C4 issue, maybe vaccum?

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1971 Spring Sports Coupe - Replica


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Hi Gents, and ladies.

Mustang 302 with new C4 (like 2k miles on it).
I am not a great mechanic myself, but use to understand how things work.
Was driving all weekend, all great. Stopped for 15 mins to grab a bite.
Started the car, felt like 1 cylinder might be off...stranger engine sound than usual and idle was higher...
And it does not shift from 1st gear! manual "1" and "2" work well, managed to get home on 2nd.

Feels like both engine and Gearbox went wrong at the same time, I assume this is this tricky lil part that connects intake with a gearbox. I assume vacuum hose or "modulator lines" or "modulator" itself.
Could you please confirm I am thinking in a right direction? I will try to check intake side myself, if not - have to show it to the doctor i guess.
 
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Sounds like the vacuum line going to your trans modulator valve came off or is leaking. This would cause the engine run rough and the trans not to shift. If that isn't it than the diaphragm in the valve might have failed.
 
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My all time favorite vehicle is our 1969 Shelby GT500
I would get a few cars each month that would come in for a harsh 1-2 upshift, and no 2-3 upshift. Most of the time the idle was too high and rough. In every case it was due to the vacuum line from the intake manifold to the transmission modulator having come off. In come cases the rubber line at the modulator had become swollen and fell off due to the diaphragm beginning to leak ATF, where the modulator had to be replaced due to said leakage.
 

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