I have a 2000 HP 500 and when it is sitting idling it sucks a lot of air through the intake. It just sounds real loud, especially when it idles down from hitting the throttle. Does anyone know is this normal?

yes,,,it's your IAC...idle air control.

Is that a part that should be replaced on a maintence schedule? Also, lately it seems when I put it in the water it takes a while to warm up. It's kinda sluggish for 15 min. or so.

LOL...but unless the ECM has a warmup sequence and a normal run...what is your engine h2o tmps??

temp hangs around 150.

see a dealer...too many things to check.

Mine always has a loud hiss at stat up, and coming off plane. My inquiries told me it was normal.

Brian,

If your sitting there, like at a dock in neutral, and hit the throttle when it idels down is that hiss there then too?

The IAC is located on the throttlebody near the front of your intake plenum. It's round in shape, about 1 inch in diameter and has electric leads coming to and from it with a small boot that goes over it. There are 2 screws that hold it in place. It's purpose is to allow outside air in on a quick slow down or initial start up to aovid an overrich condition. For example when you rev up the engine and then pull it to neutral quickly, the butterfly is essentially closed and no air can enter for the moment. That's when the IAC opens and you hear th vacuum hissing. It generally doesn't wear out, unless it gets moisture inside. When disected It's essentially a small shaft with a spring around it and a "seat" at the plunger end. When it's seated no air gets in, when it opens you hear that loud hiss noise. To check if it's working you should be able to hear it. Or use a scan tool or simply remove it and depress the plunger. If you can't or it's extremely slow to return to it's full length state, then replace it. It's an OEM part you can get from a car dealer or boat. Retail is something in the high $60 dollar range. Same one is used for the all the big block late model EFI series (and maybe more models that I'm unaware of)

If it's malfunctioning a couple of things will happen:

#1. Roug idle or no idle
#2. Rich fuel condition at idle and possible shut down with a quick closing of throttle
#3. high idle and/or it takes more time than usual for the engine to return to idle after being reved up.

Hope this helps (I've replaced 2 of 3 on my engines over the past 30 months which is pretty good service life all things considered)

Take care,

Steve

What SteveDavid said!! I think this guy knows as much or more about the 500EFI than Mercury and CRE. Yes, mine does hiss, just like you described; if when you rev it at the dock, at neutral, or whenever. Totally normal. I would worry when it does not hiss, and starts to choke itself out. If you look at those butterflies; they are VERY tight clearances! Much closer than any auto GM TPI I have owned.