Transmission flush problems, won't flush
 

Ok a few months back I drained and refilled my wife's transmission in her 04. I added the same amount that I took out and moved on to flush the fluid out of the cooler. I plugged the return hose and put a length of hose on the nipple at the bottom of the cooler. I started the car expecting fluid to come out into my drainpan and nothing happened. Stopped and started several times still nothing. Is there a thermostat or something somewhere that would stop it from letting me flush the fluid out? I let it run for 5 minutes and still nothing.

Any ideas? does this procedure need to be done when the engine is hot?

Doing a flush is not recommended with the automatic transmission you can do more harm then good.

It's not the same kind of flush, I think you are thinking of the flush where they hook up to the filler neck and force fluid through the system. I'm talking about letting the transmission pump pump fluid through the system on it's own at it's normal operating pressure and speed.