Hey everyone... I have a 97 f150 with a 4.6L just picked it up for $500 from a neighbor and its been sittin for 2 years behind a barn. The truck when cold cranks for a while before starting and when it fires it misses a little bit and runs rough, then eventually smooths out and idles perfect. When I push the gas pedal down fast it starts missing like crazy and acts like it wants to cut out and when I let it the pedal up and bring it back to idle it idles perfect... if i push the pedal down slow and gradually i can get it up to about 4 grand before it starts missing but after 4 grand it acts like its gona cut out again. The truck doesnt throw codes either. I started out by replacing plugs and wires but that didnt do anything. I put it on a scanner and it showed that 1 down stream and 2 upstream o2 sensors were registering at 0 so i replaced them and that still didnt fix anything. I used the special cleaner and cleaned the mass air flow sensor thinking that might be it but that didnt do anything. When the truck was running I disconnected the MAF sensor and the truck just about died and when I plugged it back in it came back up to idle so that seems like its still good... Im thinking intake gasket, TPS, or fuel pump but those are just guesses, if anyone could help me out I would appreciate it...

thanks

 

could be an injector,

after i bought my new 5.4 it missed, took it to the shop, showed no codes. the only thing we could come up with is that water may have gotten on top of the motor.

 

If not done already, make sure tank has fresh gas and a new fuel filter.

 

I replaced the fuel filter right away to and the old one was plugged but the new didnt make a differance either.. I also ran a tank of fresh gas through it that didnt make a differance either.

 

How about checking fuel pressure! With a gage.

 

I have had good luck with BG 44K fuel system cleaner cleaning up things. Expensive, but it works good. Might been some water in the system from sitting so long if tank was not full.

 

Alright.. well im going to see what my fuel pressure is at and if thats good I will put a fuel additive in with new fuel and try cleaning out the system... Any idea what fuel pressures supposed to be at? Thanks for the help I will try these things and let you know what happens

 

Techron is the best fuel additive - BG44K has been reformulated and is no longer worth the sky high price.

Fuel pressure should be around 38, and around 45 when the vacuum hose is pulled.

 

well i tested my fuel pressure and it lloks like I need a new fuel pump... With the key turned to the position before start i have 12 psi and with the truck started and idleing the pressure is 22 psi... I forgot to check it with the vaccuum hose pulled tho, is that a big deal?

 

Let me correct some things
Just turning on ther key before cranking will never bring full fuel pressure up.
It only runs the pump for about a second.
You have to cycle the key several times for full pressure assuming the pump's check valve holds each time..
22 psi is at least 10 psi to low.
Be cautious that the regulator may be at fault before changing the pump too quickly.
At least check for pressure change with the motor running be either removing the vacuum source to see if the pressure rises or use a hand vac pump on the regulator to vary the regulator pressure output.
Good luck.

 

Well I started the engine with the pressure gauge on the fuel rail again, and again the pressure read 22psi... i disconnected the vaccuum line on top of the regualtor and the fuel pressure never changed I hooked it back up and stayed the same... Does this mean the regulators bad, and the fuel pump may still be good?

 

I'm thinking you need a fuel pump.

 

You should read al least 38 to 40 psi on the fuel rail minimum.

 

Alright picking up the fuel pump tommorrow and hopefully putting it in I'll let you know if that solves my issues... Thank you everyone for your help so far