Hello all
I recently purchased a 99 F150 with 4.6L V8, 70 K miles.
It was a cable company truck, and I have the company maintenance records, But the truck sat idle for about a year before being auctioned off.

The engine idles perfectly but hesitates and bucks when under load at 1/2 throttle and getw worse as throttle opening increases. I can also detect something akin to backfiring sounds under prolonged Wide Open Throttle which leads me to think it may be starving for fuel.
I replaced TPS sensor blindly as it seemed locgial, but no real help. No engine codes are present and all system monitor test are OK.
Checked tha MAF but it looks clean. Have not yet checked fuel rail pressure but plan to do so tomorrow.
At this point, I do not think it is electrical, but more fuel related. I am looking for advice as to where to proceed first, fuel or electrical. Eventually, I will change plugs and wires but if not needed immediately would prefer to concentrate on the hesitation problem without throwing lots of money at it.

P.S. It got nearly 20 MPG on the 700 mile ride home from purchase location.
Thanks
Dialtone

 

Has the fuel filter been replaced?

 

I agree with racerguy on wires. Company trucks usually get aftermarket wires/plugs which (from what I've seen) don't meet spec. You may also want to check fuel pressure.

 

Try unplugging the MAF and drive it if it runs better you need a new one.

 

Ran the fuel rail pressure test today and it was in specs but pressure dropped off rapidly to 0 psi in less than a minute. So I replaced Fuel pressure regulator (now that was a real PITA) only to find it still does the same thing. At this point I am really suspecting fuel pump problems. Just for grins, I will replace the fuel filter next anyway as part of regular maintenance.
Keep you all posted.
P.S. Wires and plugs appear to be OEM originals.
Dialtone

 

Some one else out there may disagree but the electric fuel pumps I've seen go bad go completely bad. Basically on/off, no middle. Good luck with the filter.

 

Changed out the fuel filter today but did not help. I ran an extended pressure test with engine under a load and never saw the fuel rail pressure drop below 30 PSI at all, so I think I will rule out the pump.
Cleaned the MAF element.
Ran an extended diagnostic from a PC monitor system.
MAF shows air flow changes ranging from 0.7 lbs/hr to >15 lbs.hr depending on throttle opening, so it looks to be functioning fine
Both pre-cat O2 sensors look good. They switch rapidly in response to throttle movement, and cycle above and below the trim set point.
Post- cat O2 sensors are giving different readings however.
Bank 1 somtimes hovers around 0.7 volts but then will take a dive to the bottom, flounder around the recover somewhat (all this at an idle)
Bank 2 is a relatively steady 0.7 volts at idle.
Does this indicate a mis-fire or injector malfunction on bank 1 causing the post-cat O2 readings to swing so wildly?

Any help appreciated as I am getting stumped as to where to proceed next.
Dialtone

Anyone know what to make of these readings?

 

I still say try unplugging your maf, the maf has the baro sensor built in and if it doesnt read right it will not run good.

 

While I was cleaning MAF, I unplugged it and went for a short drive. It ran no better, and if anything, it ran slightly worse than when plugged in, so I am ruling the MAF out of the picture. Going for plugs/wires next, and possibly removing/cleaning injectors while I am in the area.
Dialtone

 

If you are changing the wires I recommend OEM. I was trying to track down a miss in mine and researched aftermarket wires and none that I found met the Ford spec of 5k ohms/ft. BTW: Did you change the fuel filter?

 

Well, I fixed the miss today, and it was the plugs. That was the sorriest looking mess for 70 K miles I have ever seen. Gaps ranged from .085 to .095, electrodes and ears were severey erroded and 3-4 showed signs of blow-by where the metal and ceramic meets.
Changing them was no fun job and bank 1 (1-4) was a real PITA due to the backwards angle of the holes. For those who may be contemplating doing yours, I recommend you have an air compressor handy to blow the junk out of and around the holes in the heads, and have several lengths of extensions and u-joints.

Thanks for all the suggestions
Dialtone

 

Yes, blowing out the holes with compressed air before pulling the plugs is part of the procedure. There is a very good thread at the top of this forum that covers the plug change in detail.

I have gone through it several times now on my truck, my Dad's and my preachers, all 4.6's. I can knock it out pretty fast now.

I pull all the COPS on one bank, then blow out all the holes on that side, then replace all the plugs on that bank and then put all the COPS back in place on that bank.

I have yet to pull a set of plugs with the gaps opened up beyond .060 at the max and I always check the gaps on the removed plugs just to satisfy my curiosity. My curiosity is really working on yours. Its amazing that the gaps had opened up that much.

Also I got a few locking extensions so that I don't lose my spark plug socket down in there. I got them at NAPA and they have a little slider at the end that hold onto the socket and won't let go. They are REALLY handy for this job.

Glad you got it taken care of,

 

I have a 02 4.6 that has the exact same problem, but only has 26000 on it. Sat for a little while. I noticed when I changed the fuel filter, it looked rusty, very rusty. I do not see the plugs being a problem at this point in the game. With only 26k

 

Even with low miles on the plugs you still might have a bad one or more.

 

Turned out to be the speed sensor.