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Discussion Starter #1 (Edited)
Hello guys,

As the title of my post says, my engine idle (while in drive gear) fluctuates between 550 and 750. When the fluctuation occurs, there are vibrations that come through the steering and pedals. This fluctuation occurs without any accessories on (defroster, air conditioning, etc.). I lifted up the hood yesterday to see if my untrained eyes could notice anything amiss and I heard a hissing noise. After roughly narrowing down where the noise was coming from, I think it is coming from the area around the intake manifold. There are alot of vacuum hoses there as well.

My question is, should I notice any sort of air hissing noise in that area during normal engine operation or does this seem irregular to you?

Also, my check engine light is not on. I would figure the check engine light would be on if I had a vacuum leak..?

I drive a '14 Pro4. Lemme know. Thanks in advance.
 

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How often is your idle fluctuating? Mine is about once a minute within your specified RPM. My old Malibu idles worse, so it hasn't caused me concern.
Mine has the hiss from the passenger side as well and I couldn't find anything. I had it all apart for the timing chains and didn't spot anything obvious.
 

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Discussion Starter #3
It seems like it's completely random. Sometimes it fluctuates more frequently than others. Usually, the longer the truck has been running, the more the idle seems to drop and give a rough vibration through the steering/pedals.
 

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You could lightly spray some starter fluid on the intake plenum where you think the leak might be and listen for idle change.
 

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Starting fluid (ether) is VERY explosive - You might as well be pouring raw gasoline on your running engine!

WD-40 will work, and is only mildly flammable. When some of it covers the leak, you'll hear the idle smooth out for a moment.
 

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With the plastic plenums used on these engines, there is very little muffling of the air moving through them, so, they all sounds like there's a vacuum leak. As suggested, spraying WD-40 can help find a leak; I wouldn't use carb cleaner nor starting fluid on a plastic intake plenum. A smoke tester is the best way to check, but that's typically shop equipment and not something a DIY'er would have due to their cost. I've heard of a few brake booster check valves sticking open on these vehicles.
 

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every time i hear someone talking about using starting fluid, it reminds me of a time i saw a mechanic pour 3 bottles of it into the intake on a shiny new kenworth. they found pistons in the oil pan and one of the connecting rods shot through the side of the block. when that engine blew i watched the mechanic run away like he dropped a grenade. lol. good times.
 
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