I have a 99 4.6L,auto expedition XLT 4x4
around a month ago I added some fuel system cleaner- ran the treated tank up but still wasnt getting "good" gas mileage like when I originally bought it.

I then installed new NGK V prong plugs from Summitt and oil /filter change. (the old plugs were way gapped out at 66000 miles)after initial rough idle the computer adjusted itself and truck ran great went on trip of 50 miles -ran great.

Here's where I could kick myself - the first fuel injector cleaner I used was not what I "normally" use and seemed to do nothing--so I went into auto parts the next day after the plugs were changed (buying something for another vehicle) and they had my duralube fuel injector cleaner on sale. I was thinking well it runs great but I always add this at oil changes with great results- cant hurt. so I added it to tank (this was 2 tankfulls after the last treatment)-- argggg instant miss started - then I noticed it said only treat once every 2000 miles! the miss is noticable under very light acceleration from a stop (slight hesitation) and when cruise control engaged and going up a hill/crest at highway speeds at around 1500rpms- the truck will lightly lurch a few times dropping in speed then cruise control will jack up rpms above 2000 and truck runs fine.

again the miss started as soon as I poured in the second bottle of fuel injector cleaner.

it has now been a few weeks and 3 tankfuls of nontreated gas later. yesterday I installed a new fuel filter and "reset the computer" (disconected battery for an hour) the miss is now not as frequent but still occurs under the above stated conditions.I emptied the gas tank to about 6 gallons before adding fresh gas (3 tankfuls)-- by now mathmatically there is virtually no treatment(16oz.) left in gas.(so many hundredths of an ounce- of cleaner per gallon)

maybe adding the 2nd treatment and the miss were a coincidence but the miss started instantly after adding?
any ideas where to look next?-- clean IAC? or? any particular sensor I could have screwed up?maybe I should scrap the ngks for some motorsport platinums?

also truck revs in neutral/park up to 1500 rpms without ever missing a beat(slowly accelerating to 1500rpms or quickly)--hot or cold.

also the miss while driving from a stop occurs sporadically wether engine hot or cold(10-20% of time).

the highway miss at 1500rpm up a slope I'm pretty sure always happens just sometimes worse then other times. any ideas?Thanks in advance

 

Just a thought, with the 4.6 you have the 4r70w tranny. Have you changed fluid and filter lately? The tranny can cause what seems to be a "miss" and those trannys run HOT! It causes the fluid to break down faster. If you haven't changed fluid, do so and in my opinion (as well as alot of others) drain converter and change filter.

 

Thanks for reply prcr, but full fluid change and filter done 3000+ miles ago and gear selector sensor replaced.this miss just started about 1000 miles ago and I know its engine related/fuel/air/secondary or sensor because twice engine faintly backfired during one of it's "episodes". It also is slow to adjust fuel/air/throttle calibration right at initial operating temperature when at highway speeds and white smoke briefly comes out exhaust (viewable in rear view mirror) but less then a quarter mile later computer adjusts and corrects itself--- whether the delayed reaction and my miss are related I havent a clue.- cant remember if white is lean or rich?

 

I am having nearly the exact same problem with a 2000 model with a 5.4 triton in it. Please let me know if you have come to any sort of solution or if anyone else out there has any information to offer.

I had an 89 model also with the 5.4 in it, which did the white smoke and backfire also. Turned out that it was a coolent leak which occurs when you go up steep inclines. Coolent leaks onto a sensor and creates the white smoke and backfire.

My current problem does not include the white smoke and backfire issue.

 

Oxygen sensors perhaps? If I had a scan tool I might be looking at the O2 sensor voltages; they should flip back and forth above and below 500mv every couple of seconds.

 

Sure seen a lot of threads about EXPYS's having problems when switched to Bosch or NKG. Changing plugs is a bite, but the Motorcraft Double Platinum appear to be the smart way to go. O2 sensors aren't cheap and the upstream passenger side is a battle to change...would certainly feel more confident about changing O2's if indicated by a CEL code. If the IAC is acting up it should cause problems at idle. You might consider shooting the intake with some throttle body cleaner. I suppose it's possible a COP is starting to go. Have you washed the engine latey?

Good luck...let us know how you come out

 

I had a problem with my 97 explorer, it started shaking around 65 mph and would go away after speeding out of 65. The CEL came on and the dealer said that the intake manifold gasket was busted and O2 sensors went bad. I would agree that those O2 senors are acting up. Just thought id give you my experience.

Karlen

 

I think one of your "coil-on-plug" coils is bad or a connection loose.

 

Sorry for pickin' brains.....

done hundreds of spark plug jobs (engine rebuilds, carb rebuilds etc...), this being the first with plugs mounted "on top".another excuse is I had taken out my rubber grommett in the spark plug socket cause they have a tendency to stay on the plug and pull your ratchet "setup" all apart and i haden't cracked a plug installing it in years....

with this job i needed the grommett (couldnt find) so I wouldnt have to reattach the wire on old plug to pull it out of it's chamber and basically have to many steps when youre garage is attached and youre talkin' with family and uhhhh basically I got distracted-- had dropped a plug in cylinder tunnel by itself (cause again I did not have the rubber grommett in socket and you cant hand start these plugs...) then musta went to bathroom or got a drink somethin to cause a memory lapse- and then put the wire on unstarted plug!! Daaaaaaaaa
Of course I didnt realize until I had bought ford's platinums at 7 a piece!!(thats a ridiculous price!) but oh well at least this time I found my pair of large needle nose pliers to remove them generic wires),a new socket and a 1/4 to 3/8 adapter to use my 1/4 drive to get a better "feel" and prevent binding- job took 1 hour 15 min. compared to over 3 hours the first time! this also explained my secondary missing one second and then not the next second later...
I still think the plugs placement is quite generic (my mistake wouldnt of happened on a side of head setup...) and only 3 full threads in - how ridiculous...

jfbranham if your miss sounds like mine throughly recheck/replace your plugs, clean wires (make sure theyre sunk all equally on plugs compared to one another) after that youll need an error code or take it in for analysis cause at 80 per oxygen sensor (x4) and 2 cop's its quite expensive.

now I have an extra set of ngk's for future, and in all fairness to ngk the pickup from a dead stop was just as good maybe a little quicker takeoff and that's with one not even threaded!!! so all in all I wouldnt hesitate to reinstall ngk's 50000 from now and will.

lesson: even when a job is routine avoid distractions, or maybe one should retire from car repairs when approaching 40's ? (loss of memory skills-haha) I WISH!
someone kick me..........