Sorry for the long post.

Truck is a 99 F150 4x4, 4.6 v8 with M5OD manual transmission.

This truck also has coil pack ignition, not individual COP's.

About a year ago my truck spit out plug number one. I was about a mile from home so I unplugged the fuel injector and limped it home.

Long story short, I didn't have time to work on it but I did have another truck to drive. Knowing it was going to sit for a while I added fuel stabilizer and removed the battery.

Finally almost a year later I rented a Time Sert kit and did the repair on cylinder number one. I followed the instructions and the whole process was very easy.

I went ahead and replaced all 8 plugs with brand new MotorCraft plugs and replaced the wires with new MotorCraft wires.

After that I changed the oil and filter. Then I drained the fuel system and replaced the fuel filter. I added new gas and primed the fuel system.

The truck fired right up and seemed to be running smooth. I let it idle for a few minutes and it started to develop what seemed like a small misfire at idle. The misfire feeling got way worse with the AC turned on and made the rpms drop down to about 300 at idle, this made the engine struggle to stay running.

I decided to drive around the block and see how it would run. When I left the house the truck struggled to accelerate, kicking and bucking worse the more I accelerated.

After 1500 rpms the truck falls flat on its face and has no power at all while accelerating. If I gently press the gas it accelerates smoothly enough to make it driveable but it still hesitates and bucks through the rpms. It also is making what sounds like a small backfire while accelerating, it sounds like the backfire is inside the exhaust manifold area. After 3000 rpms it seems to accelerate a little better and gains some power back.

While sitting still the engine revs somewhat smoothly with a slight misfire feeling but under load it really struggles to climb in rpm.

I have checked all 8 plugs and they are fine. I checked the firing order and it is perfect, all wires are attached snug on both ends.

Thinking it could still be cylinder number one, I unplugged the plug wire and that caused an immediate noticeable misfire. Same thing happens with every other cylinder as well.

I couldn't find any vacuum leaks at all.

I have limped it around town for about 50 miles but I cannot get it to throw any codes at all. To me its running like it has a blown head gasket but the coolant and oil both look good.

I'm not sure where to look next and don't want to waste money on parts that won't fix the problem. Any help or suggestions are greatly appreciated.

 

did you change the plug wires?
if your on a budget, check wires with a OHM meter and replace the one that's high ohm's
coils can be had online pretty cheap, I've bought them from the scrap yard too.

 

Check the fuel pressure. You said you drained the fuel system. Does that mean you drained the gas tank. If not it could just be bad gas. If so, fill it up with fresh gas and put a couple bottles of Gumout for high mileage engines in the tank.

 

It could be plugged injectors. Just to make sure, I'd pull the plugs and do a compression test.

It could be a cracked coil pack or you have plug wires crossed.

 

My money is on the fuel system. Bad fuel somewhere. Either poor fuel delivery as in a partial blockage somewhere or just bad fuel in the system. Get ALL old fuel out of system meaning tank as well. Change the fuel filter again and yes check for a blocked fuel injector.

 

I completely forgot to update this thread and it was resolved a year ago lol. Thanks for the replies everyone.

For anyone who comes across this thread in a future search it ended up being a clogged catalytic converter.

The passenger side front and rear cats were completely hollow with no catalyst materials at all. The driver side cats were very clogged up. I still have no idea how the conditions of the cats didn't throw a code.