ShootinRopes35z

In the spirit of BUP - I am offering this guidance....

I would have him check your injectors while running. He should be able to do a Drop Test and validate you are injecting at all cylinders. This will be noticed by the decrease in RPM's. Once you get to the injector that is not firing correctly you can than see if a new injector will alleviate the issue.

This sounds like every other 496 issue from circa 2004 - forward. Your cool fuel housing has began its "shedding of paint" some time back and now that you have either gotten decent gas or whatever the scale of paint through the system has started its migration towards the injectors. I would suggest the following and stay ahead of the problem...
Check Cool Fuel filters - replace. If you see black paint peeling on underside of the lid - Run the vacation and immediately get home nad start buying parts and scheduling for rails and injectors to be cleaned.

Fitchpowerboat

This right here is solid advice. We find that sometimes people instantly jump right to diagnostic software and skip the basics. We had a boat with this same problem in our shop 1 month ago it was flaking paint and plugged injectors. When we have a fuel injected engine come in with a running issue the first thing we do is check fuel pressure. Most of the time when the paint flakes it plugs the regulator screen and the fuel can’t get through the regulator fast enough jacking fuel pressure through the roof making engine run rich and foul plugs. We had a Volvo Penta come in last year with 110#’s of fuel pressure (they have same issue with their fuel pump module)

h20guy

Hi sorry I haven't posted an update recently I was trying to enjoy the rest of my vacation as well as the 4th of July weekend.. So here's the latest on my boat the overall performance is still down. A buddy camping with me helped me switch the original IAC back on to the engine. The new IAC has a shaft and a spring on it near the top and the original doesn't have those on it. The boat performed better out of the hole with the original IAC back on it but it's still not back to how it was. The Merc tech disappeared on me and stopped replying to my texts so I wasn't able to try out some of the suggestions above to test the fuel injectors and do any further IAC testing. I did read an other post on here that with the PCM 555 computer you really can't do much testing and manipulating it with the diag software with the IAC as you can with other ECMs not sure if that's true or not? I removed the Fuel Pressure Regulator to check it for debris and it's female counter part that it fits into, they both looked pretty clean and i didn't see any black paint flake debris on either of them. I did clean both of them with a clean rag and got into the orifices the best that I could. I also replaced the both the disc and element fuel filters back in mid-May and didn't see any black paint debris in the filters and in the chamber that the fit into. To me it seems like the throttle stick calibration is off as it seems to take more throttle at cruising speed than it used to take to get to the same RPM level. I have a mechanical throttle cable not digital and I also removed the flame arrestor to verify that the throttle butterfly is opening up all the way at full throttle, which it is and also closes all the way. Makes me wonder if the Throttle position sensor is not working properly? The tech said that the TPS codes went away on his screen as soon as it was reconnected to the connector. The flame arrestor looks quite clean as i can see lots of light thru it. One thing that I did notice on my last run on the water while I was camping and the wind was blowing from the back of the boat to the front of the boat was that the engine air sucking sound seems louder than it was before, any ideas what maybe causing that? I do plan on getting my fuel pressure checked ASAP to see if anything has changed with that. Any other thoughts and or ideas as what the power loss cause can be?

PA.WOODCHUCK

Check your messages...

h20guy

Thanks for the info Woodchuck. Well I just got back in from out working on my boat now that it's out of the water. I removed the side engine wall board and I was checking the plug wires on the Starboard side of the engine and i barley touched the wire closest to the transom and it fell down totally loose off of the plug. So hopefully i found the cause of my issue. It looks like the engine was only running on 7 cylinders and the Computer was trying to compensate for it. I'll find out when I get it back into the water and see how it runs which I'm planning on doing tomorrow. I'll keep you posted if this fixed the problem. Thanks to all of you who have commented on this thread!

nanzpac01

HI h20guy. I just sent you a message but your inbox is full. As you know I'm having the same 496 running rough when cold issue where my first run of the boat for the day when its cold it sounds awful and only running on a hand full of cylinders and then after a minute under acceleration it clears and runs great all day. A local Havasu mechanic told me he sees this all the time and its a failing IAC that is making the boat run too rich and fowl plugs when the engine is cold and once the boat is under load and warmed up the air leans out and the plugs come back to life, refiring the cylinders. I noticed in your post you changed the IAC and it didn't help so that may throw my mech's idea of a fix out the window but I'm curious to hear if your new heavy duty plug wires and spark plugs help. I read on other posts guys pouring some mercury quick clean in the fuel filter and firing the boat with the combination of some quality new plugs will fix this issue.