Took the boat out this past weekend and it started just fine. Left it warm up a couple of minutes, as always, and backed it off the trailer ... idled about 200 yds to the end of the no wake zone at the ramp and slowly pushed the throttle. It climbed to about 1500 - 2000 rpm, started hesitating really bad, and backfired a couple of times. Pulled the throttle back to idle and it tried to die, so bumped the throttle to about 1000 rpm and it ran ok. Let it idle for a couple of minutes and tried a couple of more times pushing the throttle and it repeated the hesitation with an occasional backfire . Got a couple of intermittent alarm beeps during all of this but no constant alarm and when setting the throttle back to 1000 rpms, no alarms at all. It finally took throttle without hesitiating and ran ok, no alarms, no gardian mode. Ran the boat for about 30 minutes and then shut it off so the kids could swim for awhile. Started right back up, and while it was idlling, it started a 5 beep alarm mode. Boat ran ok back to the ramp, but still had 5 beep alarm the entire time going back to the ramp. Book says 5 beep is IAC failure, MAP sensor error, coil pack failure, fuel injector failure, or fuel pump failure. I can't find any local shops to scan the computer for about 2 weeks ... they are all running a backlog in service dept's. Because the engine ran like chit cold, but then ran ok when it warmed up, with no misses, hesitation, or any other quirks, I'm thinking it is the IAC, but that is a $150 guess, to replace it. I unbolted it and checked the plunger, and it is not bound up. I left it connected, turned the ignition on and the plunger still moves freely ... it isn't being "held" in any particular position. So, is there a way to bench test it, or any other ideas what is wrong? It will be after July 4th before I can get anyone to scan the PCM.

I want to start by saying Im not a expert on boat drives. However I have owned a car lot for 5 years and know a ton about fuel injected motors and all their sensors. I do not think you have a fuel pump or injector problem. Im not sure if you have a IAC sensor issue but there is a small chance at that. You could have a bad coil with a crack in it. If the coil built up dampnes while out of use it would cause the problem you mentioned. Once it warmed up the coil could have dried out then run fine. That does not explain why it ran bad on the way home. I have had problems with a motor doing the same think. My problem was caused by a bad knock sensor in one case. In the other case the coolant temp sensor was bad and the car did not know how to run when it was cold. After it fully warmed up it ran fine. But when it was cold it would shake and pop through the exhaust. Check and see if you can run a OBDII car scanner on the motor. They can be purchased on sale for around $100. Good luck and let us know what it was when you figure it out.

Based on some bench tests, it appears the IAC is dead. Won't know for sure until I get it on a scanner, but based on some help from Merc Racing Tech Support, it looks like it is probably the IAC. Here's the real kicker ... if it is the IAC, they are on backorder from Mercury, and there are none in any warehouses ... expected in stock, mid July according to local Merc dealer!

If it's the same as a 496 Mag HO I have a new one in the box.

Its not the same ... but thanks for the offer! I appreciate it.

Bob

My iac failure set 2 beeps about 1-2 mins apart checked it with my scan tool and IAC was reading open 100% at idle cold (and idled like a cammed out 500) even with the motor warm if I bumped the throttle a couple grand in gear it would stall out,Replaced it and ran a solid 72 on gps with 3 people on board no more problems.Hope this helps in some way for you/ not sure if the 525's has a similar motorola ecm but....02 cents