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496 back fire cuts outs.

tbults88

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Ok ill start from the beginning. Im at a loss. Bought a 2003 24 aftershock got a good deal at 15k has 496 mag with bravo one. Took the boat out fired it up and it would idle for 5 min then die. Found water in the fuel. Took the boat home drained the tank had it cleaned by gretchens in LB they also replaced the low pressure pump and had the injectors cleaned. Next trip idles great starts to get on plain then falls on its face. Took it home replaced filters again put in new high preasure pump and fuel pressure regulator ( the one under the motor mount the cool fuel system. That was a nightmare) then took it out today. Idle great got on plane did about 30mph. Opened it up and back fire miss backbfire bring it down to idle runs great. Shut it down for 20 min fire it back up Start to get on it ran all the way around lake elsinore at 30mph no problems open it up and pop pop miss back fire. Any one had a problem like this?
 

sintax

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How fresh are the plugs? I had a bud with a similar issue of the missing and rough running. Plugs fixed it
 

Kylemenz1

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A shop should be able to plug a code reader into it to see what codes are being thrown. Throwing parts at it in hopes of fixing it might not be the best option. It could be as simple as a distributor cap cross firing, or something causing it to lean out. Trust me when I say you don’t want that. A lean condition under heavy load can do $$$$ damage real quick in a bote motor.


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Kahunajuice

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How fresh are the plugs? I had a bud with a similar issue of the missing and rough running. Plugs fixed it
2x what sintax said. I had the same problem, a new set of park plugs took care of the problem.
 

Nikwho

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What do the plugs look like? General look could tell you if its going rich or lean.
 

Nikwho

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Has the boat ever run well for you? Or did you buy it sans test ride?
 

tbults88

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No been chasing the prob since i bought it. Changed the plugs and took it out today. Same thing runs fine at 35mph but chocked out at high rpms gunna do wires this week
 

tbults88

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The old plugs did not look to bad little black ill post a pic of one tonight
 

Nikwho

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Honestly sounds like a fuel delivery issue to me! I'd try replacing fuel filter again and cleaning throttle body! Might have something stuck somewhere, from dirty fuel. I don't think that this smells like an electrical issue, just from what I've read here. Sounds like fuel starvation.
 

Nikwho

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Could check to make sure that the fuel tank is venting? Or when its misbehaving, open the fuel filler cap and listen to see if there is a vacuum on the tank. If the tank is not venting, or getting fuel returned properly, it could create a vacuum that the fuel pump will eventually not be able to overcome! I've had that with my old '59 Apache. I moved the fuel tank to under the bed, and used a flush mount marine fuel filler. Tank wasn't venting properly. About 10 minutes in to a test drive, it would start bogging and cutting out. Less time if I got on it hard. With little load on it (less fuel needed), it would get around okay. Was letting some air in, but not enough for heavy fuel useage.
Marine fuel fillers/caps don't vent like automotive caps. They can't, or else they would allow water in through the cap. Could be similar situation for you.

Honestly, this COULD answer why you had water in the fuel to begin with. If someone realized this issue and ran the boat with the fuel filler cap loose or off, water could have splashed into the filler neck, putting water in the tank(s). Im not sure exactly how your fuel system vents, or replaces fuel in your tank with air, but would at least be worth looking into and checking that the system is working properly. It would explain the water issue AND the bog with higher fuel demand.

Good luck!
Nik
 

tbults88

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A shop should be able to plug a code reader into it to see what codes are being thrown. Throwing parts at it in hopes of fixing it might not be the best option. It could be as simple as a distributor cap cross firing, or something causing it to lean out. Trust me when I say you don’t want that. A lean condition under heavy load can do $$$$ damage real quick in a bote motor.


