Mysterious small green cap and a chugging Stang

mickn8r65

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Aug 20, 2018
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Hello All,
First post so please be kind :)
I have 2003 Mustang manual transmission with a V6. I was looking for the AC recharge port when I came across this little green cap:
View media item 13797
I must have messed with it just enough to cause the car to chug violently in first gear. Less so in second and not at all above that. Any help on what it is, how I caused the problem and what I can do to fix?
Thanks in advance!!
 

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The green cap is the test port for the EVAP system. So if you messed with it and now have driveability issues it seems to me that you have created a vacuum leak.

NOTE, the EVAP test port has a left hand thread.
 
Spray a lite mist of water on it to see if you can hear the vacuum leak. Once found, fix the vacuum leak.

NOTE: the EVAP system is pressurized only to 0.5 PSI. So if you applied any high pressure air to the EVAP system you have likely blown a seal some where in the EVAP system.
 
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