Have a 1991 S-10 Blazer 4x4, 4.3 with an overdrive automatic. Has 140k miles and trans works good but the fluid looks horrible, probaly hasn't been changed in a while. I've been told to do a flush and have the filter and fluid changed. I've also been told to just do filter and fluid change and not the flush. What do you suggest? and why?

 

Fluid and filter. Some vehicles you can carefully and for a very short time, unhook the line from the tranny out put at the front cooler and use the tranny pump to push some more of the fluid out. When they back flusk, they or forcing fluid in the opposite direction that the vehicle has been use to running for a long time. A lot of the inner seals and mating surfaces probably have taken a "set" and you can ruin them forcing the fluid the other way under high pressure. I've had more than one professionally tranny builder explain this to me and have experienced losing trannies including a 4l60E not long after a power flush.

 

I've never had a tranny flush done, can't justify how much they charge. I usually hook up a large container with a pipe nipple threaded into the bottom of it, fill it with 15 quarts or so and hook that up to the return line. Disconnect the supply line and dump that into a drain container. Run the tranny through every gear until all the fluid is gone, reconnect hoses and top off. Doesn't get every ounce out, but pretty close.