I have an 08 Merc 496 with a Whipple. I came into the 5 mph zone today and the idle, in neutral, sticks at about 1800 rpm. It will not go down. The linkage looks fine. The rpms do not "rev" up and down or waver, it stays at a high, constant idle. Yesterday in the five zone it felt like I had a misfire. The same today. I'm thinking it's the IAC. Anyone have this type of issue before?

I'm just learning my 496 Whipple that I built last winter, but in tuning I noticed the TPS sensor had a profound effect on how it ran. It was very touchy, just a little off and it would misfire. When the IAC goes bad I think they throw a code?

Another thought, what's your fuel pressure at idle? I've heard the regulators aren't the most reliable.

I'm not sure what a vacuum leak would do at idle on supercharged engine either?

Good luck!

It has a 555 ECM that says Motorola. I suspect it's the IAC so I ordered one for $45 from Amazon. My friend has a scanner, but I think the 496 takes a four pin adapter. He's checking. I'll see if it's the IAC tomorrow when it arrives. I don't know if it's related, but the engine seems to run fairly rich when it first starts ... and it seems rich sometimes at idle. I'll check the fuel pressure but as I recall it was OK.

Ok. Not the IAC. Installed the new one with no change. But, this is interesting: at cold the idle is correct, about 1,000. After the engine starts to warm up, the idle goes up. So, the problem with high idle is only when the engine is warm. Any ideas?

need your friends scanner,could be the water temp sensor or a vacuum leak or fuel pressure.