Somethine weird,
 

OK, let me start by saying hey guys, it's been awhile since I've been on here, Truck has been doing great up until last month, at 186400 Transmission finally went out, which cost me $2000. But now on a another problem. Probably at about 160K my truck is acting weird, when I hit between 45 through about 70 it keeps surging RPM's, Like I'm at 1900 and it jumps to 2300 or 2500, but does not do it all the time. It's like I'm hitting the gas for a second then let off, but I don't and it's getting worse and worse lately. I'm just trying to get a starting point. It does it every time and a lot worse if I try to use cruise control.

Any help?

Thank You,

Derek

Sounds like your brake switch. Try this: get up to the speed that this happens and then tap the brake with your left foot just light enough to only light up the brake lights. What SHOULD happen is the torque converter should unlock briefly causing the engine RPMs to increase or surge briefly. If this mimics the problem surging, than your brake switch is likely the cause.

Could also be your TPS but let's check the brake switch first.

The first things I'd check would be the TPS and IAC. A Haynes manual has good test procedures for both. You'll need a multimeter.

OK,
 

Ok I will start testing and go from there. Thanks guys,

Any other help would be great,

Thanks Again.

Derek.

you've jumped all over me for my test procedure for the iac a couple times and i was just repeating the haynes manual in a few less words. nice.

anyway, i've tested my tps and even replaced it a couple times and it didn't help the surging at all. the wire that is in the tranny pan that one guy posted about a long time ago isn't rubbed through and the tps is good, so i'm at a loss also. i just got my tranny rebuilt and the guy that did it took it to the dealer and had them flash the pcm to make the tps less sensative and it helped with the surging. it was about 300rpm surge, now its about 20-25 rpm. i'm hoping that the wire re-route will make it stop since surging is one of the symptoms caused by the spark plug wires being too close together.

if it was the brake switch, wouldn't the truck kick out of cruise when you hit a bump?

Still sounds like a brake switch to me. From 45MPH and up the torque converter is locked (not slipping). for the engine to jump several hundred RPM while the torque converter is locked in top gear would require hundreds more horsepower than our trucks make. So, that means that either A) the torque converter never locks in the first place (problem) and something with the engine is causing it to jump several hundred RPM's (problem 2) or B) the torque converter is unlocking (causing a several hundred RPM jump with constant throttle) and then locking again.

I'm leaning towards B but the brake test on the highway will verify. Get up to 65MPH and while holding steady on the gas with your right foot, lightly tap the brake pedal with your left. If the RPM's jump like they have been: brake switch.

Sweet, I will try the test out tonight when I get off work.

Thanks,

if it was the brake switch wouldn't it make the cruise control disengage? the surge is the pcm telling the TC to unlock for some odd reason. i was told by my tranny guy that the tps is too sensative on these trucks and the newest flash for the tps makes it less touchy and makes the surge go away. although mine didn't go away completely i am hoping that the wire re-route will fix it.

Hmm...didn't think about the CC disenguaging. Worth a check anyway. Sounds like something is definitely telling the PCM to disenguage the TC-lock.

OK, I did the test and it is reproducing the problem, I got to about 65 and did the test and it does what it has been doing. So maybe it is the brake switch. So question, is it hard to change out and would you happen to know the cost? I'm not sure where it is located, and if it's an easy fix and cheap, then it will be worth the first thing I try out. So, I know that it's been also said that the tps could be bad, is there away to test it?

Thanks

Derek