I have a 98' f-150 4.6 auto 4X2 reg. cab , and lately I've noticed its slower off the line then normal lately , it gets noticably faster once I hit about 4000rpm's. I don't no why it doing this, any ideas?

 

try turning overdrive off. how many miles on it? Mine has about 71k (exact same truck as yours) and mine is still sweet out of the hole.

 

Mine has 38k miles, I'll try the overdrive trick but I thought that only locked out 4th gear?

 

It sounds like the IMT (intake manifold tuning) system is not working. Basically it is a valve in the intake below the plenum. It effectively changes the intake runner length, so the truck makes good power high and low rpm. It opens above 3000 rpm. Sounds like yours is stuck open. My Haynes manual says it needs to be checked with a special scan tool. You need to pull the upper intake to change it, if it turns out to be the problem.

 

Thanks for the info, so it wont throw a regular code and will have to be checked by a special scanner?

 

One thing that is quick and easy, that may or may not help, would be to "reset" the computer by disconnecting the battery. Your computer may just be working with data from slower driving.

I may be wrong but the IMTV should be located behind the alternator. When I installed the Magnacharger I had to take the entire intake manifold off. I took the IMTV out because the supercharger instructions didn't say what to do with the electrical connection. It's just a little motor with a "butterfly" type valve. My service manual says that the IMTV does not communicate with the PCM.

Let us know what you find.

 

Why would shutting OD off make any difference whatsoever off the line?

I'd check your air/fuel filters.