I just changed the transmission oil but the tranny (4 spd manual w/OD) still slips out of 3rd. Luckily for me, I can do what I need to do in it in 1st and 2nd gear, but should I be worried about losing them soon too?

Basically...sometimes it goes into 3rd, then I have to hold it there to keep it from slipping out (usually upon deceleration). Other times it won't go into third at all. It seems that it shifts into 3rd better after it's warmed up, but I could just be imagining things.

Any suggestions outside of taking it to a tranny shop? It's a 1980 F-100 I6.

 

Moved thread to transmissions forum.

"ritterpa" welcome to FTE and all it has to offer. There is plenty of help and advice to be found.
I moved your thread to the transmission forum since it is specific to the transmission instead of truck year specific. Try the search feature to see if there has been any other problems like yours.

Thanks, Rollie (FordBorn)

 

A standard that slips out of gear needs the synchros and gear replaced.