Mercruiser 5.0 MPI #3 cylinder ignition miss

[Searay205]

I would check the clocking on the Dizzy. Best method is to make the $10 tool in the bulletin

Can also be caused by yellow flag circuit breaker (50 amp) but you should not have one of those
Edit: don't know your serial number

Thanks!!!!!!!!!!!!!!!!!!!!!!! for the TSB. Mine has 260 hours so I am kinda past the window but making the "tool" and doing the test can't hurt. I do have two flag type breakers. When doing the cap I must have bumped the one by the PCM boat wouldn't start, so I reset it when I found the "flag" it was 10 amp and then I tripped the one by the battery 50 amp, it also had a flag I had to reset. Validates my theory when working on something 75% of the time you create a problem that didn't exist. Case in point I tripped on breaker by bumping it and the other intentionally why trying to determine why engine wouldn't start.

 

[dubs283]

$1700 manifolds were replaced earlier this year....... but when I pull plugs. will know.

Says the guy who's afraid to spend money on maintenance but will replace manifolds???

Again, welcome to boating and arbitrary spending, shheeesh

 

[alldodge]

Thanks!!!!!!!!!!!!!!!!!!!!!!! for the TSB. Mine has 260 hours so I am kinda past the window but making the "tool" and doing the test can't hurt. I do have two flag type breakers. When doing the cap I must have bumped the one by the PCM boat wouldn't start, so I reset it when I found the "flag" it was 10 amp and then I tripped the one by the battery 50 amp, it also had a flag I had to reset. Validates my theory when working on something 75% of the time you create a problem that didn't exist. Case in point I tripped on breaker by bumping it and the other intentionally why trying to determine why engine wouldn't start.

The bulletin posted was just to make the tool from, not for your motor. It dealt with emission motors which yours is not. The comment I was making is (not knowing what your serial number is) I don't know your complete configuration so just spitballing right now.

The breaker comment was the issue for another motor because it was not getting full power thru it and yellow flag type was the issue. Not many have issues with the red push button breakers on the motor, but it can cause issues if similar is occurring.

 

[Searay205]

Says the guy who's afraid to spend money on maintenance but will replace manifolds???

Again, welcome to boating and arbitrary spending, shheeesh

So far I spent $115 for distributor cap and rotor and $15 for fuel filter with zero to show for it. I don't feel good about feel good maintenance. Truth be told the $1700 on manifolds were a waste of $$. Long story on another post but in the end there wasn't any corrosion which could compromise the manifold integrity that would allow water in engine.

 

[Searay205]

The bulletin posted was just to make the tool from, not for your motor. It dealt with emission motors which yours is not. The comment I was making is (not knowing what your serial number is) I don't know your complete configuration so just spitballing right now.

The breaker comment was the issue for another motor because it was not getting full power thru it and yellow flag type was the issue. Not many have issues with the red push button breakers on the motor, but it can cause issues if similar is occurring.

Mine is catalyst motor. That why changing the distributor cap took 1 hour vs 15 minutes. That said my catalyst are in my attic. I have a cat simulator that prevents codes from being set. Yea I got tired of replacing oxygen sensors every year. Also removing the catalyst completely eliminated my pinging problem when using 87 octane. My engine is not equipped with knock sensors, I will have to get the serial number.

 

[alldodge]

Being a CAT motor you should be closed cool, right?
If closed cool, should never need to replace mans. If your open cool and fresh water should not need them either. Sold my 95 model this year and mans were changed. Replaced the ones on my 94 (repower needed better flow style) some years back and they looked prefect

 

[dubs283]

Being a CAT motor you should be closed cool, right?

