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496 back fire cuts outs.

SBMech

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Update. Boats Fuel psi is 43 on the trailer. Installed new iac idle surge is gone whooshing noise intake is gone. Sat is test drive day
None of your posts mentioned an idle surge, but you did say the intake sounded "loud" like it was struggling to breathe.

Sounds like Master Shlbyntro nailed it, hope the test reveals the fix!
 

tbults88

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None of your posts mentioned an idle surge, but you did say the intake sounded "loud" like it was struggling to breathe.

Sounds like Master Shlbyntro nailed it, hope the test reveals the fix!
It had a small surge in it. My father inlaw had had a mag ho in his magic. This is just the mag i just figured it didnt have as much cam in it. After changing the iac it sounds almost identical to his magic. Today was the first time ive heard the boat idle "right" sound wayyyyyyyyy sexier thanks for watching the post and helping out as well @SBMech. We will see how it goes sat. This sight is great hope i can help other people out eventually with this learning experience.
 

tbults88

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So boat idles better ran the same. Stalls out high rpm. Changed map sencor on the water. That helped boat ran for longer at high rpm. But then fell on its face again. Boat has 38lb fuel pressure at idle. But i found out if i remove the line to the regulator on the rail it bumps it up to 43lb. When the boat falls on its face it loses fuel pressure
 

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SBMech

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So boat idles better ran the same. Stalls out high rpm. Changed map sencor on the water. That helped boat ran for longer at high rpm. But then fell on its face again. Boat has 38lb fuel pressure at idle. But i found out if i remove the line to the regulator on the rail it bumps it up to 43lb. When the boat falls on its face it loses fuel pressure
Looks like a dying pump to me. The only info you are missing is amps and voltage on the wire to the pump. Without that info I would still be looking at replacing the pump, unless it's running out of fuel, some sort of pick up issue. Looks like a pump that heats up and quits.

If it was a regulator it would puke black smoke and load up. Pulling the vacuum line should make it jump up 8-10 PSI, it's the fuel enrichment map, since there is no accelerator pump.
 

Shlbyntro

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So boat idles better ran the same. Stalls out high rpm. Changed map sencor on the water. That helped boat ran for longer at high rpm. But then fell on its face again. Boat has 38lb fuel pressure at idle. But i found out if i remove the line to the regulator on the rail it bumps it up to 43lb. When the boat falls on its face it loses fuel pressure
How was fuel pressure when it fell on its face?

@SBMech he has replaced both low and high pressure fuel pumps already. Unless he got a bad one out of the box or like mentioned above, a bad connection somewhere in the harness.
 

tbults88

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Runs great you can see it loose pressure right before it cuts out
 

Shlbyntro

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And you tried cracking the gas cap open already a while back correct?

Make sure the under side of the fuel water separator bracket doesn't have corrosion where the rubber gasket of the filter seals. There should also be an inline filter somewhere between the gas tank and the engine.. was that changed also?Change your fuel line all the way from the antispyhon valve to the engine, some fuel lines have an inner liner that delaminates over time when it comes into contact with ethanol.
 

Shlbyntro

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Isn't that the one your mechanic replaced back with the injectors?
 

tbults88

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I also put a meter on the plug from the ecu and it read 11v dc when i turn the key on. So i plugged in starboard pump and unplugged port pump like u said and nothing. Pretty sure starboard pump is trash.
 

Nanu/Nanu

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I got my fingers crossed for you @tbults88 that you get to the bottom of this.


I had a 96 crown line with a 5.7lx mercruiser ran like a top then I changed the oil and it started doing something like this to. I was replacing the coil and noticed my number 8 spark plugs wire was loose. My motor was going into a lump mode. Didn't even realize it.

I know your motor is different but does anyone think this could be in the coil packs?
 

Shlbyntro

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I got my fingers crossed for you @tbults88 that you get to the bottom of this.


I had a 96 crown line with a 5.7lx mercruiser ran like a top then I changed the oil and it started doing something like this to. I was replacing the coil and noticed my number 8 spark plugs wire was loose. My motor was going into a lump mode. Didn't even realize it.

I know your motor is different but does anyone think this could be in the coil packs?
He verified on the lake test today that it was the fuel pressure dropping off and it sounds like tonight he figured out why. The infamous bad, brand new part
 

tbults88

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I got my fingers crossed for you @tbults88 that you get to the bottom of this.


I had a 96 crown line with a 5.7lx mercruiser ran like a top then I changed the oil and it started doing something like this to. I was replacing the coil and noticed my number 8 spark plugs wire was loose. My motor was going into a lump mode. Didn't even realize it.

