My Grand Prix stays in high gear. It will shift thru all gears manually but when I stop it doesn't downshift to first. I am having a vacuum problem. Could this cause the shifting problem?
Thanks

Well, I gave it the weekend for someone to pop up with an answer so I’ll give it a try.

Vacuum “Could” be the problem. Vacuum on some cars is use to “Kick Down” the transmission to a “Passing” gear when you floor the gas peddle. I would get that in order first. Next, I would try replacing the transmission oil filter. I would also add a bottle of a “Transmission” cure all. This sometime helps older transmission.

These are just cheap fixes before I tell you to take the car to a transmission shop. Pick a reliable, recommended shop. Some of the places will take you for a ride and not your car.

By the way, when was the last time you had your transmission flushed and fluid replaced? Have you ever over heated the car? Do you tow anything with the car? Boat, camper etc. These cause varnish to form on parts of the transmission causeing them not to slide freely.
That is, overheating and not flushing will cause varnish.

No passing gear. I just took it on a highway trip and noticed. Who makes the best repair manuals?
About varnish and additives.
I bought a small chevette from my aunt who said the transmission was shot. I figured since I had to replaceit anyway, I would put a cup of "Paint thinner" in the tranny. Little gas, forward, reverse, drove perfect for 5 years.
STP and other gas treatments are nothing but paint thinner. Try it for cleaning carbs too. Great stuff at a great price.
Thanks

Being in the Uk and knowing more about manual trasmissions Makes me almost useless
But when you mentioned Vacuum I do know about ventilation issues on engines due to breather Pipe problems
I imagine that there are several such pipes around the transmission casing, Check that any rubber hose is intact not perished or blocked /kinked this will affect the internal fluid flow with an air lock stopping something moving when the road speed increases
If you block the engine breather of a car. it builds up pressure rapidly (from piston blow by) Then blows oil seals and stalls fast
Block the air filter or take it away performance get poor performance or a stalled engine (like it has run short on gas)
I've just taken the same analogy to an automatic gearbox And it'll be an easy fix if that is the root of the problem?