So the situation is i have a 99 sierra with 200,000 miles on it but I'm not sure if the trans was rebuilt or not because i got it at 180,000 anyways...trans runs fine and shifts decent until i get past the 60 mph line the rpm jumps from 1500 to like 1900 or 2k and sticks there its really annoying and makes the engine really loud in the cab plus i have a corsa cat back which has no drone but gets really noticeable in the cab when the rpms jump. Did any one have this problem before ? is there something i can check
PS. Car has no Cats and the check engine light is ON but it did this rpm jumping before the check engine light. Recent tune up too Spark plugs,wires,oils,and all that good stuff

Thanks in advance

 

sounds like the torque converter is unlocking.

 

Whats the best way to fix it ?

 

my 80e was doing the same thing, turns out on mine 3rd and 4th gear were slipping really bad causing it to unlock the converter and throw a code p0984.