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TexasPilot71

Engine noise - “popping” or “missing”

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Went out today and came back after some experimenting with a strange popping sound. Happens between 3600 and 3900 RPM most notably when reducing power. I’m thinking a cylinder is missing but it is fine below 3600 and above 3900. Any thoughts?  Video below. 

 

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Popping True Popping is a too lean cylinder or cylinders.

But that narrow bad of popping can be something else. Check for loose spark plug rubber ends. Or a plug wire being rubbed right to the center conductor.  It can be in a difficult spot to check. Sounds like a vibration at those rpms is causing it.

Distributor ? or direct ignition coil for each cylinder ?  Engine info ?

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Distributor. Changed cap rotor and plugs April 2017. 350 Mag MPI motor. Sorry thought it was in my Signature. 

On another boat forum folks are saying maybe a sticking valve (seafoam or power tune to unstick it) others are saying throttle positioning sensor, my mechanic suggests the easy first...replace cap rotor and plugs again. I think I’ll try power tune into throttle body first then cap rotor plugs.  Others are saying maybe camshaft. Hope not. 

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11 hours ago, TexasPilot71 said:

Distributor. Changed cap rotor and plugs April 2017. 350 Mag MPI motor. Sorry thought it was in my Signature. 

On another boat forum folks are saying maybe a sticking valve (seafoam or power tune to unstick it) others are saying throttle positioning sensor, my mechanic suggests the easy first...replace cap rotor and plugs again. I think I’ll try power tune into throttle body first then cap rotor plugs.  Others are saying maybe camshaft. Hope not. 

I think the popping is key. If the sound is emanating from the area of the throttle body, then you are hearing a backfire which is propagating back into the intake manifold runner. Only two ways that can happen. First possibility is the intake valve is not closing properly indicating a broken valve spring, or failing valve, or most likely a cylinder is firing out of sequence while the intake valve is still open. Your engine has a roller tappet cam shaft which I have never seen fail on a production engine so I believe it's safe to take that possibility off the list. A compression test will tell you if a valve or it's seat is failing. Now as far as the cap and rotor goes, the crab cap design is notorious for pre-mature and even new out of the box failures. High voltage electricity will always take the path of least resistance. An engine cylinder on compression stroke takes a lot more power to fire it's plug than an adjoining cylinder which is on the exhaust stroke. Those caps only have to cross fire for a few hundred cycles before they carbon track and then miss-fire becomes common place.

Remove your cap and examine the margins between the terminals carefully for any signs of arcing. Carbon tracking will initially appear as as a blue spider web looking trace between terminals. A second consideration is rotor phasing which is confirmation that the rotor is pointing directly toward the center line of it's corresponding cap terminal when the crank position sensor commands that cylinder to fire. When these engines seem to be trashing caps at an alarming rate, it is usually because the distributor body is not in the proper base position. Super critical on these engines and should not be confused with "setting the timing" as on older 350's.

With the cap removed, the engine is brought up on Top Dead Center of the #1 cylinder compression stroke, and then the distributor shaft and base plate reference marks are precisely aligned. Also there are several after market alternatives to the stock Merc and Volvo aluminum terminal GM factory caps which have solid brass terminals and just flat out-perform the OEM caps and rotors. I've attached some viable alternatives below. Go real easy on the hold down screws as the distributor body is plastic. Don't be surprised if you find one of the threaded ears already broken off. I've attached an all aluminum replacement option too. Good Luck.  W

https://www.rockauto.com/en/moreinfo.php?pk=1991918&cc=1031555&jsn=566

https://www.diyforums.net/chevy-4.3l-5.7l-vortec-distributor-install-part-two-13472.html

http://www.marinepartsplus.com/catalog/mercruiser/serialize(value)/350_MAG_MPI_ALPHA-BRAVO/8M0049000/15322-50

https://www.rockauto.com/en/moreinfo.php?pk=1130207&cc=1031555&jsn=572

 

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How many hours on it?  backfire on decell?  i listened to the video a few times, and I think thats all i heard.  how about at idle smooth?  does it start ok?

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19 minutes ago, gnarlykaw said:

How many hours on it?  backfire on decell?  i listened to the video a few times, and I think thats all i heard.  how about at idle smooth?  does it start ok?

About 420 hours.  Exercised regularly.  Yes, backfires on deceleration in the 3600-3900 RPM band.  Idles perfect, easy to start, even runs in the 4000+ RPM range very smooth.

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1 hour ago, BajaDriver said:

Crossfire, and that will require new plugs wires and dist cap. Simple fix and very tell tale. 

