This is a real head-scratcher. 2001 F-150 5.4L runs great for about an hour, then will begin to experience power loss where RPM drops to 500 or so, then after a second or two will come back up to previous RPM. This happens intermittently, but the longer I run the vehicle, the more frequent the stumbling becomes.

After the turning off the truck and letting it sit for 3'ish hours, it starts fine and runs fine until the whole process repeats again after running for an hour or so.

Plugs are new, fuel pump was changed last year, fuel filter is new. I also changed the Idle Air Control (IAC) valve on a hunch that it was the problem, but that didn't help.

On a really hot day last week (102 degrees F), I drove about 25 minutes pulling my boat (ran fine), then shut off the car for about 15 minutes, then on startup, it started stumbling again, and got worse as I headed home (barely made it!).

Any suggestions much appreciated!

Thanks!
John

 

You need to put a Scanner on, look at the live data then look again when the drivability issue happens, to see what changes.
Any other way is guessing.
Good luck.

 

Bluegrass, thanks for the reply! I appreciate your time and thoughts!

I've never used a scanner. Are you referring to an ODB II scanner? Or something else?

I never got a check engine light. Would I still get some useful information from an ODB II scanner even without a check engine light?

 

With a problem like you describe you can't get a lead on the cause unless you look at operating live data with a good Scanner.
Able to use it and interpret the data.
CEL indication is only a first driver alert and tells you nothing specific except there is a system issue that was detected.
Reading a code tells you the nature of the fault. You still need to interpret the meaning of the code. It does not tell you to replace any specific part.
Looking at the live data displays the <<range>> of operation for all parameters. You still need to understand what you see
Every possible problem is not able to be detected and reported by code due to the allowable operating range built into the software or you would see every little variation and be driven nuts by it.
For your issue this is the level that needs to be used to have any chance of identifying what parameter is shifting over time, then what part of the system to look into for the cause.
Unless you have a good shop to work at this level, it all guessing.
Lacking any knowledge of the system control and a Scanner, your not equipped to solve the problem unless you would get lucky.
The above is why you came for this first level of help in that understanding, as a start.
Good luck.

 

Bluegrass, thanks for the explanation! I've ordered a good scanner. Should be here in a couple days. I'll just leave it plugged in while I drive. In the meantime, only short trips in the truck!

Thanks!

 

Read the user manual for general operation, then any Ford specific info.
You need to know how to access the Mode 6 tests and step through them as well as general info like ignition timing, OX Sensor actions, table shifts etc..
To begin with, you need to get familiar with its operation because you don't know at this time what data to look at or what is changing unless you have some reference to compare to..
I would not at this point use a graphing functions if it has it because its to much to learn from the beginning.
Only analogue data to begin with.
It will be a learning curve that will take awhile.
Good luck.

 

Have run into very similar problem on a different vehicle that turned out to be the crankshaft position sensor.

 

I've had something similar in my 02 f150 5.4 screw. It always happened on long trips in Texas heat (100+ temps). I've also changed the IAC.

I thought the IAC fixed it, but I hadn't taken any long trips since then. On a 4 hour trip last month, it came back. I swung by the first car parts place I saw, (autozone) and picked up an air charge temp sensor. ($20.00 and changed it right in the parking lot.) Drove back a few days later, and even though temps only reached 97*, I didn't have any more bucking at high speeds or loopy idling at stops.

I hope this helps. You are the first person that I've ever read about having these problems.

 

CORRECTION' Replaces IAC with IAT. its the air temperature sensor.
The ...IAT .....was known for this issue by being out of tolerance.
It will not set a code for this situation.
The tolerance difference would show up in hot temperatures more than cold because it shifts table values over a longer term.
Then drivability shows up until the truck is driven in cooler temperatures to shift the tables again, then the drivability issue goes away..

Measure the difference in resistance between the new and old.
Lets see if there is a large enough difference to tell.
The sensor changes value with intake air temperature changes.
Good luck.

 

I don't know if your truck has COP's but my 4.6 V8 does. I have seen one start messing up and run fine until it gets hot. Then it will start making the truck kind of miss. I always keep a new one in my truck tool box and I have found that if I hop scotch them I can sometimes stop a miss that never throws a code. ex... Replace #1 COP and keep the old one. Drive the truck and see if it still misses. If it does still miss take the COP you pulled out and replace #2 with it...Then #3... Keep on doing this until you pull one and the truck stops missing when its out then BINGO... I've used this several times with success when everything else has failed.. Good luck!

 

Yes, a coil with shorted turns can be temperature sensitive.
Good luck.

 

I wonder is Spovich figured out what the issue is? A few days ago, I was driving in 101* heat for about an hour and the looping idle and bucking came back.

Bluegrass had mentioned a known issue with the shift in the tables. Does anyone know if there is a TSB with the issue?

 

Guys, the coils live a harsh life being mounted on the motor, getting heated and cooled repeatedly, taking radiator heat and exhaust manifold heat after engine shutdown.
A coil's wire expands and contracts each time.
If the wire insulation rubs through a short develops that has the effect of lowering the high voltage output.
Then its misfire time when the short occurs.
(Its not a hard fault) but causes drivability issues that come and go and no code is set.
.
Use a Scanner to look at the misfire 'history' in mode 6, test 53 for cylinders with high counts out of limits.
Any 'significant' counts can even mean the boot or early model that uses plug leads is faulty.
There is more to this than many think and need ways to detect it.
Many shops don't even know it so a lot of guessing and parts are changed needlessly.
Good luck.