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5.0 hard misfire low fuel pressure...kinda

blakshukvw

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2001 Explorer XLT 5.0
So as stated in the title, my 01 5.0 is having some pretty regular hard misfire. It runs good at throttle tip in and low speed cruising and even at high speed with little load. When I approach a hill and increase throttle to maintain speed it begins to misfire/surge heavily while it waits to shift out of OD. Even after it shifts out of OD it still misfires/surges heavily while trying to accelerate.

I've been running some tests at work. Of course in all this no codes. Disconnected the vacuum to the egr and it ran the same. Started running some tests on fuel pressure and found that when the pump primes upon cycling key it only gets about 20- 25 lbs of pressure. If I cycle the key a few times it comes up to 65 or so. It crank a little longer than usual when the key hasn't been cycled to build pressure to 65 but when running it maintains 65 lbs give or take a couple. I have yet to test drive with the gauge on because it will require removing the hood but I'm interested to see what the pressure is when it's surging like it does.

So I'm gonna run a scope on the ignition tomorrow to see if there's an issue with any of the wires or coils. So what should I make of the fuel pump priming low pressure and is that related to the poor performance?
 


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RomeovilleIL

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Replace the pump. Based on your initial priming the pump is weak and is very likely the source of your issue.
 




slowstang88

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My 99 mounty had the same issue, ended up being the rubber hose that connects the pump to the fuel pump/level sender was rotted out and losing pressure in the tank.
 




blakshukvw

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I've read about that hose issue on the forums and wondered what effect that would have on performance. Did your truck run bad like mine with that issue? What effect did outside ambient air temp have on your trucks poor running? Mine seems worse when it's real cold.
FWIW, my trucks fuel pressure doesn't drop like a rock when the engine is shut off. That's not to say that all vehicles respond the same to this exact problem every time.

Priming pressure is low but it always comes up to 65 or so upon start-up. If it stays there while under load then the question would be is the hose in the tank causing the engine to not get enough fuel volume regardless of the pressure? Oxygen sensor voltages didn't indicate a lean running issue however.
 




slowstang88

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Well I bought mine and fixed in the summer so that's all I can comment on, it had most of the same issues you described, what year and engine is yours? Edit: I just read your Sig, it could be a fuel pressure regulator vale seating issue
 




blakshukvw

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So I ran the Pico scope on the ignition today in the shop with no load. Checked primary voltage to coils and secondary voltage through wires. Everything looks good so far so Monday I'll install the scope on it and drive it under load and inspect the voltages of the wires and coils when it's misfiring. If it looks normal then I will have eliminated the ignition system. At that point I'll be force to move on to just dropping the fuel tank and inspecting the pump and the hose you suggested. I'm fairly certain at this point that I'm dealing with a fuel supply problem. Meaning not a pressure problem but volume. When I've had the gauge on it and it shows 65 lbs of pressure an I release the pressure with the button, I get what seems like very little volume of fuel and it seems to have air in it. I can't wait to have my truck running at 100% again. I've been dealing with issue, be it very intermittent and not as bad as lately, for a while.
 




slowstang88

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My issue wasnt intermittent, it was constant and always ran terrible. After testing fuel pressure came to the conclusion that 15psi on first ignition, and approx 5psi added every time I switched it off and back on, there was a problem with the FPR. I couldn't find just a replacement one so I bought a complete Carter pump/FPR/float assembly and when I dropped the tank and pulled the old unit saw the rubber hose degradation. Also the rubber boot around the pump pickup was extremely soft and gummy.
 




blakshukvw

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I think we share the same issue really but I want to be thorough. Now I'm wondering if I should just drop the tank and if I find the hose to be the problem only replace that and not the pump at this time. I mean the pump must be strong to make 65 lbs even with air in it. Besides working at a shop I can drop the tank again if the pump dies later.
 




RomeovilleIL

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blakshukvw

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So finally had some time to drive the ex with the scope on it. Drove about 5 miles and recorder over 100 pages of live data. I probed 2 wires on the drivers side coil and 2 on the passenger side coil. I probed the plug wires in the rear lead locations on both coils because on each 4 pack of coils, those are the 2 leads that share a driver and such for the fronts of each. One of the shared leads is positively fired and the other lead on that same driver is negatively fired.
So what the scope revealed was that under mild to hard acceleration, one of the leads on the drivers side coil was having a problem. When the spark was released, the resistance was acceptable but when the coil was supposed to close, the voltage seemed to bounce instead of just a rapid closing of the coil. This would then cause some strange feedback into the other side of the coil sharing that driver that fires another cylinder.
The passenger side coil that I tested did not show any of the same symptoms. I didn't feel it necessary to move the test leads to the other side of both coils because I felt i had enough to safely say I found the problem, at least part of it.
So with that I've determined that the drivers side coil is shot and with 180k miles I'll probably just replace both. They only cost me $25 each so it makes sense at this point. I'll leave the leads on the wires and run another test after the installation of the new coils so to see what a nice strong coil should accurately look like for future reference.
For what its worth, I have livewires spark plug wires and after probably 20k miles that still fire great and had very decent resistance readings.
I'll let you guys know what happens next.
 




blakshukvw

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Well installed the new drivers side coil today. Left the scope leads on and I test drove the x again. Misfire still there. This time I finally got a cel. Misfire on cyl 3. Checked the plug wire for 3 and found the plug boot had been compromised due to the heat generated from the manifold. The boot showed signs of a chalky type substance and some little cracks on the side facing the manifold. Replaced the wire and truck ran awesome again.
So what was going on? Remember cyl 3 and 5 are on the same coil and share a driver on that coil. The resistance on cyl 3 was never high when seen on the scope because the spark was able to jump easily from the plug right through the boot to the manifold easily, never clueing me off to a problem. Just looked like a good firing plug and wire. While the problem was with cyl 3, it manifested itself on 5. On the scope cyl 5 was the one with the trailing voltage when closing. Obviously this was caused by cyl 3s issue. Why I'm not exactly sure. Coil packs are so hard to diagnose whether they be packs or individual coils and I've learned recently they act very strange in ford products.
 




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