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Gunna have it plugged in tomorrow at dick sheere marine
 

tbults88

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Could check to make sure that the fuel tank is venting? Or when its misbehaving, open the fuel filler cap and listen to see if there is a vacuum on the tank. If the tank is not venting, or getting fuel returned properly, it could create a vacuum that the fuel pump will eventually not be able to overcome! I've had that with my old '59 Apache. I moved the fuel tank to under the bed, and used a flush mount marine fuel filler. Tank wasn't venting properly. About 10 minutes in to a test drive, it would start bogging and cutting out. Less time if I got on it hard. With little load on it (less fuel needed), it would get around okay. Was letting some air in, but not enough for heavy fuel useage.
Marine fuel fillers/caps don't vent like automotive caps. They can't, or else they would allow water in through the cap. Could be similar situation for you.

Honestly, this COULD answer why you had water in the fuel to begin with. If someone realized this issue and ran the boat with the fuel filler cap loose or off, water could have splashed into the filler neck, putting water in the tank(s). Im not sure exactly how your fuel system vents, or replaces fuel in your tank with air, but would at least be worth looking into and checking that the system is working properly. It would explain the water issue AND the bog with higher fuel demand.

Good luck!
Nik
Thanks yeah it has a vent hose on it. Tomorrow im gunna check if its clogged up. That or im missing something in the fuels system. Feel like i have replaced the entire fuel system lol
 

tbults88

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Could check to make sure that the fuel tank is venting? Or when its misbehaving, open the fuel filler cap and listen to see if there is a vacuum on the tank. If the tank is not venting, or getting fuel returned properly, it could create a vacuum that the fuel pump will eventually not be able to overcome! I've had that with my old '59 Apache. I moved the fuel tank to under the bed, and used a flush mount marine fuel filler. Tank wasn't venting properly. About 10 minutes in to a test drive, it would start bogging and cutting out. Less time if I got on it hard. With little load on it (less fuel needed), it would get around okay. Was letting some air in, but not enough for heavy fuel useage.
Marine fuel fillers/caps don't vent like automotive caps. They can't, or else they would allow water in through the cap. Could be similar situation for you.

Honestly, this COULD answer why you had water in the fuel to begin with. If someone realized this issue and ran the boat with the fuel filler cap loose or off, water could have splashed into the filler neck, putting water in the tank(s). Im not sure exactly how your fuel system vents, or replaces fuel in your tank with air, but would at least be worth looking into and checking that the system is working properly. It would explain the water issue AND the bog with higher fuel demand.

Good luck!
Nik
Other thing is my intake is LOUD like its trying its hardest to take a breath. Not sure if thats normal for the 496
 

Nikwho

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Other thing is my intake is LOUD like its trying its hardest to take a breath. Not sure if thats normal for the 496
My 2001 Eliminator with a 496 was always quite loud through the intake, too. As was my '99 Eliminator Eagle with 454.
 

ConcreteDr

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Maybe a dumb question but how old is the fuel? Does it have ethanol in it ? I only run non ethanol fuel in all my river stuff heat and time will cause you problems if there is ethanol in it for more than 30-60 days @ 100F +
 

tbults88

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Maybe a dumb question but how old is the fuel? Does it have ethanol in it ? I only run non ethanol fuel in all my river stuff heat and time will cause you problems if there is ethanol in it for more than 30-60 days @ 100F +
Its fresh
 

Shlbyntro

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I suggested that he do a compression test just to rule that out and then go lake test the boat with a fuel pressure gauge on the engine and have somebody watch the fuel pressure when the boat falls on its face to verify that it is not the fuel dropping out thats causing the running issue. Currently I'm leaning towards a failed IAC valve thats stuck in the open position. Crossing my fingers for him and waiting on his report :)
 

Ballsdeep

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Had a friend with a similar issue on their 496, the problem was intermittent and they chased it for awhile. If turned out to be a wiring harness issue from the factory were one of the pins wasnt making good contact. Mercury ended up sending them a new harness, might be worth checking all sealed connectors.
 

