99 blazer 4.3 230,000 miles.while driving engine surges 200 to 500 rpm then drops down to where it was then does it again does not always do it.some times for a few minutes other times for the entire trip.help

 

is the motor doing it or just the tach. if its just the tach thats a sure sign of the ICM giving it up. very simple and cheap fix.

 

motor as well as tach

 

..thats very strange, done a fuel pressure and leakdown test? wondering if the regulator is starting to have intermitted leaks and pouring raw gas into the intake at random. may also be TPS related so if you have the means pickup a cheapo scanner that can read live data and see what the TPS reading is doing when surging.

 

I will scan it thanks for your help

 

Sounds like TCC operating normally. Bet it doesn't do it below 45mph or if you drive in 3rd gear.

 

I have not tried just driving in 3rd. gear .it was only doing it below 2000rpm but going home it started to do it just above 2k rpm but stopped and ran like it was new.