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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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 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?