496 backfire
 

My friend's fairly new 496 started backfiring through the exahust yesterday anytime the rpms went over 4000. We plan to check the codes later this morning with a scan tool but does anyone have any thoughts on this? Would it be worth checking the fuel pressure?

You sure its coming out the exhaust and not the flame arrestor? We had a cracked spark plug, which caused a misfire to come through the arrestor screen.

Had the same issue on a pair of hp500's. Let my son help on the tune up. and he cracked a couple on install. The back fire was thru the flame arrestor, thuogh it sounded like exhaust when running.

I believe I have the exact same problem. Ran fine last weekend but this Friday went out and started missing/backfiring at high rpm but only under load. Revs up fine and smooth in neutral. Runs perfect cruising around. Looking forward to some advice also.

Yeah, sounds just my dad's 496 in his Sunsation 288. He just replaced the plugs and it was fine. It was backfiring through the arrestor only under load. Change the plugs and I'll bet you'll be fine.

Here is a site that talks about the same thing you might be having, It turns out the $18.00 spark plugs that I believe you need to run gives out also, But from what I have been told it is not safe to run any other brand of spark plugs in that engine, Or you may have even a bigger problem. Read the post from "Gfinishline" It sound like he found it and after reading that post I have read about three other engines doing about the same things as yours, So It might be it worth looking at anyways, Good luck with it.http://www.performanceboats.com/html...ad.php?t=37580 http://www.performanceboats.com/html...ad.php?t=37171 http://forums.iboats.com/showthread....pi+spark+plugs

It sounds to me like it would be a broken valve spring on an exhaust valve.I would think that motor would have dual springs and maybe one is broken.Or the valve is hanging up in the guide at higher rpm's.If timing is all working correctly that"s about all I would think could make it backfire out the exhaust.Spark plugs fire when both valves are closed so a bad plug would make the motor miss and not fire out of time.

I think I would have to agree with Pickeljim on this one.
If the engine is truly backfiring or popping out the exhaust only at above 4000 rpms, I would suspect a weak or broken valve spring on an exhaust valve on that side of the engine. If the cylinder does not seal completely during combustion with an exhaust valve hanging open, it will combust out the exhaust system with a pop or backfire.
I also have seen a cylinder not firing and spitting a lot of unburned fuel into the exhaust where it lights of from the other cylinders hot exhaust which could be caused by a bad plug, but the engine should be missing from that plug under load.
I have seen 496's go into guardian at 4000 rpms and spit and backfire if the throttle is held WOT, but that will show up on the scanner. Good luck finding the problem. My suggestion, figure out which side is backfiring and then look for exhaust valve spring issues under that side valve cover.
I don't think spark plugs are the issue here.

Best Regards,
Ray @ Raylar

If it was valvetrain related why would it not do it in neutral? I can hold mine at 5000 rpm in neutral steady and it runs perfect. And it runs perfect almost all the way up to full throttle.

I did have a poly lock back off and actually tighten, causeing zero + lash. It ran fine at idle to about 4 grand. Then would pop out the exhaust. So must confirm if it is out the exhaust or flame arrestor.

First of all NightHawk never said anything about it not doing it in neutral. He did clearly say it was backfiring through the exhaust,not the flame arrestor and only above 4K rpm's.

Your problem could be anything. Someone that revs and holds their engine steady over 5K rpm's in neutral is just asking for problems! You can't even tell if a engine is missing on a cylinder at 5K rpm's in neutral. Your mechanic must love you.

Thanks for all the suggestions.

It's difficult to imagine a spark plug at fault if indeed the backfire is through the exahust. The engine had no fault codes on the scan tool other than the usual ones. I'm guessing a lean condition would also cause a backfire from the intake and not the exahust.

So I think the most important thing he needs to do is get the boat back in the water and verify with certainity it's coming from the exahust and which side. I'll probably need to follow him to see if it's coming from one side or the other.

I am my mechanic. I know old school carburetor and nitrous drag racing though. Just looking for advice from boat pros here. Appreciate any info, thanks.

Did you get back out to see what's going on?

The boat's owner can't get back out until Saturday. I plan to follow him closely in my boat and watch the exahust.

I got mine out today with some new plugs and it ran perfect. Used the $4.00 Autolites. Old plugs really didn't look that bad and had 228 hours. Thanks for the input guys. Hopefully that's all it is for Nighthawk's buddy. Good luck.

Do you have a newer 496/8.1 engine???

2001 496 Ho.

My friend put a fresh set of plugs in before he put the boat back in the water Saturday. Problem solved. It must have been coming from the intake.

:drink: :coolcowboy: Good to here that it was not a big problem, what did the old plugs look like??? thank's

The old plugs looked perfect. :confused:

Just curious did you shake it and here a rattle inside like one guy had said before?

Mine also looked fine. Had a light gray color. Looking at them I'd of said they were fine and could not hear a rattle in them either.

Sorry to kick up an old and dead thread, but this post helped me solve my problem. I'm a long-time lurker but figured I'd give back to the community since this saved me so much time and money.

My boat (2012 Bryant 267 w/Mercruiser 496 Mag) started backfiring through the arrestor on Sunday. After reading this thread (and a few others) I opted to swap the plugs and the fuel filter. Long story short, one of my plugs was cracked. I installed new Denso Iridium IT16 (which advance auto pricematched Summit Racing at $7.00/plug) and the backfiring has stopped and the boat runs better than ever.

I do not understand why a backfire through the arrestor would be caused by a bad plug, so I was skeptical that this would work and was expecting that a valve job was in order. While swapping the plugs I did a compression test on every cylinder, all were good.

If you are backfiring through the arrestor on a Merc 496, start with the spark plugs first!

check the fuel supply module

496 back fire
 

Just wanted to add that I also had this issue this week . Thought it was a bad valve spring but ended up being a bad plug ..