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Nitrousbird

Rough idle after idling for a few minutes

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Nitrousbird

This problem has been going on for some time but has gotten slightly worse.

Boat starts up smooth, will run smooth for a few minutes at idle, then starts getting rough.  Not "WTF is wrong with the boat rough" but not as smooth as it was.  Run her hard for a bit and the idle is smooth again.  Never an issue with firing up.  Doesn't seem down on power, though with the overkill prop I have on her for the reservoir we normally boat it would be hard to notice.  Rough idle is in gear or neutral, so not a drivetrain issue.

Pulled all the plugs; noticed #5 is wet.  Almost seems wet with water/condensation; rest are dry.  Plug does not smell like fuel.  When I say wet, we are talking drops of water on it, not water coming out of the cylinder or anything.  Plug looks like it is firing though.  Boat is partly closed cooled; coolant is fine, no water/coolant in oil ever.  I have replaced the coil pack for this cylinder (and whatever other cylinder that coil pack runs).  Replaced all plugs.  Wires tested okay but replaced anyway on that side of the motor.  Replaced #5 injector.  Same deal.

Plug being wet is concerning, obviously.  Any other ideas?

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oceanbu

Could it be a leak in the exhaust manifold on that side?

I have same setup as you as far as closed cooling in the block but raw water through the exhaust manifolds.  I run in the ocean so I always wonder, despite my religious flushing with salt away after every use, what the symptoms will be when my manifolds need to be replaced.

Replaced them once and the symptom was they would run really hot at idle due to water passages being blocked/constricted but cool down when under way as the higher rpm was enough to force the water through (that's what my dealer told me anyways).  But I wonder if a hole developed in the water jackets of the manifold could water invariably make it's way through the exhaust valve into the cylinder?

I would think that would cause it to run rough or a mis if there was just enough water to inhibit the spark.  Obviously if it was a lot of water it would be catastrophic but could your situation be signaling the start of a problem?

I would be very interested to know what you find.

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Nitrousbird

@oceanbu

Our boat has never been in salt (or brackish) water, and the water here (though dirty) isn't hard on components.  Did your manifolds show any signs of external cracking?  Mine show no external signs of wear at all; even the factory paint is fully intact.  

It wouldn't be a big job to pull one but if I'm doing that I want a new one sitting here, in case the old one is bad.  I can see where this could be the issue.    

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oceanbu
6 hours ago, Nitrousbird said:

@oceanbu

Our boat has never been in salt (or brackish) water, and the water here (though dirty) isn't hard on components.  Did your manifolds show any signs of external cracking?  Mine show no external signs of wear at all; even the factory paint is fully intact.  

It wouldn't be a big job to pull one but if I'm doing that I want a new one sitting here, in case the old one is bad.  I can see where this could be the issue.    

Mine looked brand new on the outside.  The latest ones I have on look brand new on the outside also.  It's what's inside that I wonder about.  I assumed you were on fresh water.  Did your manifolds ever overheat in the past that they could have developed a crack on the inside jackets?

I agree...I wouldn't want to pull one off unless I had a new one sitting there ready to go on.  I have been meaning to buy a new set of manifolds for this very purpose because I know in my case it's just a matter of time and not an if.  

If you ever figure the culprit please leave an update as I would love to know.

All the best!

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Woodski

@Nitrousbird:  An intake manifold gasket breach between the water jacket and the cylinder would create a miss and water on the plugs.

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oceanbu

He's got closed cooling so wouldn't he be seeing coolant then instead of water if it were an intake manifold gasket breach?  Wouldn't that cause coolant to show in the oil too?

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Nitrousbird
7 hours ago, Woodski said:

@Nitrousbird:  An intake manifold gasket breach between the water jacket and the cylinder would create a miss and water on the plugs.

Nope, partial closed cooling.  If that were the case I would see a change in coolant level (either drinking it down or getting pressurized and overflowing the expansion tank).  Coolant level is rock steady.

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Nitrousbird
16 hours ago, oceanbu said:

Mine looked brand new on the outside.  The latest ones I have on look brand new on the outside also.  It's what's inside that I wonder about.  I assumed you were on fresh water.  Did your manifolds ever overheat in the past that they could have developed a crack on the inside jackets?

I agree...I wouldn't want to pull one off unless I had a new one sitting there ready to go on.  I have been meaning to buy a new set of manifolds for this very purpose because I know in my case it's just a matter of time and not an if.  

If you ever figure the culprit please leave an update as I would love to know.

All the best!

No overheating.  Unfortunately, the LS1 manifolds are not as easy to come by online...I will have to contact some places and price one out.  I am sure the cost isn't easy on the wallet either.  

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oceanbu
5 hours ago, Nitrousbird said:

No overheating.  Unfortunately, the LS1 manifolds are not as easy to come by online...I will have to contact some places and price one out.  I am sure the cost isn't easy on the wallet either.  

No kidding...the ETX/CAT manifolds on mine are $1000 a piece!

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Woodski
Posted (edited)

@oceanbu@Nitrousbird - Yes, fully closed cooling system would draw coolant from that source, open or partially open (depending on how it is plumbed) system you may not see it.  Also, LS engines much less prone to that issue, much more common in the older small blocks.  'When' you pull the exhaust manifold you should hopefully (not really but) be able to see a water trace inside that identifies the source of the water.  

