You might also check the wiring & connections to the iac motor to make sure the connections are good. A friend of mine went to a Holley computer on his 500s & they would not idle at all, the computer was giving signals to control the iac but it would not respond. He found out he had to swap 2 wires so it would function. The computer would not throw a code for any of this but they just wouldn't idle, he switched the wires & the engines will now fire right up & idle like champs. The point being there can be more than 1 cause & it would be easier to check when your in there than have to go back again.

They make a special MAF spray cleaner that might work nice in cleaning the IAC

You would be surprised what a new ring terminal can do crimped onto some fresh stripped wire.

I thought I had a bad thunderbolt ignition on the cruiser and instead crimped new connectors on that appeared fine and she has been running better then ever all summer!

Good luck bud

I had the same problem yesterday, except the motor was idol ing at 1500rpm, I would try to get it into gear and it would stall. Through the laptop on and found the TPS was bad.

Sorry fo the thread jack but since you mentioned laptop, I'm currently in the market for some Merc diagnostic software, what are you using and how do you like it? Will it also work on outboards?

Thanks