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Discussion Starter · #1 ·
When I shift my C-4 transmission from park to reverse or drive, the vacuum gauge reading drops from 20 to 15. Is this a sign of anything?
 

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I really don't know if it signifies anything as I've never shifted my car when I had a vacuum gauge hooked up to it. The drop could be from the vacuum modulator on the trans though.
 

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I have a C4 also, but she's still in hibernation....NewEngland...I never checked myself, but you may have a vacuum leak in the line to the modulator. Does it idle rough in gear, or just drop a little?
 

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Discussion Starter · #4 ·
No, it does not idle rough at all, there is just a vacuum drop when put in gear. I went ahead and replaced the vaccum hose to the transmission modulator anyway with a new one. Now the vacuum gauge shows the lower reading of 15 whether in gear or not, it dropped with the new hose.
 

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The vacuum will drop when the engine is placed in gear. Vacuum is load dependent. The greater the load, the less vacuum. When in gear the engine is turning the hydraulic system in the trans causing a load.

You must have created a vacuum leak when the hose was replaced, double check.
 
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