return line off of the FPR, ran the pump briefly with the jumper wire, and see if gasoline comes out of the FPR before the pressure comes up.

Basically, if there is never enough pressure to open the regulator, then something is wrong upstream of the regulator. You've already replaced the filter, so we'll assume it's clean. Maybe a kink/clog in one of the fuel lines, or maybe a weak pump. If the regulator is opening prematurely, then the regulator should be replaced.

Good idea. i'll try those tests on the fpr (see if fuel comes out before pressure builds, and/or block/pinch the return line) tomorrow and post the results.

the fuel pressure definitely did drop when revving, only by a few psi, but the pressure gauge made a noticeable drop every time i revved it. i figured because more fuel was going into the engine and the pump volume remained the same. (low)

i tested my '93 just for comparison, and it went to 30 psi immediately, and would increase by a couple psi when i revved the engine.