Rosario

Have 2005 5.4 lariat with 125k miles. Shortly after I bought it, I had knock sensor issue fixed and the truck was running great. Recently I have been getting random misfire P0300 code. The plugs were changed just before I bought the truck. Truck doesn't run bad except for the occasional "reduced engine power" message I get and the engine temp gauge shoots to the red. Once I turn engine off and then immediately back on this problem is no longer. The random misfire code continues to have "check engine light" go on from time to time.

Only other thing I should add is the truck has a bad fuel tank sensor. I have the parts to fix, just haven't changed it out yet.

Could it be coils?
Could the plugs have been replaced poorly?

I'm lost.

F150Torqued

If it were me - and I didn't personally witness the plugs replacement, I would be replacing the Plugs FIRST.


You will certainly read otherwise around here, but I'm not a big fan of replacing COPs for addressing misfires as an initial step. COP BOOTS, Yes, and stretch the spring, insure COP spring contact is secure, and use a little dielectric grease on the boots. Avoid greasy finger prints on plug ceramic and replace COP primary connectors if the snap is broken.


Operational design of the 5.4L dictates that everything has to be just about perfect or you will experience misfires.


On my 2004 (the '04 through some 2007s have a unique plug design), I am now a believer in another procedure to make double dog sure the ignition system is as near as possible to the way it was when new. My hair-brained idea is detailed here, but it has served me well, even at 220k now with no more misfires whatsoever.

https://www.f150forum.com/f4/5-4l-3v...6/#post3930018


Hope this helps

Rosario

Last week I bought new plugs thinking maybe the guy never put new plugs in. Took to a buddy's shop (because he has tool to remove broken plugs) and we pulled the first plug and it was brand new and perfectly fine. I did notice my coolant was low and decided to drain and fill with new coolant. Once I did that I disconnected battery for an hour or so to try and reset the issue in case maybe by chance it was a CPU glitch. Everything has worked great until yesterday when the light came on. I didn't notice anything with the way the truck was running but after checking the light code I realized it was the same issue. This morning when I started the truck, the light was not on anymore.

F150Torqued

Yeah, misfire DTC's can heal. (The PCM constantly runs a complex misfire monitor routine.


Not to be a pessimist, but I doubt you have seen the last of it. If you can get a Mode $06 report, it contains some more useful information than the single idiot DTC light. And I am attaching TWO DOCUMENTS that might be enlightening about trouble shooting misfires down the road.


Something that hasn't been mentioned is O2 sensors. I am a strong advocate of replacing O2 sensors at around 100,000 as a preventative maintenance procedure. REASON: They are not THAT expensive, and Just about EVERYTHING - EVERYTHING is based (if not absolutely dependent) on good accurate / calibrated readings from them for the PCM to control fuel injector metering and a whole bunch of stuff. And I believe they become 'lazy' and off calibration with age - contamination from / fuel additives / oil residue from normal oil consumption and the like. They could even be contributing to your random / periodic misfires by richening or leaning the mixture trying to compensate for their own out of calibration.

Rosario

So for about 10 days the engine light has stayed off. Today the light came back on and there was no change in performance. I went to start my truck this evening and the truck wouldn't start. I tried cranking 3-4 times before waiting a minute or so and trying again and it was successful. I turned engine off and cranked again successfully. Does this give you any new theories?

Thank you!

F150Torqued

Would be interesting to know what the specific code is that caused that MIL. Before clearing it, it would be enormously helpful to get the "Freeze Frame" data.

Rosario

Yeah, I think I'm just going to give in and pay $100 to have Ford run tests. Hopefully they can 100% narrow it down to something cheap that I can do myself without tearing the engine apart.

F150Torqued

Then I would just suggest that you NOT clear the most recent code/data before taking it to them. Even for them to 100% narrow it down may be asking for a lot - ESPECIALLY that part about being something cheap. Stealerships frequently recommend you need a new engine when they get stumped!

Rosario

Lmao, by cheap I really meant like coils or bad plugs.

The engine light goes on and off by itself without me resetting it.

F150Torqued

That is NORMAL if it goes ON / OFF over the course of one or a few drive cycles for any trouble code related to a constantly monitored condition - ie misfires. BUT, Not if you are talking about it going on and off for very short durations. OR, heaven forbid, FLASHING!