I know your motor is different but does anyone think this could be in the coil packs?
At top end fuel pressure is all over the place on the gauge. There's a video of it couple comments up. But shit shes got all kinds of new part now she almost brand new
 

tbults88

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Looks like a dying pump to me. The only info you are missing is amps and voltage on the wire to the pump. Without that info I would still be looking at replacing the pump, unless it's running out of fuel, some sort of pick up issue. Looks like a pump that heats up and quits.

If it was a regulator it would puke black smoke and load up. Pulling the vacuum line should make it jump up 8-10 PSI, it's the fuel enrichment map, since there is no accelerator pump.
Bad new pump good call @SBMech
 

tbults88

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Oh and i put a new tps in it today and she got alll pissed off and wouldn't get on plane and started beeping at me. So i put the old one back on and she went back to normal. Is there a special way u have to to replace those??
 

Shlbyntro

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Oh and i put a new tps in it today and she got alll pissed off and wouldn't get on plane and started beeping at me. So i put the old one back on and she went back to normal. Is there a special way u have to to replace those??
You could have gotten a reverse sweeping one. They look identical but the internals are backwards depending on application. With those its important to get genuine Merc with the exact part number specd for your engines serial. That part isn't worth cross referencing or going aftermarket on
 

tbults88

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You could have gotten a reverse sweeping one. They look identical but the internals are backwards depending on application. With those its important to get genuine Merc with the exact part number specd for your engines serial. That part isn't worth cross referencing or going aftermarket on
And thats exactly what i did lol
 

H8Plow

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IAC is not your issue, once off idle IAC is no longer needed it only creates a controlled vacuum leak to maintain commanded idle strategy. Stuck or malfunctioning IAC give an erratic, high, or hunting idle.

Your issue is a fuel delivery issue as others have stated. You're experiencing a lean misfire and when the fuel supply is below what the engine is asking for it dies because it ran out of fuel. Fuel pressure is not the only issue but the volume of fuel is, something is restricting the flow of fuel.

A simple but effective test can be done. Go get a 5 gallon fuel jug, rig up a fuel line and electric high pressure pump (I believe your system runs on a 3 bar or 43 psi of fuel pressure. Try and run the test fuel system before your factory fuel pressure regulator. You won't have much time (5 gal can go fast) but you will be able to see if it clicks past 50 mph.

If it works I would suggest cleaning the fuel tanks, clean the backflow valves (I believe the check valves are used to keep some residual pressure in the fuel line so the engine can start quicker) with carb cleaner and blow them out with compressed air. Air should flow one direction only. Once this is all done, grab a couple of bottles of chevron techroline and dump them in so it can act as a detergent and clean the fuel rail and injectors. Make sure your fuel jug is secure for testing, and let me know how it goes.
 

tbults88

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IAC is not your issue, once off idle IAC is no longer needed it only creates a controlled vacuum leak to maintain commanded idle strategy. Stuck or malfunctioning IAC give an erratic, high, or hunting idle.

Your issue is a fuel delivery issue as others have stated. You're experiencing a lean misfire and when the fuel supply is below what the engine is asking for it dies because it ran out of fuel. Fuel pressure is not the only issue but the volume of fuel is, something is restricting the flow of fuel.

A simple but effective test can be done. Go get a 5 gallon fuel jug, rig up a fuel line and electric high pressure pump (I believe your system runs on a 3 bar or 43 psi of fuel pressure. Try and run the test fuel system before your factory fuel pressure regulator. You won't have much time (5 gal can go fast) but you will be able to see if it clicks past 50 mph.

If it works I would suggest cleaning the fuel tanks, clean the backflow valves (I believe the check valves are used to keep some residual pressure in the fuel line so the engine can start quicker) with carb cleaner and blow them out with compressed air. Air should flow one direction only. Once this is all done, grab a couple of bottles of chevron techroline and dump them in so it can act as a detergent and clean the fuel rail and injectors. Make sure your fuel jug is secure for testing, and let me know how it goes.
The boat was losing fuel pressure at high RPM after the tank was cleaned Fuel line was replaced both filters replaced Low pressure pump replace high pressure pump replaced injectors taken off service cleaned and tested iac was replaced After all that. I found that the low pressure pump was dead not running at all Even when jumped to the battery. Bad pump out of the box we're replacing the low pressure pump tonight
 

Nanu/Nanu

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Happy to hear you're making forward progress.
The boat was losing fuel pressure at high RPM after the tank was cleaned Fuel line was replaced both filters replaced Low pressure pump replace high pressure pump replaced injectors taken off service cleaned and tested iac was replaced After all that. I found that the low pressure pump was dead not running at all Even when jumped to the battery. Bad pump out of the box we're replacing the low pressure pump tonight
Well hurry up youre not the only one losing sleep. The suspense is killing me!!!! 🤣

If this doesn't work I'm pretty sure it's in the prop. Jk

Good luck @tbults88
 

tbults88

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Happy to hear you're making forward progress.

