I have a 2002 Blazer that just started bucking while I'm cruising in higher gears (around 1500rpm). It is pretty noticeable and will jump about 300-400rpm back and forth and will cause my transmission to shift gears/truck chokes. If I manually put it in 3rd gear and get the rpms above 2k and cruise it doesn't do it. It also doesn't seem affected when I'm accelerating hard. Does anyone have any idea what this could be? I recently had my catalytic converter replaced because it went bad and I replaced my o2 sensors at that time as well.

More info.. the truck has 100k on it and is throwing no check engine lights other than a rear o2 heater circuit malfunction and the problem persists even after i clear the code. It idles fine and driveability is fine except for the random bucking around 40-50mph/under 2k rpm. Nothing has been replaced except the cat and all 3 o2 sensors since I bought the truck at 72k 2 years ago.

 

I would suspect the torque converter lock-up soloniod. It's what make the converter directly drive the tranny at highway speeds. Putting it in a lower gear will disengage it.

I would look there first!

 

i had a thunderbird that did that and i was saving up for a trans and i gave this chick i worked with a ride to lunch one day and she was like "you need a tps" sure enough her husband brought his diagnostic computer in and it said tps 20 bucks later the bird ran fine so check into that first get some diagnostics done by an honest mechanic. i hate all these computers and sensors.

 

I figured out my o2 problem and got the check engine light off for good. I also found out my tranny fluid was running low so I added in a quart and one of the two helped fix the problem. Thanks for the input though.

 

The last time I met an honest machanic his shop got shut down because he wasn't making enough money by lying and robbing people like most machanics.