87_stoker

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I skimmed over the comments quickly so hopefully I’m not being redundant... but has it been checked for the paint in the cool fuel system? I had a little pop at idle from one motor and had wot marine check and I had paint residue in the fuel system. So new fuel pumps, injectors and plugs and starts/ runs great. Parts are covered by mercury just had to pay labor
 

Shlbyntro

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I skimmed over the comments quickly so hopefully I’m not being redundant... but has it been checked for the paint in the cool fuel system? I had a little pop at idle from one motor and had wot marine check and I had paint residue in the fuel system. So new fuel pumps, injectors and plugs and starts/ runs great. Parts are covered by mercury just had to pay labor
My first thought as well. But from the description of the work the owner has already performed, this engine sounds like a 1st gen 496 which still used the spin fuel water separator with a gen2 cool fuel and a boost pump mounted above the fuel filter. Imo this is mercs best fuel system and it is actually what I convert older vaporlock motors to.
 

billperkins3@att.net

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Ok ill start from the beginning. Im at a loss. Bought a 2003 24 aftershock got a good deal at 15k has 496 mag with bravo one. Took the boat out fired it up and it would idle for 5 min then die. Found water in the fuel. Took the boat home drained the tank had it cleaned by gretchens in LB they also replaced the low pressure pump and had the injectors cleaned. Next trip idles great starts to get on plain then falls on its face. Took it home replaced filters again put in new high preasure pump and fuel pressure regulator ( the one under the motor mount the cool fuel system. That was a nightmare) then took it out today. Idle great got on plane did about 30mph. Opened it up and back fire miss backbfire bring it down to idle runs great. Shut it down for 20 min fire it back up Start to get on it ran all the way around lake elsinore at 30mph no problems open it up and pop pop miss back fire. Any one had a problem like this?
Looks like a nice ride for the buck!

 

tbults88

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tbults88

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Update for those watching. Compression check done all cylinders between 140 145. Got the fuel pressure gauge and headed to the lake sat. Letting shelbynitro take the lead on helping me figure this out as long as he dose not get sick of my questions. hopefully this helps someone else out latter
 

tbults88

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I've got a brand new IAC for a 496 sitting in the garage if he wants to use it for diagnostics.
Thanks but i ordered one earlier in my frustration and my idea of just replacing every part i could find lol i do appreciate the offer though. Im building a small stalk pile of parts and buying tools left and right. U ever need anything feel free to pm me ill help you out if i can. Im in the long beach area
 

02HoWaRd26

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I suggested that he do a compression test just to rule that out and then go lake test the boat with a fuel pressure gauge on the engine and have somebody watch the fuel pressure when the boat falls on its face to verify that it is not the fuel dropping out thats causing the running issue. Currently I'm leaning towards a failed IAC valve thats stuck in the open position. Crossing my fingers for him and waiting on his report :)
I thought the IAC as well but when the intermittent changed rpm it took that away from my mindset. However didn’t even think of compression test. The fact it’s changed as a variable is what’s got me scratching my head.
 

SBMech

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I suggested that he do a compression test just to rule that out and then go lake test the boat with a fuel pressure gauge on the engine and have somebody watch the fuel pressure when the boat falls on its face to verify that it is not the fuel dropping out thats causing the running issue. Currently I'm leaning towards a failed IAC valve thats stuck in the open position. Crossing my fingers for him and waiting on his report :)
Failed IAC would make it idle at 1300 rpm without control. Pretty easy to diagnose. Backfires under load at half throttle, makes me think fuel starved, bad coil, carbon tracking in the cap, burned rotor or fouled plugs.

Not too familiar with the early injected marine stuff, I'm thinking it's a MAP sensor controlled system?
 

billperkins3@att.net

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She deff has the early 2000s paint skeme lol
I understand there's pressure to get the "best boat" but just being on water is what its all about. Plenty of time to chase the next better boat. I've enjoyed boats of all sizes over many years all ramping into the pacific after my time playing on rivers & lakes. My first one was a 23' open bow Caravelle Interceptor. I took that lower profile to Catalina many times so don't shy away from doing that. But first get that engine right.... looks like you have two of our resident tech studs already on your post.... you're in good hands.
 