Thanks!  I have a new Dist Cap, rotors, plugs, fuel filter, on the way.  Plan to do all that Saturday and see how she does.  Ordered the Rock Auto blue cap/rotor linked by @Wingnut above.  Haven't got the wires yet.  Probably buy them locally.  Just not sure what spec to ask for at the auto parts store.

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ok, your tech is right.  start with the cap and rotor.  look close at the dist when its open.  make sure there is NO corrosion anywhere.  green in color.    I use dielelctric grease, sparingly, on the wires to cap points... take it for a run and see....  if still there, try the plugs.

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One thing I noticed tonight is that the spark plugs I am using (AC Delco 41-993) come gapped at .035.  Mercruiser specifies .060.  The plugs that are in it now I never checked nor adjusted.  When I checked the new plugs I received today, they were gapped at .035.  So I used my tool and put them at .060.  So regardless, I'm changing the plugs.  Just wonder what kind of issues I might have caused by not adjusting the gap of the plugs that have been in there for 18 months now.

The plan for Saturday is to change cap, rotor, plugs, wires, and fuel filter.  If all runs well after that, I'll do my end of season oil change too.

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The tighter gap makes the plug easier to fire and actually puts less stress on things like caps, rotors, and wires. The wide gap allows the fuel/air mixture to see more of the spark and promotes more complete combustion. Running with a .045" gap and economy may suffer a tad, but as an operator you would likely not see a difference. I still suspect your issue is with the cap and rotor.   W

 

Production Years
Engine (Sterndrive, Inboard, Ski) OEM Spark Plug Mercury Part Number Gap
After April 2006 3.0L AC MR43LTS 33-862029 1.14 mm (0.045 in.) 4.3L. 5.0L and 5.7L carbureted AC 41-993 33-884019001 1.14 mm (0.045 in.) 4.3L, 5.0L, 350 Mag and MX 6.2 MPI AC 41-993 33-884019001 1.5 mm (0.060 in.)

• AC 41-932 spark plugs (P/N 33-884019) will be replaced by AC 41-993 spark plugs (P/ N 33-884019001). The spark plugs are fully interchangeable. Serial number breaks have not been recorded.

• Follow these guidelines for the correct spark plugs for carbureted MerCruiser engines: a. MerCruiser 4.3L, 5.0L, and 5.7L carbureted engines are shipped new with platinum AC 41-932 (P/N 33-884019) spark plugs, soon to be superseded by AC 41-993 (P/N 33-884019001). b. The recommended service replacement spark plugs for these engines are non-platinum AC MR43LTS (P/N 33-884019) or NGK BPR6EFS (P/N 33-816336). c. MerCruiser carbureted engines require a spark plug gap of 1.14 mm (0.045 in.). d. Mercury platinum spark plugs have a gap setting of 1.5 mm (0.060 in.). To reduce the possibility of installing the platinum spark plugs with the incorrect spark plug gap, the non-platinum spark plug is the recommended service replacement. e. The use of AC 41-932 (P/N 33-884019) or AC 41-993 (P/N 33-884019001) platinum spark plugs in a MerCruiser 4.3L, 5.0L, or 5.7L carbureted engine is acceptable if the spark plug gap is set at 1.14 mm (0.045 in.).

• Always check the spark plug gap before installing any new spark plug.

• In some owner's manuals, the AC 41-993 spark plug used in 496 and 8.1L engines was incorrectly listed as the appropriate spark plug for 4.3 MPI, 5.0 MPI, 350 Mag MPI, and MX 6.2 MPI engines. The heat range and the thread length of these spark plugs are correct. The firing tip on the AC 41-983 is longer and extends slightly deeper into the combustion chamber than the correct AC 41-932 (now being replaced by AC 41-993) spark plug. While the AC 41-983 is not the recommended spark plug, its use will not harm the engine or its performance.

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1 hour ago, Wingnut said:

The tighter gap makes the plug easier to fire and actually puts less stress on things like caps, rotors, and wires. The wide gap allows the fuel/air mixture to see more of the spark and promotes more complete combustion. Running with a .045" gap and economy may suffer a tad, but as an operator you would likely not see a difference. I still suspect your issue is with the cap and rotor.   W

 

Production Years
Engine (Sterndrive, Inboard, Ski) OEM Spark Plug Mercury Part Number Gap
After April 2006 3.0L AC MR43LTS 33-862029 1.14 mm (0.045 in.) 4.3L. 5.0L and 5.7L carbureted AC 41-993 33-884019001 1.14 mm (0.045 in.) 4.3L, 5.0L, 350 Mag and MX 6.2 MPI AC 41-993 33-884019001 1.5 mm (0.060 in.)

• AC 41-932 spark plugs (P/N 33-884019) will be replaced by AC 41-993 spark plugs (P/ N 33-884019001). The spark plugs are fully interchangeable. Serial number breaks have not been recorded.

