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Discussion Starter #1
i know nothing about cylinder compression, so i don't know if this is a problem or not. when i turn off my car the engine stops and i hear air leaking out of something from somewhere under the hood. i'm assuming it's coming from the cylinders or the intake, somewhere where air would be pressurised in the engine. is this normal? the air only takes about 3-4 seconds to discharge, then it's silent. i've noticed this since the day i bought my car, but never really thought anything of it. is this something to worry about?
 

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It may be the coolant reservoir cap relieving excess pressure. Often when a warm engine is shut off the coolant no longer is circulating so it will increase in temperature a little and expand causing the system pressure to go higher than the desired psi and causing the cap to vent. It could also be the refrigerant equalizing in the a/c system after being shut off. Just some thoughts on the possible causes.
 

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Discussion Starter #4
hmmm, never thought about that. the coolant system makes sense. but would the a/c system pressurise without being turned on? i don't use my a/c.
 

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Discussion Starter #6
but what if i don't use the defroster either? the only time i run air is in vent mode. i never use the a/c or the defroster. so this air leakage isn't a bad thing?
 

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i know nothing about cylinder compression, so i don't know if this is a problem or not. when i turn off my car the engine stops and i hear air leaking out of something from somewhere under the hood. i'm assuming it's coming from the cylinders or the intake, somewhere where air would be pressurised in the engine. is this normal? the air only takes about 3-4 seconds to discharge, then it's silent. i've noticed this since the day i bought my car, but never really thought anything of it. is this something to worry about?
This is compression leaking down into the crankcase from whichever cylinder (s) are under compression when the engine stops. The reason you hear this is because there is much less crankcase volume with no cam and lifter valley as in a pushrod engine. You can gauge the condition of your ring seal by how long this 'leak' takes to dissipate!!

Mitch
 

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Discussion Starter #9
well in that case how long should it take to dissipate? and i take it that some discharge is normal?
 

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WOT Junkie and avid corn burner
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I have a question about that?

Doesn't the AC Compressor turn on during Panel/Floor, and Floor selections?
The compressor engages when Max A/C, A/C, Panel/ Floor, Floor/Defrost, and Defrost is selected. It does not engage when Vent or Floor is selected separately.
 

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WOT Junkie and avid corn burner
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Sometimes I get no noise, sometimes I get a couple seconds worth. Stop freaking out, it's fine.
Yeah, all of the cars that my father and I have had that have either a 4.6 or 5.4 does this. Even the 5.0 on my Mustang does it too. What you hear is normal, so chill.:zbeer::cool:
 

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The compressor engages when Max A/C, A/C, Panel/ Floor, Floor/Defrost, and Defrost is selected. It does not engage when Vent or Floor is selected separately.
Don't know if its necessary or not, but to elaborate on this one.....

The A/C compressor runs in the defrost/defog mode because it has the effect of "drying out" the air, making it more effective at defogging your windshield.
 
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