2000 Blazer LT 4WD - 240K recently rebuilt tranny. Lately when driving after it gets hot it will surge/fluctuate from 4th to 3rd back and forth when cruising about 45mph and ~1600rpm. The tach jumps back and forth from ~ 1600 to 2200rpm back and forth -- but only after it has been driven about a half hour at 65-ish mph and gets warm/hot and only when under steady throttle at around 40-45mph.

Took it back to shop that rebuilt it (it's still under warranty) - the tech drove it and said the TC was unlocking -- and they replaced the TC and valve body ... and on the way home it shifted to 3rd at 65mph. Took it back - had no codes - so they cleaned and re-seated the electrical connector. Was fine yesterday but today it did the 4th to 3rd "waver/surge" thing at 40mph after a half hour cruising at 65mph and it was hot and AC was on. When it does the surge thing it is like 1 second in third then back to 4th for a second and then back to third for a second ... lather rinse repeat. IF I gag it it will straighten up for a minute or so and then right back to the surge deal.

Short trips locally no probs -- but head across town on the interstate for a half hour or so -- then hit the side streets and that's when it starts.

Any ideas what it could be??

 

I acquired a rebuilt trans when I put my blazer back together, and ended up having an issue with it not wanting to shift into forth, well as it turns out someone broke the one tab on harness...the harness that's inside of trans that is....not to mention was the source of my random leak that I had....in all it was pretty damn stupid , the harness coming out was cocked on an angle, I never would've thought that was my issue, good luck

 

Checked it yesterday on a short trip and it did it again just around locally without the interstate speeds. And it's closer to 40mph @ 1300rpm jumping to 1500-iish. It was late the other night when I made the original post. And when it's wavering/surging - if I increase or decrease the accelerator it stops for a moment until it reaches that certain speed/spot again and then it starts it again. I then took it out on the interstate at about 70-75 and it stayed locked at around 2000rpm.

Just wondering if the throttle position sensor could have a flat spot -- even though the check engine light is not on? Could a semi-faulty TPS cause this weirdness??

 

Just got back from trans shop and they pulled codes 100 thru 109 said to change MAF sensor first and MAP if that doesn't fix it. Could that be causing it? Also only does it when it's hot out with AC on and temp at straight up on the gauge.

 

I'm having the same problem with mine. It seems to do it whether it is hot or not everything else same as you described. I do have a code pop up though for engine temp p0128. Not sure if it's related. Anyone know what to do.