2006 496 Mag HO running rough and sluggish, Please help me fix this problem
 

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! :picard1:

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.

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.

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)

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?

Check your messages...

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!

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.