I have a 1999 F-150 4.6L automatic (never flushed) with 46,000 miles. I have started to notice my engine will over-rev when I accelerate from a "coast" with just a little bit of gas. I first noticed this when I was going down a hill with the cruise control on. While going down the hill the throttle was all the way up and the engine was about 1100 rpm, but when I reached the bottom of the hill and the cruise started giving it some gas the tach jumped up to about about 1700 rpm for about 3 seconds then came back down to where it should have been. Since then I notice it all the time. I don't think it is "slipping" because if I stomp the gas it will respond properly.
The only modification I have made to this truck is Magnaflow muffler (1 in/2 out) and a KnN air filter. I first noticed the over-rev shortly after these modifications. Could this have something to do with my problem? Someone suggested that I disconnect my battery for a few minutes and the computer will "reprogram" itself to the conditions of the truck on power up. Any validity to that?
Any information would be helpful, thanks!

 

You don't give quite enough details, such as speed, grade, etc.... It's sounds normal though.

If you're in overdrive and at just 1100 RPMs, the engine is not producing enough power to keep the torque converter locked when you press the gas pedal. It's perfectly normal. The toqrue converter is simply unlocking for a moment to give you enough torque to move along. The tranmsission is dropping out of O/D and into 3rd gear.

 

AjRagno,
Thanks for your response. My problem seems to be happening at just about any RPM and grade. If I am driving at 50 MPH and I let off the gas for a few seconds, when I give it just a little bit of gas my tach will jump up several hundred RPMs for a few seconds and then drop back down to where it should be. I took it out of overdrive but couldn't repeat the problem. I can believe what you said is correct "the transmission is dropping out of O/D and into 3rd gear" if I was pressing the pedal to the floor, but I'm only depressing the pedal maybe an inch. Why would it be shifting to 3rd gear so soon?

 

My truck and my car do the same thing. I notice it more on my car It's a lumina, even when I'm on the freeway going 70. If I let up the gas, then press it down ever so slightly the tach will jump up and then come back down. It is normal, it is the converter.
Dan

 

I have a 92' that did the same thing, had a cousin install a shift kit, all gone. Trans is basically a hydraulic pump that move parts around, shift kit causes trans to build more pressure before the shift, hence the fast hit from a shift kit. Also not enough rpms to shift into od at low speeds.

Just my 2cents worth, but my cousin owns a transmission shop and he tells me that anything below 60 mph and the transmission will slip while in overdrive. Too much gear for low speed/low rpm. He says youy should never put the trans in overdrive unless running 60 or better. The slippage below 60 in o.d. contributes to increased wear on transmission as spippage equals heat and burned discs. If what he says is true you might try at speds 60 and above and see if you notice the slippage, if you don't notice it he may know what he's talking about

 

It sounds like the TCC is unlocking and locking up again. That would explain the slight increase in rpms. The PCM can determine when the time and conditions are right for a shift, or when to apply or release the torque converter clutch.

Regards

Jean Marc Chartier

 

Yes, it is definately the Torque converter locking and unlocking.
Perfectly Normal.
My dad's Jaguar did it as well after he installed the Chev 305 with the Turbo 400 and locking converter.

The locking convert is what helps you get the better gas mileage. When the converter locks, you crank shaft and the input shaft to the tranny are 1:1.