I have a 99 F-150 Lariat 4.6 4wd automatic trans. It starts great and accelerating up to about half throttle it does great but seems that when I put it about half throttle it stumbles and shudders missing. I ran the tank down low and put in some octane boost thinking I might have some bad gas but that didn't help. Haven't changed the plugs or anything yet. What do ya'll think? I just bought the truck a few weeks ago.

 

Get scanner on it even if the check engine light isn't on. You can have a pending code that hasn't tripped the light. If the scanner can acess mode6, check misfire monitors.

 

Yes, check the misfire monitors.

Otherwise, the high-load missing might be a dirty MAF.

 

Anyone else wanna take a shot at my problem?? I might be giving away prizes to the winner.......(key word being "might"). I still have to take off the tint from my two side windows....MY representative decided we needed a TINT LAW!!

 

Fuel filter been changed??

 

Good point!

 

Anyone else?? I'm gonna do all these things and I just as well fix all the things that might be going bad......soooo, lets here em.

 

We had a miss on our 75K mile 5.4 Excursion last summer. We changed the fuel filter as it was needed anyway. No fix. We changed the MAF. No fix. I bought new plugs and 3 COP boots (all our Autozone had). The back 2 rear boots were very wet, rusty, and dirty. This fixed our problem. Our EX was missing very noticabely, but never gave an engine light.

Note: I was of the impression that octane booster could damage the emission controls on late model engines!

 

I changed the fuel filter but that didn't help. Gonna go with the plugs and wires next.

 

You don't have wires like the old V8s had wires. There is a boot with a metal spring the runs from the COP to the plug. These run around $5 a piece. 8 would be cheaper than a set of good plug wires.

 

Thanx. I'll try that first. I haven't had any experience with the newer Ford engines. I liked it better when you could raise the hood on a vehicle and actually see an engine (although I can see a lot more engine on this vehicle).

 

My 54 has a 239 ci V8 that I can climb into on either side. It weighs as much as a 460, has 140 HP, and gets about 8 MPG. Sometimes newer is better (until something breaks)!

 

I had a similar problem with mine. Replaced the fuel filter and cleaned the fuel system and it helped but continued to be a problem. Replaced the plugs (Motorcraft) and plug wires and it runs great now.

 

I cleaned the MAF and went to Autozone and had them check it and it came back #3 misfire. I was gonna buy the boots but my engine doesn't have the COPs. It has plug wires with a big boot that goes down to the plug and has a solid plug connector just like the old wires. Any idea why that is? Oh yeah, the plug wire was loose and I reconnected it and now the engine runs fine.

 

If you have plug wires, you need to replace them. The boots are for the COPs.