i have a 99 f150 with 5.4, it has given myself, technisions, and friends a real brain buster. It looses power sputters/misses from idle to 3000. i have new plugs, fuel filter, have had the coils checked, replced 2 leaking fuel injectors, and still the truck runs like crap . it idles fine, and revs without a load like nothing is wrong, the power loss is the worst when u drive about 45 and the truck shifts to overdrive it surges and misses the worst. if anyone has any ideas i would love to hear them.

 

Welcome to the forum. Sounds like it is running way too lean. I would first clean the MAF. Then check the PCV hose elbows for cracking, especially the one where the tube goes into the back of the intake. Have you scanned for codes?
Has a compression test been done?

 

I have a 99 Expy 5.4 with very similar symptoms. I had it to Pep Boys today for a diagnostic (no check engine light). They said, "yes, it's missing, but since there are no codes, we can't diagnose it. Maybe you should bring it to a dealer."
What I did is I went to Auto Zone and picked up 1 coil pack. I will start swapping them
one at a time to see if I clear up the problem. I've already replaced #4 COP twice. I will do all the easy COPs first (1, 2, 3, 5, 6, 7).
Keith

 

its been pressure tested with all 8 cylinders within 7% of 125 p.s.i., i have taken it to a dealer where i "know someone" who let me use the shop and tools after hours, when hooled up to the computer it showed all coils, injectors, and sensors to be working within parameters and without a laptop i could not hook it up and drive to check when the power loss/miss/sputter occurs.

 

i have a 98exp5.4 that had very similar problems. it was giving me a P0305 (#5 misfire) code, but only once in a while. then i finally got a nice loud hissing noise coming from the engine and many more codes. car was idling very rough and driving real bad. long story short, grabbed a can of carb cleaner and started spraying around the carb and intake and found a bad vacum leak from the elbow joint that connects from the IAC along the firewall. it was ripping over time and finally gave way. replaced with an end from a spark plug, since no one has the part. problem fixed. moral of the story, don't count out a leak somewhere. good luck

 

My mis-fire on my 5.4 was in the range of 1500 rpm and was very intermittent but when it happened the truck ran like, well you know. Very frustrating. The dealer could not pick it up on the diagnostic so I had to go the individual COP replacement myself. Took about two weeks but I finally did change out a COP that corrected the problem. You would think that there would be some way to check these COPs individually.

 

ya i am about to buy an autotap or something like it to plug into my laptop to see everything that is goin on while i drive, its just something that i feel will be minor but is being a reel pain to diagnose

 

Try to hook the MAF up to a scan gauge to see if the barometric pressure functionality is working...not all scan tools can check for this, if I'm not mistaken. AutoTap doesn't check this (at least I don't think it does...I've got an AutoTap).

 

I bought a COP from AutoZone, but decided before I change the COPs to find the problem, I was going to change all the plugs. I did a complete plug change 7000 miles ago, but I used Bosch Platinum plugs. I heard from others on this forum that the best plugs are OEM, so I went to Ford and bought 8 Motorcraft platinum plugs for the Expy 5.4. So far, I have replaced 1,2,3,5 and 6. The problem is not resolved yet, so it is either plug 4,7 or 8 or one of the COPs.
Keith

 

For you folks having miss problems and COPS, I would suggest before spending any money to use LOTS of silicon on the COPS. If you don't it will leak electrical energy causing a miss. I don't have the TSB number but it is a far too common problem not using enough di-electric silicon.

 

I usually squeeze some dielectric grease into the plug end of the COP before putting it back on the plug. I also use anti-seize on the plug threads.
Keith

 

I resolved my misfire problem.
After swapping out plugs 1-6 with OEM plugs, I started swapping COPs. I placed a new COP on 3 and tested. No good. Placed COP from 3 onto 2 and tested. No good. Placed COP from 2 onto 1 and tested. No good. Today, I went to swap out COP 4. Before I did that, I noticed that the radiator hose above COP 4 was damp. I thought that I had checked the tightness of that clamp before, but this time, I used a 8mm socket and tightened it a little more. I then put the COP from 1 into 4. BINGO!
I took a test drive and it ran like a champ.

 

I replaced all of the COPs with no change in the way it's running. Try unhooking and rehooking the MAF, if it smooths out for a couple of seconds, then it is bad. I tried cleaning mine and it didn't help at all. I'll going to buy a new one.

Just an idea.