I have a 1990 f150 with a 5.8L engine. I just changed out the IAC valve(three days ago) and noticed immediate improvement in the shifting and generally how my truck runs, until today.

While getting onto the freeway the engine seemed to make a slight downshift. The RPM kicked up about 500 for a few seconds, and then drop back down. It continued to do this intermittantly up the one grade I drive to work everyday, but not while heading downhill or on the flats. It did it smoothly and it didn;t seem the same as when it will drop out of overdrive on a steep grade. My experience with this truck is when it does drop out of overdrive it is much more abrupt and the increase in RPM is higher.

It hadn;t done this over the last few days since I changed the IAC valve, but I am owndering if there is any corralation between chagning the valve and this, or if they are completely unrelated.

Prior to changing the valve I had idle problems that I had been ignoring for awhile. When you let something like that go on, does the computer try to compensate, and now it is trying to readjust?

Any feedback, help, guidance is greatly appreciated!

 

1. check trans fluid
2. reset computer (remove neg. batt cable for 15 min)
3. test drive and call me in the morning

 

Try resetting the computer.

 

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just a thought, could you be noticing T/C unlocking and re-locking? You did notice better shifting qualities.

 

Thanks for the suggestions. I will reset the computer, as that seems the most logical. The tranny fliud level is good, but in need of a change so I will get on that. I thought about it being the T/C, but the increase in RPM is much more than when the T/C unlocks and re-locks. We will see what happens.

Thanks again!

 

T/c?........................

 

It's the torque converter unlocking, and relocking. On slight inclines where the speed starts to drop it will unlock to reduce strain on the motor. 500rpms is about normal between lockup and unlocked.

 

Does it do it in any other gears at all?

 

Well, resetting the computer didn;t do the trick, in fact it is worse now. This morning it started fine, but when I got on the freeway it seemed to be reving a little high. Did a quick step on the gas and it acted like it was about to kill. Got to the grade and it was still reving high (2600 @ 65mph, vs 2100 at 65mph) and then it dropped down to what I would consider normal till I crested the hill and the RPM's jumped up and the CHECK ENGINE light came on. For the next couple of miles the CHECK ENGINE light went on and off intermittently, seemingly in reaction to how I gave it gas or let it coast. Then the CHECK ENGINE light just stayed on. After leaving the first stop light after exiting the freeway it shifted very rough, and proceeded to shift rough the rest of the way into work.

I am at a lost, and will probalby have to take it into the shop as I don't have the equipement to run any kind of tests.