Had a similar problem this weekend and it was a loose spark plug wire... You can always dump your water seperator in a coffee can and see what you have there.

It sounds more like your loosing the head gaskets. The head gasket material between the cylinders gets very thin on the 575's and will burn out causing cross firing. Especially if they are original Mercury gaskets.

Be carefull, it could cost you alot of money.

Quiet storm is correct...if you have over a 100hrs change them to atleast felpros 17048 gaskets. I just had mine changed and they were pretty much toasted between the cylinders. While you have it apart I would send the blower out for a refresh also. Already been down that road also.

What about a possible vacuum leak?

575sci help
 

The head gaskets and heads were done @ 260hrs. I put felpro head gaskets on at that time. It has 395hrs on it now. The Iac reads 36 at idle. Injector on time is 2.4ms at idle. Doesn't the IAC only control idle? It runs rough at idle and at speed. It back fires through the throttle bodies when it back fires. I have replaced the following so far:
Distributer, module, wires, plugs, rotor, cap, solenoid.
Electric fuel pump. I took apart the mech. pump The diaphrams were fine. I even installed a temporary electric fuel pump to eliminate the mech. pump and a gas can to eliminate the tank and fuel line to the tank.
New MAP sensor.
Alternator (it actually was bad)
Fuel filter, fuel/water separator.
I have checked the vacuum line from the cool fuel regulator to the throttle body. I also checked the vacuum line from the map sensor to the intake. Is there some other vacuum line I'm missing? Any response or help would be appreciated.

Thanks again,

Try gapping the plugs down. Almost sounds like detonation to me.

They are at 35. What do you recoomend and what would this tell me?

Thanks

I would try 32 and see if that improves it. Cant really recommend a gap, but im familiar with issues like this on turbo applications.

575 Mef3 computer
 

Anybody ever heard or seen a Mef3 computer go bad? Who can test this? Jim @ Arizona speed said it would take about 2 weeks before they could get to it.

Thanks for all the ideas!!!

Get the IAC back down to 15 and try it. Betcha it will run properly.
575SCi's are picky about IAC setting.

If you had scan tool plugged in, why didn't you adjust it?
It is adjusted by what looks like idle screw on fron throttle body. Careful-you don't need to turn it too much. Let it settle - then blip throttle - let it settle and then check reading. Repeat as needed.