Good afternoon all! I am working on a 2003 F150 with the 5.4l. I own a F150 with the same motor, so when the owner of the truck i am working on said he needed help I figured it would be a breeze since I know this motor fairly well.

Well, the truck I am working on has me scratching my head. It has a code P0171 and a code for a misfire on cylinder 3. I started by trying to diagnose the misfire. I checked both the coil and the plug on that cylinder and both look good. I then swapped coils from cylinder 2 and 3 to see if the misfire moved to cylinder 2. It did not. Now I am stumped. I checked to make sure the cylinder 3 coil pigtail connector is functioning correctly with a test light, and it is. What else could it be? The misfire only happens under load. Could it be a faulty/clogged injector?

Please let me know what you think! My brain is too hot to continue thinking of this issue!

 

Swap plug
Swap injector
Check injector fires....test light or noid
Check conector to coil fires
Check coolant in plug well

​​​​​​Above fail....compression test and look for coolant in cyl
​​​​​​

 

So you can actually feel and hear the misfire? Since you have a reader, the freeze frame might be interesting. It should show conditions when the code was set. Might as well look at a few more plugs also, just to see how they compare. If the ECM is adding fuel to compensate for a clogged injector then the other cylinders should be getting more fuel, and might show it. Easy to check.

It has been proposed that the PCV hose on some engines can leak and lean out just one bank. This proposal usually causes some argument. Some say it is a thing though. Not sure about the .54.

 

p0171 is usually caused by a vacuum leak

 

Thanks for the replies guys. I didn't get a chance to work on the truck yesterday as I was trying to remove some birds from a bathroom vent.

I am certain the vacuum leak and the misfire are related. The owner of the truck mentioned that the motor was recently rebuilt by a less-than-reputable shop. I am wondering if they didn't completely bolt down the upper intake manifold. I will do some investigating on it today hopefully.

 

yep check that

 

Ok guys finally had a chance to look at this a little more today. I checked all the upper intake manifold bolts and all were pretty tight. I also removed all vacuum lines and checked them with the vacuum tester. I repaired any issues and reinstalled. Drove the truck and still a misfire on cylinder 3.

Came back and swapped the injector from cylinder 2 to 3 and vise versa, and the cylinder 3 misfire is gone! Also, no misfire on cylinder 2! Now the misfire is coming from cylinder 7, but i suspect this is electrical since the previous mechanic swapped in a new pigtail coil connector, but made the connection with wire nuts!

What is the fuel injector life on these motors? On my personal 2003 F150 SCREW with the 5.4 I have never had any issues.

 

The fact that the swap fixed the problem seems like you had a vacuum leak right at the injector and the swapping fixed that leak, by accident.

The wire nuts make the vacuum leak not surprising.

 

Life of vehicle basically.
You can replace injectors.
You can also clean injectors. should be all this required usually... Unless you got one that's leaking.

Mine are fine, 2003...new style idle smooth as silk at 285k.
I've thought about replacing them, but just can't bring myself to do it when there's no real reason. I might still replace them at some point.

I did spend some money to have them power cleaned once, don't know if that was worth it or not. And Ive run a few bottles of techron and b12 chemtool through the fuel too. Ethanol make some black crud that can Lodge in the injectors and foul them

 

Ok finally got it solved after spend a few minutes on it each night. It looks as though the shop (yes, professional shop) that did the rebuild on this truck for the owner spliced in 4 new pigtail connectors for the COPs on cylinders 1, 4, 7, and 8. The only issue with this is that the shop wired them backwards! Also, they just crudely twisted the wires together and slapped on some electrical tape.

About 30 minutes (had a couple Corona Extras) and 8 butt connectors later, the motor is running strong and the owner is happy!

Just disappointing that a shop who does this for a living made that large of a mistake. It took a redneck with a case of beer to get it right.