'89 delta 88 trans problem

Everytime the engine starts cold, it takes forever to shift between gears. It shifts into drive, reverse, neutral, etc. fine, just always at a very high RPM. This is causing me to get very bad gas mileage (about 17mpg) and just recently the engine stalled. If anyone knows what is wrong with it please help.

Engine- 3800 fuel injected, 160,000 mi, newly installed used trans

Welcome to AF!!

Check the vacuum modulator on the transmission. This sounds like the modulator is not working and that is why the trans is not shifting. It is easy to change and doesn't cost a lot of money. You may need to purchase a manual for your car to show you exactly where it is located. Once this is done it should cure your shifting issue.

One thing, if you haven't changed the filter and fluid lately you may want to change it too as a blocked filter can sometimes cause issues although this should have been done when the trans was changed.

IIRC, that particular transmission uses a throttle cable to control shift points. Any good transmission shop should know how to adjust it for you. It is really quite simple to adjust, however, it is best to set it while monitoring trans line pressure!