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Fuel pressure test questions


EvilTheCat

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So I used to be fairly active here when I got my explorer years ago, but stopped getting on because I fixed everything and modded enough stuff to make her suit my needs. However, lately she's been running like shit.

RIG:
I have a 97 Explorer Sport with the pushrod 4.0L OHV and a 5 speed manual.

PROBLEMS:
Hesitation, or maybe a misfire, when ascending hills or when throttle is applied more than half way, at highway speed or taking off from a dead stop. No hard starts really and it usually idles smooth and steady. No Check Engine light on either.

STUFF DONE:
*Started by putting in new plugs and wires, and also a fuel filter. No improvement noticed. The plugs were not too bad. A little light colored, so maybe slightly lean burning, but none fouled. Replaced with NGK platinum plugs (not gapped as per plug instructions)
*Coil pack spark and resistance checked.
*Injector cleaner added to 1 full tank of gas suspecting a clogged injector... what a waste of money.
*Seafoam down the intake through the brake booster, MAF sensor cleaned, and IAC valve cleaned at the same time. Smoked like mad, what a hoot tearing through the neighborhood! Some improvement noticed. The hesitation seemed to occur less frequently and at closer to 3/4 throttle now. Plus my exhaust doesn't stink anymore! I love seafoam!

FUEL PRESSURE TEST:
I rented a gauge from O'reily's to test my fuel pressure to see if maybe my pump or fuel pressure regulator is failing. From 0 psi on a cold engine it slowly climbed to about 25 psi once started. Seriously took about 10 seconds to climb to that. Revving engine resulted in zero change to pressure. Taped the gauge to my antenna and drove around to see where the pressure was when the hesitation occurred. The pressure would hover around 28 psi while driving, but would climb to nearly 40 psi at WOT. I shut off the truck and went to go pee and get another beer. The pressure didn't drop a single pound in 5 minutes. Then I drained all the pressure off and just turned on the key. It jumped right up to 25 psi again. I then unhooked the vacuum line to the FPR and plugged the hose with a .22 shell I found on the floor of my truck which is a perfect fit by the way. It fired right up and the pressure quickly rose to about 38 to 40 psi. I went for another drive and it ran about the same, but the pressure never fell below the 38 psi mark.

I guess my question is what does it all mean? Is the Fuel Pressure Regulator bad? The fuel pump seems to be able to maintain 38 psi when the FPR is open. Am I missing something on the fuel pressure test? This is the first time I've done it. Usually I can tell when my pump is bad. It just stops. Any other suggestions for things to check? I've read somewhere that the Crankshaft Position Sensor can go bad and not send a signal to the ignition when it's supposed to causing a misfire. Anybody heard of this happening? Any way to test it? I'm getting tired of messing with it, because winter is nearly here and I don't have a garage to work in.
 
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koda2000

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A '97 should have between 35-37 PSI of fuel pressure. Weak fuel pump, clogged fuel filter, pinched/restricted feed line or maybe the FPR.
 




EvilTheCat

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Thanks Koda. I'm trying to narrow it down more. I changed the fuel filter a week ago, so that is not the problem.
 




EvilTheCat

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Anybody know what the fuel pressure should be unregulated?
 




Macgiobuin

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My guess would be that the black wire in the fuel pump module is failing at the pump terminal or the intermediate wire terminal in the tank.
 




koda2000

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IDK what the unregulated FP pressure should be, but how many miles on your FP?
 




EvilTheCat

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190k on the truck. I've put on 30k since I got and haven't messed with fuel pump yet. I've owned it about 5 years I think.
Judging by the amount of maintenance that I had to do to it when I got it, it's probably still running the Ford factory fuel pump.
 
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koda2000

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190k on the truck. I've put on 30k since I got and haven't messed with fuel pump yet. I've owned it about 5 years I think.
Judging by the amount of maintenance that I had to do to it when I got it, it's probably still running the Ford factory fuel pump.
Hmmm, for what it's worth all of my OE fuel pumps started giving me trouble at about your mileage (170K-200K) though my problems were mostly with intermittent operation. Our 1997 SOHC Sport had 260K + on it's original '97 dated fuel pump. I changed it this past spring and couldn't believe it lasted that long and was still working! I use Bosch replacement fuel pumps exclusively. I buy them off eBay for around $60. Having changed 7 of them, I can now change one in about 90 mins.
 




EvilTheCat

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Sunday I'll be dropping the tank. I'm pretty sure it is the fuel pump now after much reading and consulting friends and coworkers. Sunday will be the nicest day in the forecast at 50 degrees. Did I mention I only have a single stall garage that is too small and full to utilize for repairs. I hate Nebraska. At least it's not snowing. I guarantee it will take more than 90 minutes though. Thanks Koda.
 




ZWilson07

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Hmmm, for what it's worth all of my OE fuel pumps started giving me trouble at about your mileage (170K-200K) though my problems were mostly with intermittent operation. Our 1997 SOHC Sport had 260K + on it's original '97 dated fuel pump. I changed it this past spring and couldn't believe it lasted that long and was still working! I use Bosch replacement fuel pumps exclusively. I buy them off eBay for around $60. Having changed 7 of them, I can now change one in about 90 mins.

You need to come do my Mountaineer lol. I`m fairly certain mine is original with 280,000 miles and starting to get intermittent.
 




koda2000

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You need to come do my Mountaineer lol. I`m fairly certain mine is original with 280,000 miles and starting to get intermittent.
No thanks, but I give you step-by-step instructions.
 








EvilTheCat

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Sunday sucked because Saturday we got snow. It was still cold and wet. I tried but ran into issues. A Mexico fuel pump was ordered instead of one for the U.S. built model which does make a difference. The US model has 2 electric connectors instead of 1 and costs about $50 more. Plus the hose clamps for the filler hoses were too corroded to get off and the tank strap was rusted completely through. I gave up and tried again today after securing the proper fuel pump, a new strap, some new hose clamps, 3' of 3/4" vent hose and 2' of 1" filler hose. The job only took 3 hours fixing all the above mentioned issues. Unfortunately, it still runs like shit. So back to the drawing board. I hate throwing parts at it. It's expensive and frustrating when it's cold and I have no garage. What's next?

Just to recap: Truck hesitates, stutters and stumbles when accelerating. It however doesn't do it when it's not under load. I can stab the gas and the RPMs will shoot up to 5k without a miss, but if i apply more than half throttle in gear it misses repeatedly.
 




koda2000

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While replacing the fuel pump was a hassle, I think it is time & money well spent. Have you replaced your plugs and plug wires and (if not replaced) removed your coil pack and cleaned the rust off the bottom? In my experience, missing under a load has always been ignition related. My last instance was on a '95 GM 3.1 V6. Turned out to be corrosion/rust in the coil's spark plug wire sockets. Cleaned the sockets out and reinstalled the plug wires with dielectric grease - problem solved. If you don't want to throw more money at it w/out knowing where the problem is, there are ways to test coils and even plug wires.

In re-reading your original post, I see you say you've done most/all of these things already, so I don't know what else to tell you, but I'm thinking it's a spark related issue. Check to make sure that you don't have a plug wire which has melted on a header, also examine the engine while running at night to see if you see any spark arcing on the wires and spark plugs. I've replaced spark plugs and have managed to crack an insulator. The engine ran fine until I put it under a load. Then it ran like crap. I found it while looking at the engine while it was running in the dark. Sometimes misting water around the plugs/wires will induce an arc.
 
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