Well hurry up youre not the only one losing sleep. The suspense is killing me!!!! 🤣

If this doesn't work I'm pretty sure it's in the prop. Jk

Good luck @tbults88
Boats at the shop to warranty the low pressure pump they put in last summer
 

Cole Canadian

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I know it's a pain in the but ... but ethanol was known to deteriorate some older fuel lines. You might want to replace your fuel lines complete from inside of tank and any others in fuel system.
I had one that partially separated inside of fuel line and was only an issue under full load when it would fold over and partially restrict the fuel to the pump, as soon as I backed off things were good.
You should be able to diagnose this with a pressure gauge on the fuel rail.
 

tbults88

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I know it's a pain in the but ... but ethanol was known to deteriorate some older fuel lines. You might want to replace your fuel lines complete from inside of tank and any others in fuel system.
I had one that partially separated inside of fuel line and was only an issue under full load when it would fold over and partially restrict the fuel to the pump, as soon as I backed off things were good.
You should be able to diagnose this with a pressure gauge on the fuel rail.
Put new lines on yesterday. Lake test sat boys 😵
 
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tbults88

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Did you ever check the anti syphon valve in the tank? I have seen them stick or get debris stuck in them.
Yup forgot to put that in there. Replaced it with a air craft free flow fuel check valve. Mine was sticky though. This boat sat for 3 years before i bought it. So i wasn't against changing anything
 

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So much for the smoking deal...... but at least it should be a damn good runner from the point this is done on! But seems you’ve got her figured out now.
 

tbults88

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So tbults88 .... how ya liking our RDP Tech brain trust thus far?
Cant complain....... @Shlbyntro been helping alot along with @SBMech . looks like i found a new trust worthy shop with @BDMar ill be calling him soon for a ecm print out. posting here was the best choice i made since i started working on this boat. Now who knows some gel coat guys in so cal???
 

billperkins3@att.net

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Did about 6' of my keel where beaching had scrapped over years... $450.... highly recommended by folks storing at Dunes
 

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Cant complain....... @Shlbyntro been helping alot along with @SBMech . looks like i found a new trust worthy shop with @BDMar ill be calling him soon for a ecm print out. posting here was the best choice i made since i started working on this boat. Now who knows some gel coat guys in so cal???
OC Boat Repair (714)-827-6247
8351 Monroe Ave
Stanton CA 90680
 

Yoshiro

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Looks like you are getting some good solid advice on your engine trouble. The 496 is a common engine, but it's not widely understood as it has some unique parts. Sounds like a fuel issue to me like you have been chasing. I was thinking replace the fuel lines which you did, and I removed my antisiphon valve altogether. IAC is a common one too. You are checking the boxes. Again good advice here. It will be interesting to see if it pulls any fault codes as it doesn't sound like you are hearing any warning beeps nor is it going into guardian mode? I'm anxious to hear what the final fix is.
 

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I don’t recall anything about an alarm going off so the scan may be a waste of time. Probably fuel related with no alarm sounding.
 

Cole Canadian

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Sounds like you have pretty much changed the entire fuel system.
If were to be in my shop I would do as H8PLOW said and hook a pail of gas up to the fuel rail with it's own fuel pump. If the problem goes away , you know you have missed something.
As a side note, a cracked fuel line or fitting (corrosion) before the high pressure pump will not always leak fuel under low pressure from the lift fuel pump but will allow air to enter the high pressure fuel pump and cavitate it causing a loss in fuel pressure. It could also be a crack before the lift pump.
Have you tried doing a fuel volume test yet? If you do not get an uninterrupted fuel flow without spurts of air coming out line, you might have to start looking at fittings. It doesn't take to much of a crack to get air into a fuel system.
 

SBMech

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Sounds like you have pretty much changed the entire fuel system.
If were to be in my shop I would do as H8PLOW said and hook a pail of gas up to the fuel rail with it's own fuel pump. If the problem goes away , you know you have missed something.
As a side note, a cracked fuel line or fitting (corrosion) before the high pressure pump will not always leak fuel under low pressure from the lift fuel pump but will allow air to enter the high pressure fuel pump and cavitate it causing a loss in fuel pressure. It could also be a crack before the lift pump.
Have you tried doing a fuel volume test yet? If you do not get an uninterrupted fuel flow without spurts of air coming out line, you might have to start looking at fittings. It doesn't take to much of a crack to get air into a fuel system.
His system has a low pressure pusher to the high pressure injection pump. It would leak visible fuel when the system primed, and whenever it was shut off, so you would notice a raw fuel smell.
 