Here is another way water can get in to the combustion chamber, as a hot engine cools it creates a draw on the exhaust side and can draw in water or moist air which would show as water in the combustion chamber.  Cats run hot so that aggravates that issue.  It will draw water in to the combustion chamber this way through the open exhaust valve at shut down, have you checked to see if it moves from cylinder to cylinder depending on where the engine stops?  You could find the wet cylinder, then pull the valve cover to see if that valve is open.  Boats with hot cams are more prone to this also at idle they can draw water back in to the combustion chamber.  Question, is you boat sitting lower in the water for some reason or listed to that side during the cool down cycle?  This theory may not be valid as your issue happens after running for a while, this scenario would be more at start up and then it cleans up as the water burns off.

Edited by Woodski

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95echelon
41 minutes ago, Woodski said:

@oceanbu@Nitrousbird - Yes, fully closed cooling system would draw coolant from that source, open or partially open (depending on how it is plumbed) system you may not see it.  Also, LS engines much less prone to that issue, much more common in the older small blocks.  'When' you pull the exhaust manifold you should hopefully (not really but) be able to see a water trace inside that identifies the source of the water.  

Here is another way water can get in to the combustion chamber, as a hot engine cools it creates a draw on the exhaust side and can draw in water or moist air which would show as water in the combustion chamber.  Cats run hot so that aggravates that issue.  It will draw water in to the combustion chamber this way through the open exhaust valve at shut down, have you checked to see if it moves from cylinder to cylinder depending on where the engine stops?  You could find the wet cylinder, then pull the valve cover to see if that valve is open.  Boats with hot cams are more prone to this also at idle they can draw water back in to the combustion chamber.  Question, is you boat sitting lower in the water for some reason or listed to that side during the cool down cycle?  This theory may not be valid as your issue happens after running for a while, this scenario would be more at start up and then it cleans up as the water burns off.

I actually had the issue where my hot water shower was plumbed incorrectly and filled an exhaust manifold to the point that I hydrolocked. Could that be possible for you? Although the #5 cylinder is higher up than a couple others right?

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justgary

Thoroughly clean the throttle body.  You'd be surprised what that can do for clearing up idle problems. 

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Nitrousbird
4 hours ago, Woodski said:

@oceanbu@Nitrousbird - Yes, fully closed cooling system would draw coolant from that source, open or partially open (depending on how it is plumbed) system you may not see it.  Also, LS engines much less prone to that issue, much more common in the older small blocks.  'When' you pull the exhaust manifold you should hopefully (not really but) be able to see a water trace inside that identifies the source of the water.  

Here is another way water can get in to the combustion chamber, as a hot engine cools it creates a draw on the exhaust side and can draw in water or moist air which would show as water in the combustion chamber.  Cats run hot so that aggravates that issue.  It will draw water in to the combustion chamber this way through the open exhaust valve at shut down, have you checked to see if it moves from cylinder to cylinder depending on where the engine stops?  You could find the wet cylinder, then pull the valve cover to see if that valve is open.  Boats with hot cams are more prone to this also at idle they can draw water back in to the combustion chamber.  Question, is you boat sitting lower in the water for some reason or listed to that side during the cool down cycle?  This theory may not be valid as your issue happens after running for a while, this scenario would be more at start up and then it cleans up as the water burns off.

Does it with a minimal crew and no ballast (other than 500lbs of lead always on board).  No cat's on the LS1 motors, so not a contributor.  We leave for vacation on Sunday with the boat for a week; I don't anticipate any issues as this is a long-term problem and not a lot of idling where we are going.  

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Nitrousbird

I pulled the starboard exhaust manifold.  It looks nearly new.  Zero signs of any sort of cracking or even corrosion on it for that matter.  Much better condition than I anticipated, at least on the inside.  A couple other things of note:
- No exhaust manifold gasket.  I guess maybe that is normal on a boat motor; never pulled a boat manifold before.  But Indmar put such a tiny amount of silicone on there for a seal that I'm highly surprised it wasn't leaking big time.  I am amazed at how little they used
- #5 plug was dry this time (as were 1, 3, 7).

I did notice an ever so slight amount of corrosion on the bottom part of the head under #7, so my guess is there was a super small leak on that port.  I don't think this is the cause of the problem.  We just got back from a week at Norris, burned about a tank (most time was spent in my buddy's 247 when he got there).  It did have a bad idle, as usual, during the week.  It spent the last 10 minutes idling to the ramp, so my assumption it was acting up then when she was put on the trailer.  Unsure as my wife drove it from the house to the trailer and she doesn't tend to notice these types of things. 

I'm not sure where to go from here.  Perhaps a compression test?  She has power.  I guess I could always change out the MAP sensor as well; I'm sure I can score that pretty cheap.  

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UWSkier

What were the conditions last time before you observed the wet plug?  Was it right after the engine was running or after it'd sat for a bit?  Have you checked gaskets and tolerances both hot and cold?

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justgary
On 6/5/2019 at 2:11 PM, justgary said:

Thoroughly clean the throttle body.  You'd be surprised what that can do for clearing up idle problems. 

^^^^^ I suspect your IAC valve is cruddy (as are the idle bypass ports in the throttle body), and those don't really come into play until the engine warms up fully.

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Nitrousbird

I'm tired of guessing, so I am just spending $200 on the diagnostic software to know what is going on:

http://mefiburn.com/mefiscan.asp

Mine has the CEFI-3 ECU and they confirmed the MEFI-3 setup will work with CEFI-3.  Hopefully it arrives this week and I can diagnose on the water next weekend.  I have these kinds of tools for our other vehicles, so its time the boat gets the same treatment.

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