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Old 05-09-2011
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Join Date: Apr 2011
Location: League City TX
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Question about fuel rail pressures

OK, so my 2003 XLT 3.0 w/AC and manual 5-speed has recently had a problem, or maybe it has had a problem for a long time, and now it is manifesting itself in a much more noticeable manner.

First off, since I have had this vehicle (bought at CarMax w/28K on the odometer) it has always had issues with starting, to wit, sometimes it takes ten or fifteen seconds of good cranking to get the thing started when cold. I always wondered if the fuel supply had an issue (more on this in a moment). But the truck has run like a champ, change the oil with Mobile 1 every 5K, change the radiator every 50K and keep running on.

Alas, last month I noted that occasionally it would hesitate when accelerating, as well as having a nasty ping on the freeway. The hesitation only happened a couple of times, but since I was planning a trip to Alabama, I got concerned and took it to the dealer who could find nothing wrong with the vehicle (basically, just put the computer onto the OBD port and said nothing was wrong they could see). So, prior to the trip I replaced the plugs, wires, and ended up replacing the coil module as two of the wire plugs showed signs of corrosion and heat. I also replaced the MAF (hoping to get rid of the pinging) and TPS, just in case. And, I bought my own Acutron OBD-II reader to keep handy.

Anyway, on the trip, the truck ran fine for about 300 miles then started back into the brief, but noticeable, hesitation (usually followed a few moments later by another less noticeable one). I ran only Chevron gas the whole trip, and added in a bottle of Techron at every fillup (AutoZone loved me before my trip). Given that I had a trip to make, I soldiered on, figuring that if the truck was going to die, it would, and AAA and me would figure out what to do. But, it made it fine (with a lot more of the hesitations). I plugged my OBD-II reader in for most of the trip, and never saw any indication on any readings (at least that I could tell) that I had a problem. After getting back home, the thing ran absolutely fine for two weeks.

Then this past weekend, the hesitations started again, in earnest. I cannot drive it on the freeway for fear that the drop-out in engine power and subsequent recovery will damage the timing belt.

So, tonight I bought a fuel-rail pressure gauge (again, Acutron) and plugged it into the rail. Doing the KOn-EOff test, the pressure jumps up to about 55 PSI then rapidly drops down to 10. KOn-EOn test shows a pressure of 58~ PSI jitterey.

The long and short of this is, since I get no codes on my truck, but am seeing the pressure decay on KOn-EOff, should I begin by assuming that my fuel pump unit has probably got a problem, given that since (as mentioned above) I have had the vehicle, it has had a problem starting when cold?

Thanks in advance for any advice...

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Old 05-10-2011
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Fuel rail pressure for 2003 is 64 +/- 8 PSI, so you're at or below the low side of spec. That could be a regulator or a weak pump, both of which are part of the intank pump assembly. The fast KOEO drop to 10 after the initial prime could be a problem with the check valve which is also part of the intank assembly. A leaky check valve can influence crank times somewhat but 10~15 seconds sounds like something else.

If I were diagnosing this, I'd like to know what the fuel pressure AND the voltage across the fuel pump are doing when the power loss or delayed starting occur. This would take some effort but it would help narrow the possibilities.

There is a problem area with 2000~2003 Rangers that can be checked fairly easily. Underneath the truck, directly below the driver seat, you'll find a large, square 40-pin connector that joins the cab harness to the rear frame harness. Sometimes this connector will get contaminated with water and cause a variety of problems with circuits aft of the cab. Unscrewing the center bolt will separate the connector halves allowing for inspection.

If that connector looks OK and the voltage accross the pump never drops out, I think you will ultimately end up changing the fuel pump assembly. The fuel filter should also be changed as a matter of course but I doubt that it could be the root cause of an intermittent problem like this.
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Old 05-10-2011
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Join Date: Apr 2011
Location: League City TX
Posts: 2
Thanks, Bob, for your input. I measure the voltage at 13.8 across the relay (although, not constant). An interesting addendum to the problem that may be helpful or may not...

This AM, I scrounged around with different parts stores to see if they had the fuel pump (none did, but a Motorcraft is on order). So, I decided that while calling around, I would drain the radiator and change the coolant. That went swimmingly, until I left to head up to work, when the truck suddenly had a mind of it's own.

Coming up to a stop-light, the truck didn't want to slow down like normal, so I pushed in the clutch and saw the RPM's jump to 3500. Yikes! Thankfully, it is a manual so I was able to turn the beast around and head home, where I spent ten minutes in my driveway, with the OBD-II reader set to watch the TPS position, and noted that the RPM's ran up and down, in no particular pattern, between 750 and 3500. The TPS indicated 18.8% the whole time.

SO, figuring that I either had a spirit now inhabiting my truck, or a cranky computer, I elected to pull the plug on the computer, disconnecting the battery positive and jumping it to the ground. I let that set for, oh, roughly an hour (I thought it was fifteen minutes, but when you are on the internet, time flies by).

Now, the truck runs like a top. No hesitation on starting, no hesitation while running, and no wild RPM deviations from throttle position.

Although I have ordered a new fuel pump, and figure since I do lose rail pressure that that is a necessary change-out item, I am beginning to think I may have more of a PCM issue than a physical one. Given that I program those beasts for a part of my living, I can understand how that may happen. Any thoughts or comments appreciated...

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