Drivetrain C4 Trans Issue


Feb 6, 2022
Hey guys and gals! Having an issue with my C4 trans and I'm hoping someone on here has some ideas.

I finished my 351w swap into my 1984 mustang over the winter and I'm using the C4 trans I had bolted up to the 302 that was in the car before. When I first started driving the car after the swap It was shifting great with no issues but now it doesn't seem to want to go into gear.

I'll go and start the car and I shift from park to reverse. It shifts great into reverse and catches right away but when I go to switch it into drive it takes some time. After shifting into drive it takes maybe 10 seconds for it to shift, once I feel it hit then it will drive but it won't go over 10km hr and will just rev up without going faster. I'll stop the car and then shift back and fourth between park, reverse and drive. It will sometimes take 5-10 minutes but eventually it will shift properly into first gear and then it runs and shifts great. I can go on a 2 hour drive, stop at the store, turn it off and then come back and drive it fine all day. Once I leave it parked for a few hours I have to restart the process again.

I have the vaccume line spliced into my break booster line that also goes to my Holley sniper. Fluid levels are all good as well. Not sure If the trans is starting to go or if it's a vaccume issue or something else I'm not thinking of!

Any advice would be great. Thanks guys!
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Active Member
Feb 19, 2018
Make sure it is full of fluid, my c4 will drive home fine, leak for a couple weeks, reverse out of the yard, and fail to shift into second. Some fluid always fixes it.

Otherwise it runs mid 7 sec 8th miles.

Next, drain and refill the fluid. While its out pull the tailhousing, remove and clean the governor, and reinstall. I have inline filters in place between the cooler and return line. This helps if the governor is getting crud in it.

Could be a stuck 2nd gear shift valve. Cleaning the vb may help.

Next up are the sealing rings on the shaft, when they get tired, its low on pressure when cold, but still seals when hot.

May need a freshen up.