Changed the IAC. no change with the motor.
Spits, sputters, rough running. dies in idle, whistles or makes a strange hissing noise. Almost if it runs on 4-5 cyl. but firing in random.
The problem is, I don't know what needs to be done. All of the sudden the strbrd engine runs rough, dies at idle, spits and sputters and noise, almost whistling sound is evident. I need to read the codes first and see what's going on. Possible valves, rings, or whatever. ?

Possible head gasket, does your coolant and oil look normal? Has it ever been over heated?

sounds like you may have sucked in a intake gasket of a oring? forgot if the 525 efi had plenum gaskets or orings..
with the engine running at idle if you can hold it their take a propane torch and with just the gas not lit move the tip od the unlit torch around the edged of the intake,,if it smooths out you know you blew one of the gaskets or orings..check the pcv hose at the back of the intake as well..if the engine popped at all it could have blown off the hose of map sensor??

Not about to use a blow torch, but thanks. No heating issues, how do I know if I got some gasket parts in the engine? and where would it go? BTW no O ring, typ. head gaskets on 525's. Oil is is ok.

sounds like an intake gasket failure

How is your fuel pressure, I recently had a gauge suddeny start reading off hen I first fired my boat up for the year and adjusted my fuel pressure until it was reading correct and at that point my boat started running just like that , I chased other problems until I put a actual gauge on and saw dash gauge was off 8 psi, Smitty
Look at your IAC counts too if possible and make sure they are in the range

You can spray carb cleaner around the gasket area , if the idle smooths out and stabilizes , you have a leak .

I know what the problem is. You need to have a mechanic take a look at it and get the problem fixed.

Have you scanned codes? That's the first thing a tech would do.

Does it run fine other than idle? I had issues with an engine that would run fine until it idled for a few min, then would start running rough and missing. It ended up being cracked cmi headers. Once it idled for a few seconds it would start filling with water. Pull spark plugs and make sure there is no water in it.