Fuel system FPR, pump, relay questions

[Bwana Bob]

I'm still having trouble with my '92 XLT. If I accelerate hard with the rpm's below 2000, it jerks and bucks until the rpm's get above 2000. I checked the plug wires, cleaned the MAF, replaced O2 sensor and fuel filter. Still no joy.

I suspect that the fuel pressure regulator (FPR) is not boosting the pressure when the vacuum drops under hard acceleration because the engine acts like its starved for fuel.

It idles fine and there is no CEL. I measured fuel pump current and its 3.2 amps. When I disconnect the FPR vacuum line the current increases to 3.6 amps. The FPR is not leaking fuel into the vacuum line, as some have reported. I would like to check the fuel pump relay for excessive voltage drop but I can't find it. I think its near the power distributio box, but I popped the box out and looked around and don't see the relay, unless its inside the plastic housing that is underneath the power distribution box. If so, it will be hard to get at.

I still suspect the FPR, but I don't have a fuel pressure gauge to check it. Both the FPR and fuel pump were replaced a few years ago.

Any help is appreciated. Thanks.

Bwana Bob

 

[MrShorty]

The 0.4 A increase in current draw indicates that the FPR is increasing the pressure as it should. On mine the current will jump from 2.8 - 3.2 A under hard acceleration. Unless the fuel pressure is low to begin with, your current draw is consistent with an adequate increase in fuel pressure. I would recommend you get a fuel pressure gauge and see what the fuel pressure is.
An alternative down and dirty test would be to remove the vacuum line from the FPR, plug it, and then see if that clears up the stumbling. This causes the FPR to behave as if it's always at WOT (normally, engine management is able to correct for the increased fuel pressure). If it still stumbles, then the stumble isn't caused by the FPR not increasing the pressure (though one would still wonder if there is enough pressure to begin with).
The fuel pump relay is indeed located in the plastic housing under the power distribution box. Look at it carefully and you should find that the housing, like the power distribution box, will slide up and out, allowing access to the relays underneath.

 

[Bwana Bob]

Thanks, Mr. Shorty.
From your current readings, it seems that mine are not out of line. Yes, I tried running with the vacuum line disconnected from the FPR, but it didn't help. However, it seemed to clear up another problem: after sitting for a couple of hours, it takes more cranking to start, but overnight it starts immediately.

The acceleration problem only occurs after the engine has warmed up. This may be another clue.

Its a mystery to me, but I'll check the pump relay and try to borrow a pressure gauge. I checked and inspected the spark plug wires this weekend and they look fine.

Thanks again.

Bob

 

[Bwana Bob]

Problem Fixed

Well, I finally fixed it! Because the hesitation only occurred under acceleration at rpm's below 2000, I deduced that it was fuel starvation. I bought a fuel pressure gauge and fuel pressure readings were in spec, under all the test conditions. This ruled out the FPR, pump, and fuel filter. Since I had already checked most of the sensors, the only possibilities left were air starvation and spark. It was too late in the day to take apart the air box and look for a blockage, so I just swapped in my old original spark plug wires. Hesitation gone! It was the lowly spark plug wires.

The "bad" wires were aftermarket low resistance, spiral wound,silicon types. They had good continuity, looked fine, but my records showed that I bought them in 1996 and they must have about 100,000 miles on them. I looked at them more closely and the only thing I saw was a tiny cut in the outer jacket of one and a couple of tiny cracks in one of the boots, not obvious at all. If I get the time, I'll hi-pot test them.

I replaced them with NAPA/Belden wires which cost $33 and appear to be very good quality. They are marked "high temperature", the mfr claims that the jacket is tougher than silicone, and they carry a lifetime warranty.

So now the X (aka The Silver Goose) no longer chokes and jerks and again flys like the wind (well as fast as the old OHV engine, goes, anyway).

Thanks Mr. Shorty for your inputs.


Bob

 

[RickOTR]

NAPA electrical parts are not very good as they use to be, in fact, many shops will only use their mechanical parts from unless a customer asks for them. I had their wires on my X for about a year before they developed seperation in the windings so I replaced them with OEM Ford.

 

[Bwana Bob]

Wires

Thanks, Rick, I'll keep an eye on them. I measured and recorded the resistance and compared them to the OEM wires before I installed them. The plugs and boots are the same as OEM, but the wire definitely isn't. So far, my only bad NAPA experience has been with fan clutches. I'll advise the forum if I have any trouble with the NAPA wires, which are made by Federal Mogul. Right now I'm just happy to have the Ex running right again. (Now to tackle the front 4X4 hub problem, the door hinge, and the rust!)


Bob