496HO Engine performance issue.... opinions needed

[HubbaHubbaLife]

Guys.... I took out my boat twice in past three weeks and it runs solid all the way up to around 4-5 rpms where it feels sluggish and under powered. Kind of missing some umph. It idles fine and never stalls around docks. I was thinking maybe bad fuel so I put some fresh 91 octane in and got some speed runs done but still struggling and missing at higher rpms. It did the standard 100 hour Mercury tune up around 150 hours back. Anybody have any clues..... my mechanic moved to NC and I have a new one referred but before I pull him in I'd like to hear from our inmate mechanical types on opinions. Thanks fellas.

 

[Wizard29]

Try regular 87 octane. That’s what they are built for and like most. With 91, the computer adjusts the timing and the fuel burn isn’t optimal, which is why a lot of people get black soot on the transom and can actually see less power. That’s where I’d start.

 

[707dog]

what year 496? does it have the cool fuel system? asking because there is 2 fuel filters in that system a lot of people only change the one canister but under that is another waffle filter that gets overlooked. if not change your fuel/water separator filter the new gas on the market has a lot of ethanol in it and attracts a lot of moisture when stored

 

[Marios Metalworks]

What’s the rectal temp read at idle?

 

[NicPaus]

Spark plugs changed at last 100 hour service?

 

[HubbaHubbaLife]

Try regular 87 octane. That’s what they are built for and like most. With 91, the computer adjusts the timing and the fuel burn isn’t optimal, which is why a lot of people get black soot on the transom and can actually see less power. That’s where I’d start.

Sorry to say that wouldn't be the cause due to I always run the 87 octane .... I simply put the 91 in on last run thinking that'd help or perhaps I just got some bad fuel... but regardless it persisted. Thanks for the shout back.

 

[HubbaHubbaLife]

what year 496? does it have the cool fuel system? asking because there is 2 fuel filters in that system a lot of people only change the one canister but under that is another waffle filter that gets overlooked. if not change your fuel/water separator filter the new gas on the market has a lot of ethanol in it and attracts a lot of moisture when stored

2006..... I've used same certified Mercury mechanic for many years and several Merc powered boats.... I'd say he's very thorough. Thanks.

 

[dribble]

These engines eat spark plugs. Change them first.

 

[HubbaHubbaLife]

What’s the rectal temp read at idle?

Ha, nice one.

 

[Wizard29]

Sorry to say that wouldn't be the cause due to I always run the 87 octane .... I simply put the 91 in on last run thinking that'd help or perhaps I just got some bad fuel... but regardless it persisted. Thanks for the shout back.

Ah, got it. Change the filters and the plugs then as others have suggested. Not a whole lot more beyond that coming to mind for me if there are no alarms going off.

I had one of mine (have two HOs in my boat) blow a head gasket a couple of years ago for no apparent reason. I think someone else here had the same thing happen recently. Much as I hate to say it, you could have a cylinder getting weak, but I really hope not.

 

[HubbaHubbaLife]

Spark plugs changed at last 100 hour service?

Not sure really.... I'd say that I assume plugs get changed at every 100 hour service but thats a guess.... I see another comment about changing plugs so that could be it. I'm no mechanic but the sensation feels like that kind of issue to me. Thanks.

 

[HubbaHubbaLife]

Ah, got it. Change the filters and the plugs then as others have suggested. Not a whole lot more beyond that coming to mind for me if there are no alarms going off.

I had one of mine (have two HOs in my boat) blow a head gasket a couple of years ago for no apparent reason. I think someone else here had the same thing happen recently. Much as I hate to say it, you could have a cylinder getting weak, but I really hope not.

Appreciate that.... always hoping for cheap solution. I've got 530 hours of mostly 1K rpm usage as I'm in a no wake zone for about an hour each way before entering ocean. I think most folks believe these 496 engines can go for much longer hours without expensive fixes if properly maintained but who knows.

 

[Wizard29]

Appreciate that.... always hoping for cheap solution. I've got 530 hours of mostly 1K rpm usage as I'm in a no wake zone for about an hour each way before entering ocean. I think most folks believe these 496 engines can go for much longer hours without expensive fixes if properly maintained but who knows.

