1998 5.4 bucking, hesitating, shuddering on highway (mostly uphill) after driving a half hour or so and gets worse....then the check engine light flashes for a couple minutes and goes out @ about a hour and a half of highway driving. On the return trip I kept it out of overdrive and it ran fairly well, but the check engine lite lit up again (blinking) right about a hour and a half of driving, then went out just like before.

Did a scan the next day with NO codes coming up. Even did the KOER and KOEO tests....everything is perfect! It runs fine today around town (as usual), but I haven't taken it on the highway again yet.

What could be wrong? The plugs were done @ a dealer about a year ago, with a couple coils. I added expensive fuel system cleaner about 5 months ago.

What would make this happen only on longer highway runs?

 

Sounds like a low grade misfire caused by a cop going bad. A code would help pinpoint
which one, or get some off ebay and change them all. They have visteons for a good
price, and they are motorcraft.

 

Yes I have read that many times, low grade misfire. Does a cop sometimes start going bad only when they really heated up on a long highway run?

It actually feels more like a transmission shuddering or binding....then letting loose and running free again. How are the Transmissions on these, are they prone to trouble?


I have only owned it (a 1998) for 7 months, and this was the first 100 mile trip I took with it....it ran fine all winter around town. To get a fault code does it leave you stranded and you have to get towed???

 

I would definitely trouble shoot the COPs first. A mis-firing COP can easily be inferred as a transmission issue because of the shuddering and/or surging you may feel. How many miles are on the truck now?
As for your transmission question, the transmissions are rock solid.
With regards to fault codes, no you will not be stranded. Some codes will put the truck into "limp" mode whereby the computer will retard the timing and may even limit your speed and available gears all in an effort to protect the power train. What you are describing is actually a very accurate description of a low grade mis-fire as these tend to occur at highway speeds. Try a search for low grade mis fire and I think you will find many posts that match your symptoms. COPs are pretty cheap on eBay, you could get a set for $100-$125 from Global Automotive if they are still on line. I would address the ignition system before the transmission system.

 

Well today I took it out for a long highway run (1/4 tank) w/ overdrive, cruse & air on. It ran flawlessly....I had a scanner attached and NO codes while driving and then after I parked.
I think I know what happened....it was raining the day I had trouble, and today is was hot and sunny. I am thinking I have a cop that is effected by the moisture from highway driving in the rain. I have read that moisture travels along the air intake and that plastic engine cover and drips right on the cop's?

I also read that spraying WD-40 (or silicone spray) on the cop's can fix that problem? What do you think???

I am not keeping this truck (112K miles) so I do not want to put much money into it. But I want it to be OK for whoever buys it.
I'm a truck guy anyway, not SUV's.