2003 Ford Expedition Engine hesitates during acceleration w

2003 FORD EXPEDITION
143,000 MILES • V8 • 4WD • AUTOMATIC
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RIVIKESOJOSH
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My Expedition hesitates during acceleration when the engine is warm. It feels like a misfire. I have replaced the plugs, carefully gaping them to .54 as required, replaced the fuel filter, have cleaned the throttle body and have run a Lucas fuel injector cleaner, and run 93 octanes every 3rd tank. I suspect my fuel pump is going because I hear an intermittent knock from the rear of the vehicle, where the fuel tank sits. The original plugs were replaced at 130 K, when the problem began, and I just replaced them again 200 miles ago, because the problem re-surfaced. The original plugs were heat-range 7a, the ones I'm running now are Motorcraft 9c heat range. The hesitation only is most noticeable when the engine is between 1000 and 2000 rpms, and the engine runs fine at a steady speed, but will hesitate at any speed when I step on the gas, no matter how slightly I do it. I will appreciate your advise.

Thanks!
May 27, 2009 at 8:13 PM
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RASMATAZ
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Dirty fuel injectors (cleaning the injectors often fixes this).
Bad MAP (manifold absolute pressure) sensor
Bad TPS (throttle position) sensor
Bad or dirty MAF (mass airflow) sensor
Low fuel pressure (leaky fuel pressure regulator or weak fuel pump)
Vacuum leaks (intake manifold, vacuum hoses, throttle body, EGR valve)
Bad gasoline (fuel contaminated with water or too much alcohol)

Sometimes, what feels like a hesitation is actually ignition misfire rather than lean misfire. The causes of ignition misfire may include:

Dirty or worn spark plugs
Bad plug wires
Weak ignition coil
Wet plug wires
May 27, 2009 at 8:18 PM
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RIVIKESOJOSH
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[quote:4885842d95="rasmataz"]Dirty fuel injectors (cleaning the injectors often fixes this).
Bad MAP (manifold absolute pressure) sensor
Bad TPS (throttle position) sensor
Bad or dirty MAF (mass airflow) sensor
Low fuel pressure (leaky fuel pressure regulator or weak fuel pump)
Vacuum leaks (intake manifold, vacuum hoses, throttle body, EGR valve)
Bad gasoline (fuel contaminated with water or too much alcohol)

Sometimes, what feels like a hesitation is actually ignition misfire rather than lean misfire. The causes of ignition misfire may include:

Dirty or worn spark plugs
Bad plug wires
Weak ignition coil
Wet plug wires[/quote:4885842d95]

I'm not getting a check engine light, so I have to assume neither the sensors, nor the vacuum, nor the COP's are the problem. My truck has no plug wires, and all the COP boots are clean and free of cracks. The plugs are new (5 days old). Based on that, and in light of the intermittent knocking noise I hear from the rear left side of the truck; is it safe to assume the problem will, most likely be either the fuel pump or the fuel pressure regulator?

Thanks again!
May 27, 2009 at 8:30 PM
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RASMATAZ
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most likely be either the fuel pump or the fuel pressure regulator? To find out check the fuel pressure
May 27, 2009 at 10:31 PM