https://youtu.be/qdwlNfZM2Xw

I had the boat out the other day and fried the IAC. Replaced it and pig tail the other day and was able to clear the code with my diacom software. When I went to fire it up it does what was shown in the video. The key was on and it was randomly cycling. It will always crank however I cannot read the pcm with the scan tool as it acts like it is not getting power when it is doing this.

The boat did fire up and run for about 10seconds as normal but then lost power again.

Has anyone came across this before? I was thinking maybe bad ignition switch? I thought maybe main power relay but it will always crank and I didn't know if that relay controlled power to that starter.

Any help would be appreciated!

Start with your grounds on battery and also verify the grounds on the back of the block.

Then, if it does the same thing check the power lead connection that has the 50A breaker on the engine and your ignition switch.

Could be a bad key switch but I doubt it. Check the stuff Keith mentioned. MPR provides power to the engines system, not the slave starter relay. You can swap the MPR with one of the others which are next to each other, they are the same.

I'm not at the boat right now so it might be a dumb question. But which one is the MPR?

Isnt the 496 IAC issue like the 600SCI and 700SCIs? On the 600 and 700s if the IAC goes it it usually takes the computer with it.

Don't know your serial number but you will either have 2 to 4 small square relays. If only two relays then one is the MPR the other is for the fuel pump, just swap one for the other. If you have three relays then the third is the slave start relay, and all three can be swapped with one another. If 4 are there then you have digital shift controls

If when you turn the key you should hear the fuel pump run for a few seconds. If nothing happens it could be the computer like VoodoRob mentioned. Don't know right now, just need to find out what is going on.

http://www.mercruiserparts.com/bam/s...31819/12826/30

Not that I know of and I have lost a dozen IAC's over the years.

It kills the IAC driver in the 555 pcm when the iac is bad and the owner continues to operate the engine. It can also melt the iac wiring plug/harness and start a fire.

If it kills the Iac driver does the pcm need replaced or can it be saved?

My experience also , Send the 555 to Jeff at Whipple and have him fix it.