I'm looking for some direction. My 1999 4X4 Lariat with a 5.4 and 75K miles stumbles a bit at low rpm. It's fine at idle and it's fine under acceleration. It's when I'm coasting at low speeds, in parking garages, etc that it seems to hesitate. If I sit in the driveway and slowly rev it up, it gets to 1500 rpm then drops about 250. I'll slowly bring it up again, and again, it'll drop. If I fire it up and then drop to 1500, it seems to be fine. I replaced the fuel filter and PCV valve about 12,000 miles ago and the plugs yesterday. (Oh, what fun) It's done this for a little while now, at least through a couple tanks of gas. Any suggestions would be greatly appreciated?

 

I cleaned the air filter, and was careful to follow the K&N directions, about 2000 miles ago. The stumbling started before then.

 

One thing I would suggest, especially since you did a plug change yesterday: Disconnect the battery for 3 hrs. This will allow the computer to clear its memory and to reset. Since everything (bold/underlined) is computer controlled, resetting the computer(especially after mods such as FIPK, exhaust, and better plugs) is worth it, imho. Your truck will take a couple of days while it relearns your driving habits but you will hopefully notice an improvement in a couple of days.
Another suggestion: The problem u describe could be as simple as your fuel filter needing replaced. Since it needs periodic replacement anyways, may not be a bad idea ($8). This is especially true if you are in the habit (like me) of letting the fuel get low regularly. The reason I suggest fuel filter is that when the engine is at low speeds, the pressure may not be sufficient to overcome a slight blockage in the filter, but as the pressure builds (w/ speed) it will overcome.

But then again, it could be the dilithium crystals...

 

One more thing: I don't have the FIPK but I keep seeing where the FIPK's oil will foul the MAS pretty easily. I saw where you just cleaned your filter but what about oil buildup on the MAS prior to this cleaning? Maybe the FIPK-equipped guys can provide input... But the more i think about it, the more i lean towards the fuel filter.

 

Well,

The fuel filter change didn't do anything but rebooting the computer helped. No more stumble but now the Service Engine Soon light is on. Reminds me of that old Gilda Radner bit. "If it's not one hting its another..."

 

Did you fill up with gas? If the fuel cap is not on tight, the light will come on.

Only other thing is to have the code read so you can check whatever part it says is bad.

Maybe just try to reset it again. If it comes back on again, then something is amiss.

 

I changed my dilithium crystals just the other day, and man, what a difference! My door cracks are going away, my pinging is getting better and my mileage has been over 22 mpg in the city since the change, thanks Scotty.....

 

I have a 97 f150 5.4 the problem sound like the one I had. I replaced the EGR Valve, it seems to get dirty and does not colse all the way. Before I replaced it I would disconect the hose to it and attach anther one to it and suck on it several times you can here the valve closing when you stop sucking on it. My truck also would miss in park or neutral at about 1400rpm. Hope this helps

 

Hmmm.

I'll give the EGR thing a try. It does have gas. If that doesn't work, I'll reboot again. If nothing else, it's sure running smooth now. Can complain there.