Mine had about 120 hours at the time. I don’t run them hard, I do maintain them well, and time above 4000 RPM is very minimal. Other engine has had zero problems. Guy who fixed it (namba860) said that sometimes they just do that and there isn’t really a specific identifiable reason for it. I’ve heard of these engines going 1,000 hours plus with no issues. Who knows, but I just hope the other one doesn’t decide to give me the same problem and hope yours doesn’t either.

 

[NicPaus]

Not sure really.... I'd say that I assume plugs get changed at every 100 hour service but thats a guess.... I see another comment about changing plugs so that could be it. I'm no mechanic but the sensation feels like that kind of issue to me. Thanks.

I don't think they do. Had similar issues with mine about 20 hours after it was serviced. Changed plugs and it fixed it. Running great since.

 

[Shlbyntro]

As mentioned before, try fresh spark plugs and changing both the disc and cartridge fuel filters. I recommend acdelco 41-983 plugs.

Pay careful attention to the spark plugs to see if there is any kind of uneven burning between different cylinders, I also always so a compression test with my tune ups for piece of mind.


That being said there is a service bulletin on your motor where paint likes to come loose from the inside of the fuel pump after the filters and gets sent down the line on the fuel system causing higher fuel pressure by clogging the fuel pressure regulator and sometimes causes erratic fuel injector behavior to where some injectors can be dumping fuel and others can not be giving enough fuel. All of which can cause the running issues you're describing

 

[LargeOrangeFont]

Spark plugs changed at last 100 hour service?

Yea change the plugs.

 

[Heylam]

I say find yourself a new mechanic rather then just throwing parts at it you hope will work.

 

[Mandelon]

If no one mentioned spark plugs, start there! I had similar issue and new plugs took care of it.

 

[QC22]

I've had issues with plugs and even plug wires (falling off) on mine. Same with a friend's 496. Plugs first. Could be anything really, as it hasn't been diagnosed, but plugs are cheap.

 

[02HoWaRd26]

If it ran fine last season and is just not the same this year I’d do one thing first then the other and test between.
First I’d buy the fuel filter kit and see if that helps. It’s three bolts then pull the handle, use an angle pick, pull the coffee filter out, drip the new filter in then go to your bench or whatever and replace the canister filter and the orings. It’s a 35$ 10 minute fix. Today’s ethanol fuel destroys those filters when you let the boat sit.
Fuel Filter Element & Screen Kit... https://www.amazon.com/dp/B07ND3JTN5?ref=ppx_pop_mob_ap_share

Second if doing that didn’t help at least you have fresh filters for the year, go order a set of NGK TR55IX -- 7164 plugs and replace all 8. Take a cereal box or a something box and put 2 rows of 4 holes marking the front and rear and as you pull the plugs out in the corresponding hole so you can look at them or show someone after. As if you have certain characteristics on the plug it can further diagnose other issues.
8 New NGK Iridium IX Spark Plugs TR55IX # 7164 https://www.amazon.com/dp/B0033I4W6E/ref=cm_sw_r_cp_api_i_KQ0IEbG2SXEGS

 

[guest hs]

Depending on the year of your boat it may not have Gen III Cool Fuel it might only have a spin on water separator.

 

[wzuber]

As mentioned before, try fresh spark plugs and changing both the disc and cartridge fuel filters. I recommend acdelco 41-983 plugs.

Pay careful attention to the spark plugs to see if there is any kind of uneven burning between different cylinders, I also always so a compression test with my tune ups for piece of mind.


That being said there is a service bulletin on your motor where paint likes to come loose from the inside of the fuel pump after the filters and gets sent down the line on the fuel system causing higher fuel pressure by clogging the fuel pressure regulator and sometimes causes erratic fuel injector behavior to where some injectors can be dumping fuel and others can not be giving enough fuel. All of which can cause the running issues you're describing

I gotta ask......why on earth would they paint the INSIDE fuel passage of a fuel pump or fuel system? Seems that's just asking for trouble. Crazy...

 

[CobraDave]

I believe at 500 hours plugs are due to replace per the manual. They likely haven’t been done. Pretty easy to do on your own.


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[Singleton]

I gotta ask......why on earth would they paint the INSIDE fuel passage of a fuel pump or fuel system? Seems that's just asking for trouble. Crazy...

it’s crazy, got my entire fuel systems replaced due to that. Merc payed for all the parts, I paid service.

