my 03 f250 has a 5.4 and i cant seem to get rid of a misfire in cylinder 3. i had a p0353 code but that no longer shows up. i replaced all the coils with Accel coils, replaced the plugs with motorcraft platinum. i have checked the wire all the way back to the PCM and it checks out. i replaced the pcm with one that has been reprogrammed. i have swapped the injector and coil with another position and still get misfire at #3. Replaced the coil connector with a new one. I have good fuel pressure. I no longer get the code but the misfire is still there, unplug the coil and injector while running and no change. Unplug a different cylinder and you can tell the change. I checked compression and its good. I am lost here!!!!

 

bad connector at computer. broken wire at connector at computer.
stuck valve. ? bad camshaft lobe... not opening the valve.

 

Thanks for the reply, I have continuity all the way from the coil to the PCM, everything inside the connector looks good, wouldn't the valve show up with compression test? One thing i am going to try is run a new temp wire from the pcm to the coil.

 

Yes, bad valve seal, bad piston ring seal or mechanical damage will show up on a compression test.

A leakdown test will also show you if you have a combustion chamber sealing issue, and let you know in more detail what the issue is, but if you have combustion chamber sealing issues you're going to at least pull the heads to repair it anyway.

 

I will do the leak down and see what i come up with...

 

air
fuel
spark
compression


1 or more is missing.

 

I have also seen in other post about the accel coils being junk. Any views on this?

 

I have read the same thing, most recommend staying with Motorcraft coils. But if #3 is the only cylinder misfiring and the misfire stays on #3 even after switching coils & injectors like your previous post said, then it sounds like it isn't the coil.

I have also read where someone recommended replacing the spark plug on any cylinder that had been misfired. Junk the plug and put a new one in. As I am not clear on the sequence of events regarding exactly what order the parts have been changed, you should verify that a NEW spark plug and a known good coil are all on #3.
Also, none of your posts mention the coil BOOTS, just that you have replaced the coils. One might infer that the boots were changed with the coils, but there is an old saying assuming facts not in evidence that recommends against assuming. The boots should all be fresh and new. An ill fitting coil boot can allow spark arcing and misfiring.
Moisture in the plug well is also a source of misfire even with all new parts.

If all the coils, boots & plugs are all new, my guess would be a bad spot in the wiring harness. Either it has been rubbed, crimped, or burnt resulting in wire damage; or else there is some EMI shenanigans.

 

A cam problem wouldn't necessarily show up on a compression test. At this point I'd pull the valve cover of the suspect cylinder and check out the cam.

A wiped lobe would leave the affected valve closed, so it would still show compression on a gauge.

 

Thank You.. I have seen it happen several times... at the dealership. (not Ford)


and a leak down test will not show a camshaft issue or a stuck valve.

 

Hi I am sorry to hear you're having trouble. Correct me if I'm wrong but it sounds like we're not sure if you're getting good spark to the cylinder. I would start by getting a spark tester and seeing how much ( if any) spark you have from the spark plug.

Then I would take another look at fuel which it sounds like it's an unlikely cause of your issue but also a much cheaper repair if it is.

Then I would be looking at the mechanical aspects of the engine. How many miles are on your 5.4? Has it pulled trailers a lot?

 

Sam I Am.. i have seen in another post that the harness can be as you say and at idle give you a good reading but at higher rpm have a weak spark, so i am going through the harness next. New plugs and the boots where changed also. I am trying all these before tearing into the covers.. thanks all for the replies!

 

Always the easies and cheapest is the best way to start...


is there spark.. spark tester. next
remove spark plug, crank engine .. is fuel getting into cylinder. remember fuel pump only runs about 2 seconds per crank. so several cranks maybe necessary.
at this time you can test/check for compression..


next is remove valve cover. and check mechanical operation of valves

 

With no codes being set I would suspect a mechanical problem, if compression and leak down are okay, then a wiped cam lobe, or another mechanical valve train problem, is a very good possibility.

 

They make a tester for the injector wire from the computer...flashing light. cheap.
Pull the coil out and slip in a plug see if you have spark. If both of these are good then fuel isn't getting to the cyl...cam or linkage or blockage. Parts are going to make you broke do simple checks first. Code takes a while to set...Also almost forgot vacuum leak will cause a cyl not to run at all. Good luck