Hi all, I have a 2008 South Bay Pontoon with a Mag HO I/O.
I put it in the water yesterday and it appeared to run fine for about 3 mins then it developed a miss throughout the RPM range. Almost like it is running on 6 cylinders. It got worse the further I went trying to get to my dock.
When I switched the captains call to straight out the back I could hear a popping or cracking noise through the exhaust when under load.

Seems to idle ok.

I have been reading on line and apparently plugs going bad are common, I also want to do a compression test while I have the wires and plugs out but wanted to know how do I stop the fuel pump from running. Where is the relay or fuse I can pull to stop the pump from spraying through the injectors into the cyl while I am have all the plugs out doing a test?

I also noticed yesterday an occasional metallic noise coming from the starboard side front of the engine up high. Broken spring common or a possibility?

Cheers
Chris

Are you getting any alarms? First thing I would do is get the motor scanned. That might not detect certain mechanical issues but it will find a lot of other important info.

To de-engerize the fuel pump:
There are a couple of large relays on the top of the engine, under the plastic cover. I can't remember if its the furthest one forward, or furthest one aft.

I'd definitely have it scanned as Dave said. There are so many things it could be. You may not have bad plugs, but it never hurts to change them. I have seen burned-through plug wires which symptom wise, matches what you describe.

See some of the other posts about the 496HO and using the Rinda & Techmate scanners to troubleshoot these motors. Buying your own scanner is a big nut, but it's an invaluable tool.
In your current case, you could use the scanner to shut down each cylinder one at a time to find out which one is is having issues.

Hopefully member "BUP" will chime in here. He usually does on these kinds of threads.

Just to make sure as 2008 was also a Catalyst Version 496 for all California Models so do have a Cat engine or NON CAT engine? Do you have DTS ? Do you have Smartcraft gauges ? Please answer these questions so I can possible understand the problem better. THX

More info.

I would do a leak down test, examine the spark plugs real good (yes they can be a problem ) and comp test. Like any engine popping or back firing thru the exhaust can mean exhaust valve / train problem. Also a vacuum gauge test would be good to do as well - Mercruiser even lists this test in their 496 Service manual to perform.

Also coils can be a problem but you have the 2 nd gen version coils in which held up a lot better over time but you can never rule out ignition & electrical components .

Have to ask does your engine vibrate at idle ? then some what seems like it smooth outs but feels like its down on power as RPM's increase ?

Also scan the engine and look at available power % and report what that is - this is important as the engine can be in a form of reduction mode from 5 % to complete shut down. Just saying as that can be why you feel like its only running on 6 cylinders (down on power)

Next in the scan - you need to know all the parameters of the scan to properly understand it & see if anything is off base EVEN IF YOU ARE NOT SHOWING ANY FAULT / CODES or have no alarms. A lot of times your 5 V reference signals / sensors can be off / or a problem and this can include all of that is linked to the smart transom harness / connections / sensors. I have seen this break up the engine (like popping) especially after half throttle running.

Scan it would be the easiest thing to do first and make a copy of all the parameters to show whomever for possible problem(s) then move on to internal engine testing. ie: leakdown testing / comp testing / Vacuum testing / pull valve cover spin over engine to see any valve train related issues or possible were the noise is coming from

Thank you all so much!!! Ok so had a guy kind enough to hook up his machine quickly and it popped out 3 codes. 94 Pitot Pressure Circuit Low, 75 Fuel Level 1 Circuit High and 191 Fuel Level 2 Circuit High. All are active and source was PCM03DTS.
It isn't a Cat motor, it does have DTS but just Faria electronic gauges. I am in the process of changing all the filters and spark plugs to see if that helps.

Those codes are not important. They have to do with sensors your builder chose not to incorporate.

Be sure to inspect your spark plug wires very closely when doing the plugs.
www.rockauto.com sells parts very reasonably. You may just want to replace them if you have easy access to everything.

if any of the plugs have rust on them then theres a good chance its got a reversion.

Swapped all fuel filters and the right bank of plugs so far and no change. Every plug I removed was finger tight, I unscrewed all without a ratchet on the end of my extension. Took 3 hrs to do 4 plugs, not an easy task!

No rust. What's reversion??

When you were changing SP did you at least do a compression test ?

How long have you own this boat ? if this is a new boat to you possible loose SP's were because someone did a comp test and never tighten the plugs afterwards knowing there was a problem. Just thinking out loud unless this has been your boat since new or for years.

Also with DTS do no spray any cleaners into the throttle body and do not worry about IAC or IAC muffler because DTS does not have them.