Cole Canadian

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His system has a low pressure pusher to the high pressure injection pump. It would leak visible fuel when the system primed, and whenever it was shut off, so you would notice a raw fuel smell.
Where is his low pressure pump located? Some are in the tank, if it is a small pin hole he may not detect a leak as he looking for obstacles in the fuel lines.
The most common places for this to happen is inside the fuel tank because of not draining the tank during storage. Due to our hot cold cycle up here, condensation forms inside the tanks and will corrode metal fittings , lines ect. Since he stated that water was in the fuel system, this might be something to look at. Having said that , he should start with the 5gal. pail first and if that solves the issue, it is something else he could look for.
If it doesn't. move on to the fuel rail.
 

SBMech

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He's already replaced the lines.


Where is his low pressure pump located? Some are in the tank, if it is a small pin hole he may not detect a leak as he looking for obstacles in the fuel lines.
The most common places for this to happen is inside the fuel tank because of not draining the tank during storage. Due to our hot cold cycle up here, condensation forms inside the tanks and will corrode metal fittings , lines ect. Since he stated that water was in the fuel system, this might be something to look at. Having said that , he should start with the 5gal. pail first and if that solves the issue, it is something else he could look for.
If it doesn't. move on to the fuel rail.
Put new lines on yesterday. Lake test sat boys 😵
 

tbults88

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Where is his low pressure pump located? Some are in the tank, if it is a small pin hole he may not detect a leak as he looking for obstacles in the fuel lines.
The most common places for this to happen is inside the fuel tank because of not draining the tank during storage. Due to our hot cold cycle up here, condensation forms inside the tanks and will corrode metal fittings , lines ect. Since he stated that water was in the fuel system, this might be something to look at. Having said that , he should start with the 5gal. pail first and if that solves the issue, it is something else he could look for.
If it doesn't. move on to the fuel rail.
When the prob first started. We tried to run the boat on a 5 gallon Jug with fresh gas. Didnt change anything. After that we found water in the tank. Then started the longggggg list that we have gone threw. The first part i changed (low preasure pump) we later verified was not working. Its in the shop now getting it replaced threw the warranty.
 

H8Plow

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Something you might want to add is a fuel pressure and a water pressure gauge on the engine. Very helpful
 

Cole Canadian

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When the prob first started. We tried to run the boat on a 5 gallon Jug with fresh gas. Didnt change anything. After that we found water in the tank. Then started the longggggg list that we have gone threw. The first part i changed (low preasure pump) we later verified was not working. Its in the shop now getting it replaced threw the warranty.
OK, new info, if running directly from fuel to fuel rail did not clear up the issue, then the next step would be to take the rails off and have them checked for obstructions and cleaned and ensure no deposits are left in them.
If it runs fine at low/ mid range rpm's I doubt it's an injector issue, but more likely something is restricting the fuel to get to them.
Since you have replaced everything up to the rail , it's very possible the restriction is inside the rail itself before the injectors.
Have you done a fuel volume test on the system? If not now would be the time to do one before you take it out to see if you have enough fuel flow as well as pressure.
Keep looking and you will find it.
 

02HoWaRd26

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OK, new info, if running directly from fuel to fuel rail did not clear up the issue, then the next step would be to take the rails off and have them checked for obstructions and cleaned and ensure no deposits are left in them.
If it runs fine at low/ mid range rpm's I doubt it's an injector issue, but more likely something is restricting the fuel to get to them.
Since you have replaced everything up to the rail , it's very possible the restriction is inside the rail itself before the injectors.
Have you done a fuel volume test on the system? If not now would be the time to do one before you take it out to see if you have enough fuel flow as well as pressure.
Keep looking and you will find it.
They found that the low pressure pump wasn’t running.
 

Cole Canadian

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They found that the low pressure pump wasn’t running.
So that would be a restriction that a volume test would have picked up. Testing easier and cheaper than just throwing parts at the problem.
Hope it fixes issue for him.
 

H8Plow

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Well finally got the boat back from warranty low pressure pump. Ran it on the trailer. Shes still idleing at 38-40 psi. Id like to see 43 but well see what happens this weekend.
That’s perfect pressure. Your injectors are rated 43.5 psi. If your fuel pressure regulator is vacuum referenced then at wot you will see 5-8 psi increase. More fuel pressure more fuel, think of this as what a power valve would be for a holley carb.
Sounds like the problem is fixed!
 
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