Hi I have been experiencing some engine issues with my 2006 496 Mag HO running rough and sluggish when the weather is a little cooler. I live the Detroit Metro area of Michigan and since the mid-May I have noticed that my engine is running rough and sluggish when I first get on the throttle to get on plane and then the issue goes away. The engine idles fine, tach reading is good at idle not bouncing around, and has never stalled out. When it's a little cooler out it does this when getting on plane and up to about a minute after that, then I can feel and hear something change and the engine smooths out and has full power it's almost like it shifts into 2nd gear. I get No alarms, beeps, nothing indicating an issue.

I'm now on vacation in the upper part of the lower peninsula of Michigan and it's now running rough the entire time and this issue doesn't clear up and go away pretty quickly, like it always has in the past. The water is a little cooler up here but the air temp isn't that cool. I have replaced both fuel filters at the end of May, made sure that there are no loose spark plug wires, added some sea foam to my gas tank, i had the spark plugs replaced in 2018 and have put on less than 50 hours since then. I just disconnected the negative wires on the batteries for about 15 minutes to see if that would clear up anything that may be computer related. I can tell that something is off when it gets into this mode the exhaust sound is a bit different and it just doesn't have the get up and go pep that it normally does, it's rough and some vibration getting on plane just feels different. Please help me with this issue as I would like to salvage the rest of my vacation and be able to pull my kids on a tube, right now I don't think that it's a good idea pulling a tube around with the engine not operating properly.

My 2002 496 HO runs much better since having the injectors cleaned by BUP! Also scan for codes first.

There are many things that could be causing the issue(s) and from your posting it seams to be getting worst. Might be a MAPT or other things. Need to get the motor scanned to see what shows up

So an update on this issue. I found a Merc tech and had him hook his Merc Diagnostic laptop up to my engine. First off he found that the TPS connector to the sensor was disconnected (probably wasn't seated tight from the last tech that worked on it almost 2 years ago). I find it very hard to believe that the TPS wasn't connected for almost 2 years and the boat ran as well as it did, I think that it was connected but not locked in by the connector and it came off. Once connected the motor started running a lot smoother and better. He also saw that he wasn't able to adjust the IAC with the MERC
Scan tool. So he ordered a new IAC and we put it in today. The original IAC Visually looks fine and just like the new IAC. Luckily he's right near the lake I'm camping on so it's easy to test on the lake. the new IAC didn't make any difference on the water test and he reattached his computer to the engine and the IAC is still not adjustable with the MERC DiAG screen like it should normally be. Does anything special need to be done after installing a new IAC? The engine is still not running as strong as it was before coming on vacation and the TOP speed RPM is down by around 250-300 RPM the top speed is down a good 5-6 mph. The MERC computer says it's running at 100% and no fault codes are showing including nothing for the IAC. The boat idles fine no issues and doesn't stall or hesitate when I put it into gear. Anyone have any other ideas of what to check and which pins on the computer that the IAC is connected to? Tech says the next step is to check the wires running from the IAC to where it connects to on the computer.

I guarantee you know who knows what’s your problem is 100% too bad he’s been banned!

To follow along with this, the IAC step counts shows as 0 ? Or literally, he cannot find the IAC line on the screen of computer ?

I have a 2006 32 ft with twin 496; I had exact same symptom's. I did IAC , Then TPS just as you have . Still loosing RPM & Speed. I was another victim of the wonderful Cool fuel paint delamination disaster !!!
Easy way to check is pull fuel regulator off top of cool fuel cell; It will have loose black paint plugging screen and in body of chamber on cool fuel block. Very common issue with most Mercruiser engine with cool fuel #3 on engine 2006-2010 ,
Then the fun begins, potentially flush fuel rail , injectors , change cool fuel cap and regulator assembly. VERY common and lots of information on web about this well know expensive issue.

The IAC info shows 34% duty cycle on his Merc Diag screen at idle but he can't adjust it at all with the scan tool like he normally should be able to with a properly functioning IAC . Apparently the PCM thinks it should be set at 34% at that idle speed.

For testing purposes he can command the iac to open )more duty cycle) or close further (less duty cycle) Is this what was happening ?

But he can’t for target idle rpm. The diagnostic computer can’t program, just test.

If you open tblade up for more ‘min air’ the ecu, trying to adjust iac for target idle rpm, will open it less (less duty cycle) since it doesn’t need to let in as much air as before.

Have to be careful though, opening/closing tblade also changes TPS reading. It’s all a fine balancing act and all three things (tps/iac/min air:/idle rpm)must be set within Merc’s set parameters for idle.

Does the IAC duty cycle change from 1st start cold thru warm up ?

Fixed it for you.