Missing under load, worse at low RPM

I need some advice here... Here is a ton of details.

Vehicle-
97 F150 4.6l, PI intake, Ported PI heads, custom crane cams, Accell coil packs and wires, 75mm TB, 80mm mass air, BBK fuel pressure regulator, headers, dyno tuned Diablo Sport chip, Made 300hp at the wheels when tuned. 77k on motor, 20k since engine work done.

Issue-
Misses under load. Improves at higher RPMs. I have to slip the clutch to get it moving in first. as the RPM goes up, I can apply more and more throttle. By the time I am at 3-4k, I can be at about half throttle. If I don't push it too hard, it runs smooth and strong. As I get past that point it starts missing a little, then it will fall completely on its face and bogs. I can let off the throttle till it stops missing and then roll on it again and it will run fine. It will even miss a little when I crack the throttle from an idle in neutral.

Fuel Pressure gauge doesn't show any drop in FP when it is running bad. A/F gauge doesn't show it going lean when it is missing (the gauge will look like it is "vibrating" as it swings back and forth when it is missing, let off and it will stop right away).

It was throwing an occational P0305 (cyl 5 missfire). I replaced plugs 1 and 5 last night. They both looked pretty good. drove it to work today and got a P0305 and a P0300 (random missfire).

I was sitting for a few years, I finally got the crappy gas out and filled with fresh gas and some fuel injector cleaner. It is running better than it was, but not right. I have replaced the TPS, cleaned the MAF meter twice, and very very carefully checked for any vacuum leaks (to the point that I even pulled the upper and lower intakes).

The engine runs smooth otherwise, so I think it is likely either a fuel or ignition problem, but am having a hard time determining which it may be. I don't want to throw a lot of cash at parts without being able to fairly confidently determine which it might be. I was hoping some fresh gas and injector cleaner would fix the problem, but it definately isn't "fixed".

Any advice is appretiated

-MrCarlson
02 GT (mild 4.6)
84 SVO (moderate 2.3T w/n2o)
71 Convertable (351c 2v)
69 Fastback (moderate 331 stroker)
97 F150 (4.6 w/mostly mustang engine parts)

The fuel pump was one of the first things I thought of, but I put a gauge on it and the pressure stays steady (even when its missing) and when I am driving and the engine misses, it isn't nessisarily running lean according to the Air Fuel ratio meter.


-MrCarlson
02 GT (mild 4.6)
84 SVO (moderate 2.3T w/n2o)
71 Convertable (351c 2v)
69 Fastback (moderate 331 stroker)
97 F150 (4.6 w/mostly mustang engine parts)

I bought the truck not running (I didn't do the mods myself). The previous owner said that he replaced the fuel filter trying to fix it (he didn't have a Fuel Presure gauge).

It does still have O2 sensors in front of the cats but the ones behind the cats are gone (which is kind of funny, because it does actually have high flow cats). The O2 sensors seem to be working fine because my Air Fuel ratio meter is tapped into them and cycles properly.

-MrCarlson
02 GT (mild 4.6)
84 SVO (moderate 2.3T w/n2o)
71 Convertable (351c 2v)
69 Fastback (moderate 331 stroker)
97 F150 (4.6 w/mostly mustang engine parts)

02nightmareGT-

How did you determin it was the O2 sensors. Watching the gauge mine seem to be working well (they are responsive and go to both ends of the scale).


As far as the comments on the fuel pump... I guess there is the potential of that problem. When I floored it on the drive home yesterday to get it to stumble hard it went lean... but it doesn't make any sense how I could be in 3rd gear at half throttle and 4k accelerating hard fine, but when I put it in 5th gear and accellerate lightly it misses, it requires a lot more fuel when it was running good than when it was missing in 5th.

-MrCarlson
02 GT (mild 4.6)
84 SVO (moderate 2.3T w/n2o)
71 Convertable (351c 2v)
69 Fastback (moderate 331 stroker)
97 F150 (4.6 w/mostly mustang engine parts)

Fixed!

I got fed up and called my mechanic friend who has done a lot of 4.6 performance work. I was going to bring it in to him, but after describing the problem he said to replace the plugs/wires before I brought it in. Sure enough, that fixed the problem! Runs great now and is a lot of fun to drive.

Thanks for the help guys.

-MrCarlson
02 GT (mild 4.6)
84 SVO (moderate 2.3T w/n2o)
71 Convertable (351c 2v)
69 Fastback (moderate 331 stroker)
97 F150 (4.6 w/mostly mustang engine parts)