Mornin' , Stumbled across this site, while looking to solve a problem with the wifes Blazer. Sounds like i found the right place. The Blazer is a 4.3, 4x4 auto 2000 LS with 174K The problem seems to happen hot or cold. If it is coasting @ a steady speed, the tach will fluctuate. The tach is the only thing that moves during this. When it happens it feels like the truck looses power for a second and then jumps back to life. This will happen weather I'm doing 30 mph or 80 mph.She starts up and idels great It has a new fuel pump ( injectors checked out great @ that time),new 02 censor, new plugs , wires, cap, and distributer, new voltage reg. ,Cat has great flow. The tranny fluid is clear ( with conditioner add).No leaks @ manifold or vacume hoses. I did notice last night after a 60 mile trip , That when I shut the truck off , there was a ( tin can) sound I've never heard before. It sounded like a decompressed can popping back to shape.What do you guys think, Thanks, Dan

 

What are your tach readings and how much is it fluctuating?
Is your A/C or defrost/defog on at the time? The compressor cycling on and off will cause a slight change in RPM's.

 

Mornin', The fluxsing will happen weather the a/c /heat/blower is on or off. At first that is what I thought also. I should have put that in the first post, Sorry. It jumps about 200 to 300 rpm, just enough to loose power and return to normal.

 

is it downshifting?

im running 31's on my truck and ever since, the torque converter doesnt lock. so when im cruising, it downshifts for a second once in a while. was told i could deal with it, because its not hurting anything, or get a programmer, and tell it that it has 31's.

this bothered me at first, but was told it was a chevy thing... now i dont really notice it.

 

I dont think so, because it's not at the shifting range. It 's more like a I loose power for that second . For some reason it reminds me of " Vapor Lock " . and we noticed it does happen more when the tank is below a 1/4. Last night I heard the (thunk) from under the truck again when I stoped to gas up. It sounded like it came from the gas tank. I'm going to check the that th filler hose vent is not crimped and change the gas cap. Maybe something happened when installing the fuel pump ??????

 

I own a 2000 gmc Jimmy 4.3 vortec and I have the same problem with mine. I have no clue what it is yet. I replaced all my breaks because i though something was catching. That did not do it, I just replaced my hubs and cvs so everything is not catching. I'm going to try cleaning my M.A.F., EGR, and my IAC. Will post what happend after doing all the cleaning.

 

i did all my cleaning and so far it has not done that. I cleaned my mass air flow sensor, Idle control actuator, and my egr. So far so good. I hope this will work for you also.

Stephen

 

Mornin' Steve, You are doing everything I ended up doing. Unfortunatley, it turned out to be the senser in the transfercase and the tranny trashed itself before I could get to it. With over 150 k on the motor and everything else that nickle and dimed me ( all @ the same time) The wife ended up getting an Equenox LS. But it's OK because I still have my 91" Blazer and she ,at least , stayed with a Chevy. Good luck my friend.

 

Thanks for the info, Ill be sure to keep an eye on my tranny. I really hope mine dose not do that lol. Thanks for telling what the problem was! Sorry it had to be the tranny. How do you like the Equenox?