I've got a 2004 Lairat Screw with the 5.4 and 100k on the clock. I have been chasing this problem myself for about a year now and am getting ready to call in the pros, but before I do I thought I would see what you guys think in all your infinite wisdom.

First of all, I can only get the CEL to illuminate when towing (even a light load) going up a decent grade and pushing over 4k rpm. When I do this, the CEL will flash (misfire indication). However, the CEL goes away a minute or five after I am done climbing the hill so it took me a while know when it would flash and be ready for it. Now I will have my code reader already plugged in and ready to go when I know I will be towing over a hill. I was getting a P0306 (misfire cyl 6). I replaced all of the spark plugs about 5 months ago with zero change in the symptoms. I then replaced the coil on cyl 6 a few weeks ago. Again, no change. I then try cleaning the cyl 6 fuel injector a few days ago. Just yesterday I was towing very light load and got it to throw three codes (P0300, P0305, and P0306). So now I have a random misfire (whatever that means) and one on cyl 5 and cyl 6. I am beginning to think that I have a blown head gasket between cyl 5 and 6 or an intake manifold leak. I was about ready to buy two injectors and another coil to have fresh plug, coil, and injector on these two cylinders, but now I'm not so confident that this will fix my problem.

Any help or insight is much appreciated.

 

First thing ---i don't think you have a blown head gasket because i assume you would have noticed a loss of coolant in the tank or possibly the engine overheating


Second---I would guess that maybe you have a low grade misfire on multiple cylinders that will not necessarily throw a code but would be caused by coils that are beginning to fail...which you would be able to find out if you scanned in mode 6 to see the live misfires

Any other things besides the misfire that you experienced like the truck is shaking...poor mileage...anything out of the ordinary which would help more with finding out what it is?

BTW....I'm just saying what I'm thinking it is....I don't know much about diagnosing stuff, which is why I mainly ask questions here....

P.S take it to the dealership they will recommend you purchase a new engine or transmission

 

Good to hear that it is likely not a blown head gasket, but would I have overheating and coolant loss issues if the gasket was just blown between two cylinders only? I don't have any overheating or coolant loss problems.

Unfortunately, my scanner is a cheapo unit and I can't scan in mode 6. Just recently I have had some violent shaking (two separate times) when going about 55-65 mph at low RPM. This truck has never got good mileage since I bought it (2009 with 76k miles). On my commute to work (no freeway and about 7 traffic lights) i average about 11.2 mpg. Mileage isn't noticeably worse than in years past. Nothing else much out of the ordinary. I did just replace a clogged cat on pass side (opposite side from the misfires which is a bit strange since misfires usually ruin cats).

Thanks for the help. I have two new injectors and another new coil on their way (should be here tomorrow). If those don't solve it then I don't know the next step to take. My fuel filter was new at 80k miles so I don't think that could be a cause.

My local stealership wants $120 to diagnose so I'll try a few more things before I take it in.

 

Two things come to mind:

1- What fuel do you use? Ford 5.4 engines respond well to 91 octane. (and hate the cheap stuff) Misfiring under a load is a tip off.

2- Perform a compression check in all cylinders, look for even ness.......(is that a word?)
(eliminates the head gasket worry)

Let us know.

Miz

 

5.4's run fine on 87, that's what they are designed for. Misfiring under load is usually (but not always) an ignition issue.

 

I buy the cheapest gas I can, so regular unleaded from Arco, Costco, Space Age, etc. I have always bought regular for this truck an didn't start having issues until more recently so I am thinking that is not my issue.

I have yet to put in the two new injectors and other new ignition coil since I was out of town all weekend and working on a hot truck after work is no fun. If after replacing these I still have a misfire issue, I'll check the compression and hopefully find no issue there.


I tend to agree

 

Granted they were designed for the cheap stuff, but here in Arizona, the dry air and temperature are not ideal.

Miz

 

Costco is top tier . Most of the gas I buy comes from costco and I have ran premium i my truck once that I can remember and it runs perfect.

 

Sounds fuel system to me. You need to have it scaned with a real good scan tool..that can read engine system performance..