This really bugs me. I got my hummer about 3 months ago. It drove around town great. No problems. Then we decided to take it to the mountains to play and show it off in our one horse home town. The trip went fine on the way there. It's about 600 miles one way. On the way back we noticed the transmission was not switching gears properly. We checked the tranny fluid level. Smelled the tran stick. It was not burned fluid. So when we got back home (400 miles later) we took it to jiffy lube and did a tranny flush. The next day we took it to the GMC dealer for a complete checkup. They told us the transmission clutch plates were going out. So we decided to start planning on getting a new tranny.

So it's 3 weeks later driving around town. We keep checking the tranny fluid. It keeps going down faster as time goes by. Next thing we see is a milky substance coming out the radiator cap. So we googled it and found out we needed to replace the radiator. I and a couple kids were able to do this. We also replaced the trans cooler.

It works better now with no fluid loss. I did put Dextron fluid in it. It still seems to be in overdrive mode. When I step on the gas from a stop it doesn't change gears quickly. The rpm go up past 25 like it's in overdrive mode. It doesn't happen every time. Is there a faulty vacuum line somewhere?