my cousin has a 98 sonoma and just called telling me his rpms are jumping suddenly then coming back down to normal. i really dont know what could cause this?
can anyone help me out?

 

how much of a 'jump'? are we talking 1000 rpms, or just 200 or so. If it's a small jump, it could be the torque converter locking/unlocking randomly

 

Does it happen when he mashes on the gas??? If so it could also be a crankshaft position sensor issue!!

 

is there a way to test the crankshaft position sensor? Or is it more a set of symptoms that = bad sensor?

 

The crankshaft position sensor will pop a code and there is a few things that can tell you its bad without a code, if you mash on the gas and the truck starts to buck and you hear a popping noise under the hood its the sensor also what I am going through till I replace mine is a rough idle as well!!

 

around 500

 

ok he says the rpms jump when hes just cruising, the PCM isnt throwing a code or anything either
he says other than the random jumps it runs fine

 

Then I would say it might be the cable, it might be slipping at its attatchment point on the tranni, I have never heard of them coming loose but it might be a possiblility!!!

 

hmmm his trans was just replaced by a not so renowned shop lol.
thanks man!

 

Not a problem man hope thats the problem!!