Hi, I have a 1999 F-150 4x4 with the 5.4. It was missing bad and had misfire on #2 and #3. Found bad injector on 2 and bad coil on 3 replaced them and plugs and miss went away. Its running good but about every 100 or so mile light comes on and I have code 303 and 171, misfire on #3 and lean on bank 1. Can't find anything wrong, thinking injector. Any input would be appreciated. Thanks in advance.

 

most missfires are due to COPS... even if you just installed one.
code 171 is normally vacuum hose leaking, or intake manifold gasket leaking. Look for hard or cracked vacuum hoses. The two small ones on the EGR above drivers valve cover is a good place to start.

 

Thanks for the info. Will look for vacuum leak tomorrow. Let you know what I find.
Merry Christmas

 

Looked for vacuum leak but never found one, a few of the hoses look suspect old and brittle looking but no leak found. tried listening for leak with a stethoscope definitely no big leak. Would a small leak cause problem and what's the best way to locate?
Truck has about 100 miles on it since last code if it does it again I will swap cop with another cylinder and see if misfire follows and try to find better way to locate vacuum leak.

 

I've heard some use a propane torch to find vacuum leaks.
Turn the torch on lightly, DO NOT LIGHT IT, and go over your hose's
and listen for the engine to respond,
Listen for a change in your engine’s idling. The idling should change immediately after you pass over a leak with the tip of the propane torch. The reason the idling changes is because the engine will suck in the propane through the leak, providing the engine with more fuel.

 

Clear the missfire first.
Swap coil with another location to see if the missfire follows.
Don't run with a missfire because the cats are put in danger if raw fuel passes through and burns in the cats.
After misssfire is cleared, clear codes to see what comes back.
Then go after that code.
Good luck.

 

Thanks for the response. Haven,t had time to work on it due to holidays. Will get back at it next week and let you know what I find. Thanks again hope yal had great Christmas.

 

swapped coils from #1 and #3 today and cleared codes, will see if I get misfire on #1 now instead of #3. Also looked for vacuum leak again. Didn't find any leaking hoses but found bad cpv valve. Will replace it as soon as I can get one and see if that fixes the bank 1 lean code. Will update again with results. Thanks for the responses.

 

leaking manifold gasket
soon it will start over heating

 

Vfast, hope that's not he case but might be. After replacing CPV valve and swapping cop on #1 and #3 I cleared codes. After a little over 100 miles cel came back on, checked codes and got 171 again but no 301 this time. Going to try and rig up a homemade smoke machine to check for vacuum leak. Have tried carb. cleaner and propane with no luck. The old CPV valve still rattled but I could blow through it both ways with no resistance, the new one( from autzone) I can also blow through it both ways, it does have some resistance one way but doesn't seal off, is this correct or could that be my problem? Thanks again for all responses and Happy New Year!!!!!

 

see if under oil cap is milkey

 

Not milky under gas cap. Got smoke machine going and found a vacuum leak. It was on the very back of intake toward firewall. There is some kind of sensor mounted to back of intake, has 2 bolt flange mounting it to intake, right beneath that there is a 3/4" hose barb coming out of intake, this is the hose that was leaking. This hose reduced down to 1/4" and ran down firewall and back across passenger side valve cover and was plugged off. Can you tell me what this hose was supposed to go to, wondering what previous owner removed? I removed broken hose and plugged it off, smoke tested again and found no leaks, reset codes. Will update once I drive and see if codes comeback. Thanks for all the input. Really curious to know what that hose originally went to.

 

I found a pic. of hose routing on another post and it looked like the hose I'm talking about connected to a tee that went on the cpv valve and to intake. Is this correct and if so do I need to get a new hose and tee and connect this or leave it plugged off? Thanks again for any input

 

Been home and back to work about 90 mile round trip and no cel yet, hopefully that vacuum leak was the problem. And thinking maybe it was the connector on my #3 cop causing misfire. Time will tell. On the vacuum routing diagram on truck (partially missing) it looks like that vacuum line originally went to a canister that also had a vent line from gas tank. Canster is not there, haven't got underneath and looked for vent from tank yet. Does this sound correct and is it needed?

 

Just wanted to say thanks to those that replied. Truck has been driven over 150 miles now and still no cel and it definitely idles smoother. Glad I found this site, found several fixes by reading older posts, has saved me lots of time and money already.
Thanks again