To start I have a 1999 Ford F250ld with the 5.4 and 123k miles
I did a quick search and found some common problems to mine and some answers but not all.
I noticed many 5.4 owners are having rough idle issues. I am battling the same thing right now, I am a muscle car builder, I work at a shop and do nothing but old GTO's and chevelles, I can tell its missing/idling rough but no check engine light comes on. I tell the tech next door at the repair shop and he says its fine if no light. But thats **** I know its rough, I know it shouldnt do it, and I think my gas mileage is suffering because of it.

Also, these have the 10.25 rear gear in them? Anyone make a gear for that application?
What gear comes in these stock?
Also has anyone had a problem with trans/ converter lock up issues?
Mine is very noticeable locking and unlocking, almost feels like od is kicking on and off when it happens. I know my dads f250 v10 doesnt do it.
How long are these transmissions and converters usually good for?

Also as far as modifications to make this thing a little more powerful, maybe a little more efficient. I have a magnaflow exhaust now, I would like to put high flow cats on it and a K&N intake.
Anyone have anything else? Chips? Supercharger? converter? gear?

THanks very much for your time everyone.
Steve

 

wonder if the spark plugs have ever been changed. at 125k its time for one or two to fail. change them out and see if the miss stops. the "miss" needs to be somewhat excessive to set off a code. if it gets bad enough, it will tell you which cylinder is missing.--- on the transmission, wonder if the oil was ever changed? new fresh oil and filter will change the shift charactoristics on most. also get any crud out that is causing spools or solenoids to drag- delay.

 

The idle is probably a bad IAC (Idle air control) valve, my '97 does it off and on, but I haven't installed my new IAC yet. I want to say the Ld/7700's all came with 3.73 rears but I could be wrong.

For performance I like tuner/efans/Ud pulleys as a good combo, although it is good to get a tuner last so you can have it tuned for all your specific mods. Efans/Ud pulleys are the only mods I have done that gave me better mileage, 1.8mpg to be exact.

I've always thought my TC doesn't lock up as much as it should, when it is cold it will but otherwise I rarely notice it. The E4OD/4R100's last a very long time, these are the same tranny's they use behind the 460's, and PSD's so you know they have to be strong to begin with.

 

I have done the plugs recently, put the good double platinums in it. I have done a trans service recently, never a flush but I have a new filter in it.
I didnt think of an iac.
thanks guys, keep em coming

 

Try the IAC, I cleaned mine 2 & 1/2 years ago with kerosene and then penetrating oil. Still works fine. What kind of plugs did you choose? Some people have trouble if they don't use the specified plug, some don't. Most of these came with 3.73's, but 4.10's were available. If you got your foot in it good, the t/c lock /unlock is very noticeable.

 

I used one of the recommended plugs for it I cant remember at this time exactly what it was though. Excellent for the gear I shouldnt have to change now.
And the T/C lock is noticeable all the time i'd say, it gets annoying sometimes because it feels like its up and downshifting.
ill try the IAC tomorow, because everyone I talk to says its a COP and they want me to buy one then switch em around till I find the culprit.
F*** that, id rather spend the $400 do em ALL once and save myself the time and aggrivation.
But ill try the IAC hopefully it works!

 

If it were a bad COP it should stumble under light throttle, then when you give it gas it will disappear until it gets worse. If it is only at idle I doubt it is a COP.

 

..and a bad COP will usually throw a CEL.