Fox Got a problem... What is going on?

79 Fox

Active Member
May 21, 2019
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A van down by the river
So here's what happened: Earlier yesterday morning, my dad and I took the mustang around for a few hot laps. I made sure to let it warm up before really getting on it. No problems then. Later yesterday, I took my mom out for a couple laps, just as I did earlier. First lap, no problems. Second lap, as I got up to about 40 or 50, the car would not shift into third. It sounded like it was bouncing off of the rev limiter we set. As I was pulling back onto my street, the damn thing wouldn't shift into second. I did not try to manually shift it into second, and in hindsight, I probably should have tried that. Park, neutral, reverse, etc. are all fine. What is happening? (The car has a C4 automatic)

Edit: forgot to mention that the trans was professionally rebuilt, and has a 2500 stall converter.
 
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79 Fox

Active Member
May 21, 2019
122
14
28
16
A van down by the river
Vacuum modulator or tvs?
I'll bring this up to my dad after school. Are these easy fixes? Could I not be getting enough vacuum? I have a B303 cam with 1.72 rockers. I know that a bigger cam can screw with vacuum.

This Wednesday we will bring the car to the shop that built the trans. The shop told us to come in and they'd adjust stuff for us.
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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polk county florida
Easy fix? Couldn't say. Could be vacuum problem, is there a vacuum canister with a check valve?
your trans guy should be able to tell you more.
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
14,542
4,600
193
polk county florida
I remember needing a vacuum canister on a car with a cam that had a lot of overlap so the vacuum was low at idle so maybe it would have no effect on a modulator valve since the trans is doing nothing at idle.
I would look to see it the vacuum line has not come off (if it in fact has a modulator valve).
 

79 Fox

Active Member
May 21, 2019
122
14
28
16
A van down by the river
I remember needing a vacuum canister on a car with a cam that had a lot of overlap so the vacuum was low at idle so maybe it would have no effect on a modulator valve since the trans is doing nothing at idle.
I would look to see it the vacuum line has not come off (if it in fact has a modulator valve).
I'll check after school. It's a line going to the trans from somewhere on the engine, right?