 

[Shlbyntro]

Merc outsourced the fuel pumps, I believe to Walbro? And that's just what they did to stream line manufacturing. Of course it wasn't an issue and was discovered to be a mass issue until 5+ years down the road after the motors were outside of warranty.

You have to hand it to Merc here as they are not legally required to do anything for the consumer but volunteered to cover the parts anyway just because.

You all know the horror stories with Volvo Penta fuel pumps and they do nothing except continue to jack up the prices on them every year

 

[riverroyal]

I’d try a new prop


Or flat top marine in havasu

 

[wzuber]

Merc outsourced the fuel pumps, I believe to Walbro? And that's just what they did to stream line manufacturing. Of course it wasn't an issue and was discovered to be a mass issue until 5+ years down the road after the motors were outside of warranty.

You have to hand it to Merc here as they are not legally required to do anything for the consumer but volunteered to cover the parts anyway just because.

You all know the horror stories with Volvo Penta fuel pumps and they do nothing except continue to jack up the prices on them every year

Kuul they stepped up and at least covered the parts. Kinda hard to understand why they accepted that as an acceptable specification. Did they come from china? Haha
Were they experiencing corrosion issue's w/previous model pumps to warrant a coating of the casting material?

 

[Bigbore500r]

Plugs and filters are cheap. If it feels like its misfiring i'd do plugs first. Even if they were supposedly done 100hrs ago. Plugs could be a crappy brand. Plugs could have been gapped incorrectly. Plugs could have a cracked insulator. CHECK THE PLUGS!
If plugs and filters don't fix it, its Covid

 

[DaveH]

Try regular 87 octane. That’s what they are built for and like most. With 91, the computer adjusts the timing and the fuel burn isn’t optimal, which is why a lot of people get black soot on the transom and can actually see less power. That’s where I’d start.

can you confirm this? i know of no "octane sensor" or even a flex fuel sensor used in in merc systems.

 

[QC22]

can you confirm this? i know of no "octane sensor" or even a flex fuel sensor used in in merc systems.

The knock sensor does allow for timing adjustments as a result of octane differences. I have no idea if Merc uses it for this.

 

[Streetmoto]

This may sound crazy but have you checked your oil level since the service? Had a friend with a 496 that had similar issues 2yrs ago and after a lot of research and replacing parts he found out that the block was overfilled with oil. Turns out nobody drained the old oil before adding the new on his last service so the crank windage prevented it from RPMing. Idled totally normal, seemed fine mid-throttle. Wasn't puking oil out anywhere. Worth checking out. Good luck!

 

[HubbaHubbaLife]

Mine had about 120 hours at the time. I don’t run them hard, I do maintain them well, and time above 4000 RPM is very minimal. Other engine has had zero problems. Guy who fixed it (namba860) said that sometimes they just do that and there isn’t really a specific identifiable reason for it. I’ve heard of these engines going 1,000 hours plus with no issues. Who knows, but I just hope the other one doesn’t decide to give me the same problem and hope yours doesn’t either.

Hey appreciate that info Wizard29.... you're not really running twin 496HOs on a 29'Wizard are ya?

 

[DaveH]

The knock sensor does allow for timing adjustments as a result of octane differences. I have no idea if Merc uses it for this.

sorry, not buying this. aside from no other fuel sensors....(for that matter a 496 doesnt even have fuel pressure) the knock is a retard function only when knock is detected. there is no "run it till it knocks and then back off the timing function" in the ecu. knock can come from many sources.....not just fuel quality/quantity......engine Temp, inlet air temp, chamber contamination (soot, oil or other additives) and so forth. meaning that the calibration would never ASSUME knock was from the type of fuel only and advance timing accordingly.

furthermore, higher octane fuel is less knock prone then 87. even if the calibration was developed around 87 octane, use of 91 would just promote a higher safety threshold over the use of 87.

 

[HubbaHubbaLife]

I don't think they do. Had similar issues with mine about 20 hours after it was serviced. Changed plugs and it fixed it. Running great since.

No THERE'S some comforting news. Thanks my man. I'll continue reading all these great info postings from folks experiences and direct my new mechanic accordingly. I don't know him well, he briefly worked our launch ramp service as he built up his customer base. I hear good reviews on him.