Shlbyntro

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Failed IAC would make it idle at 1300 rpm without control. Pretty easy to diagnose. Backfires under load at half throttle, makes me think fuel starved, bad coil, carbon tracking in the cap, burned rotor or fouled plugs.

Not too familiar with the early injected marine stuff, I'm thinking it's a MAP sensor controlled system?
Yes, its a MAP engine. No distributor tho, individual coil packs mounted on the valve covers with hi tension leads running down to the plugs. Bad coils or wires are a possibility but he's mentioning the engine is falling on its face on plane which does not sound like a coil or wire issue on his engine as it would be rough uneven idling and running with poor performance all the way through the range. Merc fuel injected engines tend to barely idle with a bad IAC and fall on their face at a certain higher RPM as the measured inputs confuse the ECM. Im not saying that's definitively whats going on, but once fuel is completely ruled out, it becomes my prime suspect
 
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tbults88

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I understand there's pressure to get the "best boat" but just being on water is what its all about. Plenty of time to chase the next better boat. I've enjoyed boats of all sizes over many years all ramping into the pacific after my time playing on rivers & lakes. My first one was a 23' open bow Caravelle Interceptor. I took that lower profile to Catalina many times so don't shy away from doing that. But first get that engine right.... looks like you have two of our resident tech studs already on your post.... you're in good hands.
Oh yeah. I grew up on boats. Was a deckhand for 10 years on sportfishing boats had multiple little boats 16' regatta 17 avenger 2 smokers and a couple 18-19' fishing boats. But this is my first big block "river boat" wife was nervous with the kids on the lowprofile outboard boats.
 

SBMech

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Yes, its a MAP engine. No distributor tho, individual coil packs mounted on the valve covers with hi tension leads running down to the plugs. Bad coils or wires are a possibility but he's mentioning the engine is falling on its face on plane which does not sound like a coil or wire issue on his engine as it would a rough uneven idling and running with poor performance all the way through the range. Merc fuel injected engines tend to barely idle with a bad IAC and fall on their face at a certain higher RPM as the measured inputs confuse the ECM. Im not saying that's definitively whats going on, but once fuel is completely ruled, it becomes my prime suspect
Heh, basics. Compression, Ignition, Fuel. One is going away haha or failed. You or I in person could find this in 30 minutes or less, "can you hold the phone closer?"

:p

A dead coil is easy to find like you said always present, something that fails under load, harder.

Sounding more like a bad map or loss of fuel volume under load, without having tools hooked up, it's all guessing.
 

tbults88

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Heh, basics. Compression, Ignition, Fuel. One is going away haha or failed. You or I in person could find this in 30 minutes or less, "can you hold the phone closer?"

:p

A dead coil is easy to find like you said always present, something that fails under load, harder.

Sounding more like a bad map or loss of fuel volume under load, without having tools hooked up, it's all guessing.
Shit where u guys located what do u cost ill pay plus beer
 

tbults88

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So i just pulled my barb fitting that my fuel line plugs into off my tank. It has a check valve inside. I blew air threw it. If you blow threw it very lightly it flows well. If you put pressure on it its harder to blow threw almost as if the air is fluttering threw it. Worth replacing it before test day? @Shlbyntro
 

Shlbyntro

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So i just pulled my barb fitting that my fuel line plugs into off my tank. It has a check valve inside. I blew air threw it. If you blow threw it very lightly it flows well. If you put pressure on it its harder to blow threw almost as if the air is fluttering threw it. Worth replacing it before test day? @Shlbyntro
Thats your antisyphon valve. They're generally not a problem for engines with boost pumps like yours but if it was exposed to water like the rest of the fuel system, it is always a possibility that the check ball inside is sticking. It sounds like yours is operating fine tho but they are only like $10 so your discretion.

Austin, TX
 

BDMar

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A Certified Merc mechanic with the CDS can check fault seconds and diagnose what's happening without throwing parts at it and save a lot of time and money
 

tbults88

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The place i trust yeah there lot is PACKED. I like keeping the boat going to one shop if any. Less hands in the cookie jar
 
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