• Follow these guidelines for the correct spark plugs for carbureted MerCruiser engines: a. MerCruiser 4.3L, 5.0L, and 5.7L carbureted engines are shipped new with platinum AC 41-932 (P/N 33-884019) spark plugs, soon to be superseded by AC 41-993 (P/N 33-884019001). b. The recommended service replacement spark plugs for these engines are non-platinum AC MR43LTS (P/N 33-884019) or NGK BPR6EFS (P/N 33-816336). c. MerCruiser carbureted engines require a spark plug gap of 1.14 mm (0.045 in.). d. Mercury platinum spark plugs have a gap setting of 1.5 mm (0.060 in.). To reduce the possibility of installing the platinum spark plugs with the incorrect spark plug gap, the non-platinum spark plug is the recommended service replacement. e. The use of AC 41-932 (P/N 33-884019) or AC 41-993 (P/N 33-884019001) platinum spark plugs in a MerCruiser 4.3L, 5.0L, or 5.7L carbureted engine is acceptable if the spark plug gap is set at 1.14 mm (0.045 in.).

• Always check the spark plug gap before installing any new spark plug.

• In some owner's manuals, the AC 41-993 spark plug used in 496 and 8.1L engines was incorrectly listed as the appropriate spark plug for 4.3 MPI, 5.0 MPI, 350 Mag MPI, and MX 6.2 MPI engines. The heat range and the thread length of these spark plugs are correct. The firing tip on the AC 41-983 is longer and extends slightly deeper into the combustion chamber than the correct AC 41-932 (now being replaced by AC 41-993) spark plug. While the AC 41-983 is not the recommended spark plug, its use will not harm the engine or its performance.

Thanks, but everything above seems to reference carburated engines.  I find the same information in the maintenance manuals, but it goes on to say the MPIs require the .060 gap.

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50 minutes ago, TexasPilot71 said:

Thanks, but everything above seems to reference carburated engines.  I find the same information in the maintenance manuals, but it goes on to say the MPIs require the .060 gap.

Correct. You are on the right track. Good luck and if it were me I would take the extra time to confirm rotor indexing as wear in the timing chain can change the relationship between the camshaft and crankshaft which retards the rotor placement relative to TDC.  W

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Well, dist cap and rotor didn’t help. Lake is white capping but went out anyway to test it out. Came back replaced the fuel filter. It was pretty filthy but no water. Just black on sides. Might have been slightly restricting fuel flow. Plugs and wires next but man are they hard for this 6’4” 250lb guy to get to them. Uggg. I did seem to hear some wheezing when throttling up. Suction leak of some sort maybe?  Didn’t notice it last weekend. 

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I am about ready to check for intake  manifold leaks .  I do the oil squirting on the joints of each cylinder head to intake joint junction. Can check each one for tightness. Check oil levels of tilt, engine & power steering when warmed up.

 

Does it do it in neutral & reverse ?

 

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17 minutes ago, cyclops2 said:

I am about ready to check for intake  manifold leaks .  I do the oil squirting on the joints of each cylinder head to intake joint junction. Can check each one for tightness. Check oil levels of tilt, engine & power steering when warmed up.

 

Does it do it in neutral & reverse ?

 

I haven’t yet taken it to 3600 RPM in reverse. :)  It only does it in that RPM range. 

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Four plugs replaced on starboard side. Man they look bad!!!  Changed the wires too. Resting before I tackle the port side plugs and wires. 

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Replaced all the plugs and wires. Took her out and same symptoms. Starting to think maybe fuel related or maybe valves sticking. I just don’t know. Waiting for my mechanic to call me back and likely put him to work. 

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WAs the port side black also ? If yes ?   I quit.   Over rich  black plugs after only a few minutes on the plugs ?  The sound is not from running lean.

Give it to a good mechanic.

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1 hour ago, cyclops2 said:

WAs the port side black also ? If yes ?   I quit.   Over rich  black plugs after only a few minutes on the plugs ?  The sound is not from running lean.

Give it to a good mechanic.

No it was the old plugs that were looking bad to me. Others say they didn’t look bad at all. Shows what I know! 

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15 hours ago, TexasPilot71 said:

Replaced all the plugs and wires. Took her out and same symptoms. Starting to think maybe fuel related or maybe valves sticking. I just don’t know. Waiting for my mechanic to call me back and likely put him to work. 

Sticking valves are old school, as your engine has plenty of hours that tight valve guides are not an issue, and modern oils prevent the effects of dissimilar metal corrosion between stem and guide. Still sounds like an ignition issue so your tech may be looking hard at the ignition coil and crank position sensor next. W

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