 

[HubbaHubbaLife]

If it ran fine last season and is just not the same this year I’d do one thing first then the other and test between.
First I’d buy the fuel filter kit and see if that helps. It’s three bolts then pull the handle, use an angle pick, pull the coffee filter out, drip the new filter in then go to your bench or whatever and replace the canister filter and the orings. It’s a 35$ 10 minute fix. Today’s ethanol fuel destroys those filters when you let the boat sit.
Fuel Filter Element & Screen Kit... https://www.amazon.com/dp/B07ND3JTN5?ref=ppx_pop_mob_ap_share

Second if doing that didn’t help at least you have fresh filters for the year, go order a set of NGK TR55IX -- 7164 plugs and replace all 8. Take a cereal box or a something box and put 2 rows of 4 holes marking the front and rear and as you pull the plugs out in the corresponding hole so you can look at them or show someone after. As if you have certain characteristics on the plug it can further diagnose other issues.
8 New NGK Iridium IX Spark Plugs TR55IX # 7164 https://www.amazon.com/dp/B0033I4W6E/ref=cm_sw_r_cp_api_i_KQ0IEbG2SXEGS

Appreciate that comment.... I run boat year round here out of Newport Beach. I am pretty consistently taking it out twice a month depending on weather/ winds. It never sits longer than 3-4 weeks really.

 

[HubbaHubbaLife]

I've had issues with plugs and even plug wires (falling off) on mine. Same with a friend's 496. Plugs first. Could be anything really, as it hasn't been diagnosed, but plugs are cheap.

That;s actually a great thought.... due to the way I use boat out in ocean with lots of jumping and banging around. especially rough these past two outings when issue began. Thanks.

 

[HubbaHubbaLife]

I’d try a new prop


Or flat top marine in havasu

Thanks.... boat never leaves Newport Beach. Ocean only use.

 

[HubbaHubbaLife]

Much appreciated guys.... this forum is fantastic for feedback like this.... I've now got several directives to share with my mechanic. Hated losing my last guy of 12 years and two boats.... he was so frigging anal and smart and you could tell he wasn't gonna charge unnecessarily. I'm a bimbo at mechanic's mercy in this arena relying on good character. So we'll see how this new guy works.

 

[Wizard29]

can you confirm this? i know of no "octane sensor" or even a flex fuel sensor used in in merc systems.

I’ll see if I can explain this accurately. My understanding is that upon throttle up, the ECU advances the timing temporarily to purposely induce a knock as a test of the knock sensor. If the sensor reads, then the ECU is good and restores timing to where it is supposed to be according to the conditions. If the sensor does not read, then the ECU throws a code and retards the timing in order to protect against any damage. This equals a loss of power.

If you are running 91, it is possible that the knock sensor would not activate during the temporary advance in timing and the ECU would respond accordingly.

In any case, the 496HO is designed to operate on 87. The black soot experienced by many is basically unburnt fuel. If you aren’t burning all the fuel you throw in the hole, that means a loss of power.

I’m not saying that’s definitely the case here, especially since the OP later indicated he normally runs 87, but it can be a problem when running higher octanes.

 

[Wizard29]

Hey appreciate that info Wizard29.... you're not really running twin 496HOs on a 29'Wizard are ya?

No, the Wizard was my old boat. 34’ Eliminator now.

 

[THE WIDGE]

When you replace your plugs, take a look at the insulation piece around the center wire at the bottom of the plug. I had a discussion this year w my mechanic about this, tho I have a 525. He said he has seen numerous 496bplugs were you can spin the insulation piece , just spins around the center wire and it should not. Something about the harmonics of the motor as he has seen it on nearly every plug, some with little hours, some w a lot. He had a box of them that were broken

 

[DaveH]

I’ll see if I can explain this accurately. My understanding is that upon throttle up, the ECU advances the timing temporarily to purposely induce a knock as a test of the knock sensor. If the sensor reads, then the ECU is good and restores timing to where it is supposed to be according to the conditions. If the sensor does not read, then the ECU throws a code and retards the timing in order to protect against any damage. This equals a loss of power.

If you are running 91, it is possible that the knock sensor would not activate during the temporary advance in timing and the ECU would respond accordingly.

In any case, the 496HO is designed to operate on 87. The black soot experienced by many is basically unburnt fuel. If you aren’t burning all the fuel you throw in the hole, that means a loss of power.

