Dipstick pops out

68383GTS said
So the transmission would vent through the engine crankcase? Don't think thats even possible, or am I wrong?
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you are correct not possible. When I read your post the first time I assumed Engine. Then I re read your post...
68383GTS said
My transmission dipstick
Click to expand...

Simple answer is rubber grommet / gasket is no longer large enough dia to keep side tension on the tube.

OR

the trans vent is clogged

OR

both

You might not have oil coming out the filler but air pressure might be and pushing the dip stick out.

upload_2018-9-22_14-53-37.png

Final thought...

is the dipstick hitting the bottom of the trans pan? check with long straight rod and compare to dip stick length.