I’m not saying that’s definitely the case here, especially since the OP later indicated he normally runs 87, but it can be a problem when running higher octanes.

there is no way it purposely "tries" to knock the engine to "test" the knock sensor. No.Friggin.Way.

i do agree with you the soot you see on the back of boats its from excessively rich mixtures...something that is well known merc does. However...thats doesnt necessarily equate to a loss of power. Often times rich mixtures are used under heavy load to combat high cylinder temps, and power would suffer if ran leaner.

 

[Trash]

I’ll see if I can explain this accurately. My understanding is that upon throttle up, the ECU advances the timing temporarily to purposely induce a knock as a test of the knock sensor. If the sensor reads, then the ECU is good and restores timing to where it is supposed to be according to the conditions. If the sensor does not read, then the ECU throws a code and retards the timing in order to protect against any damage. This equals a loss of power.

If you are running 91, it is possible that the knock sensor would not activate during the temporary advance in timing and the ECU would respond accordingly.

In any case, the 496HO is designed to operate on 87. The black soot experienced by many is basically unburnt fuel. If you aren’t burning all the fuel you throw in the hole, that means a loss of power.

I’m not saying that’s definitely the case here, especially since the OP later indicated he normally runs 87, but it can be a problem when running higher octanes.

The old 454/502 Mag EFI motors circa 1994-1995 did that. There was a Merc bulletin about this. It was a very small subset of production. Does not apply here. http://boatinfo.no/lib/mercruiser/manuals/mercruiser16.html#/538

If I were you I would pull the injectors and have them cleaned and flow tested. When the flow poorly it can cause the symptoms you describe.

 

[HubbaHubbaLife]

No, the Wizard was my old boat. 34’ Eliminator now.

Thank goodness... I understand the river guys enjoy their top speeds but I would imagine a 30' Magic running twins might be a bit much, lol. Then again I see some crazy center console boats in my harbor sitting around with 4 outboards way over powered so I guess that's a state of economy alpha male indication. There used to be something called hull speed where a certain sized hull can only go so fast regardless of power.... heard that back in 90s though so perhaps physics changed .... either that or male testosterone/ progesterone levels, ha.

 

[HubbaHubbaLife]

When you replace your plugs, take a look at the insulation piece around the center wire at the bottom of the plug. I had a discussion this year w my mechanic about this, tho I have a 525. He said he has seen numerous 496bplugs were you can spin the insulation piece , just spins around the center wire and it should not. Something about the harmonics of the motor as he has seen it on nearly every plug, some with little hours, some w a lot. He had a box of them that were broken

Good tip .... thanks, I'll share this.

 

[HubbaHubbaLife]

The old 454/502 Mag EFI motors circa 1994-1995 did that. There was a Merc bulletin about this. It was a very small subset of production. Does not apply here.http://boatinfo.no/lib/mercruiser/manuals/mercruiser16.html#/538

If I were you I would pull the injectors and have them cleaned and flow tested. When the flow poorly it can cause the symptoms you describe.

Much appreciated Trash... will do.

 

[Hypnautic]

what year 496? does it have the cool fuel system? asking because there is 2 fuel filters in that system a lot of people only change the one canister but under that is another waffle filter that gets overlooked. if not change your fuel/water separator filter the new gas on the market has a lot of ethanol in it and attracts a lot of moisture when stored

Pic of fuel pump housing and filter set into bottom. That bottom filter I think is like $25. You can see the water inside the housing.

 

[adam909]

when changing the fuel filters on a 496 ho. is there anything special you need to do? shut fuel off? empty fuel? never done it before so.

 

[XX14]

Sorry I’m late to the party. I have a 2005 Rage with the 496 HO. Agree with all the advice on the plugs. Mine has 300 hours and I’ve done them twice. Has anyone addressed the paint flaking off the cool fuel system? My mechanic spotted it last year..... when they first came up with the cool fuel system they painted the fuel regulators and the paint chipped off and got into the injectors. It could cause serious issues. With the help of Nordic Mercury Marine warrantied the issue my 13 year old boat. Best of luck resolving the issue.


Sent from my iPhone using Tapatalk Pro

 

[robby dmax]

when changing the fuel filters on a 496 ho. is there anything special you need to do? shut fuel off? empty fuel? never done it before so.

Just make sure you bleed all the air out of the system With the Schrader valve pretty simple