Not always, merc has a good reputation with open cooled ec systems

 

[Fun Times]

$1700 manifolds were replaced earlier this year..,

Possibly you may have accidentally compromised a spark plug, plug wire or wire boot end while in there doing the manifold change out service as it’s easy to do without initial notice.
Maybe try running the engine at night to see if you see any sparking light down there under the manifolds… When warm too…

 

[Searay205]

I have a 5.0 mpi seawater (salt water) cooled catalyst engine. 260 hours. Of the 260 hours 259 are in 100% salt water and 1 hour in brackish. I also have a Rinda which shows the misfires on 3. #3 spark plug wire was removed, terminal inspected, ensured wire snap on when reinstalled. (I had a misfire on #4 years ago and found the terminal corroded it was cleaned and reinstalled with dielectric grease and never an issue again).

I have new plugs arriving so I will install. If misfire doesn't go away I will switch wires between cylinders.

As far as manifolds, at least on the catalyst manifolds Mercruiser uses an EPDM coating, it works. When I removed mine i knocked some light scaling off a cooling water passage. Underneath was perfect coating. That was after 250 hours and 11 years and Saltwater.

 

[Searay205]

Just a quick update/confirm/*****. As I always knew certified mechanics/techs are idiots. In my 56 years of living I have only hired one contractor in my life to replace my roof. My boats, atv's, wave runners, cars, pluming, electrical I do myself, never a hostage to their schedules. Well I went to change plugs today and idiots who paid idiots to work on my boat before I purchased it from the first idiots put in wrong plugs, lol. the last owners paid $2400 for a couple stupid items, bilge pump, plugs, starter and everything they touched I have officially redone. starter would grind when key released, aftermarket pos bolt pattern was off from OEM, bilge pump would blow fuse (idiots didn't know what type of switch to use) and now I find the plugs. Anyway we will see how it runs, they used NGK TR55IX vs NGK ITR 4A15 (alternative) vs what I used from my manual AC 41-993. Yea the AC 41-993 are Iridium and amazon had them for $12/4plugs or $25 total vs $75 for the NGK. The AC deco is actually specified in my 2011 5.0 mpi manual to boot.


The hits keep coming. I pad $26 with tax for plugs, and GM is giving me a $24 rebate so new plugs cost $2.00. Its like people who loose weight and keep it off for 10 years, less than .5%. One thing they all have in common, never ever take a day off, never eat junk because its Christmas or Thanksgiving. Same with the rich, never take a day off, don't care if its $250K or $3.00 you always want the best deal....

 

[dubs283]

Preach







And wisdom, from a sea ray owner..........

 

[Searay205]

Solved, My AC deco 41-993 spark plugs eliminated the #3 miss and eliminated the heavy gas smell in exhaust (I liked as it was bad for the environment) and it completely eliminated the flat spot at max load in the 3-4Krpm range. Again I purchased 8 off Amazon for $25 (now $30) and got a $24 rebate. Soooo idiot before used the Iriduium IX which is a low hr plug and never specified for my engine. My recommendation is replace plugs every 4 years or 125 hours.

I refilled after my 100 mile trip and it took 25 gallons. I am sure with the new plugs it would have been 20 gallons. Again since the specified plugs were not in engine I really don't have hard data on the new AC iridium plugs.

Boat runs like scalded ape, best $2.00 I ever spent on an engine......

 

[QBhoy]

Solved, My AC deco 41-993 spark plugs eliminated the #3 miss and eliminated the heavy gas smell in exhaust (I liked as it was bad for the environment) and it completely eliminated the flat spot at max load in the 3-4Krpm range. Again I purchased 8 off Amazon for $25 (now $30) and got a $24 rebate. Soooo idiot before used the Iriduium IX which is a low hr plug and never specified for my engine. My recommendation is replace plugs every 4 years or 125 hours.

I refilled after my 100 mile trip and it took 25 gallons. I am sure with the new plugs it would have been 20 gallons. Again since the specified plugs were not in engine I really don't have hard data on the new AC iridium plugs.

Boat runs like scalded ape, best $2.00 I ever spent on an engine......

Ok. Glad your boat is running better first off and most importantly. But having had a few beers in me…curious around the actual issue.
So are you thinking that iridium ngk plugs were the issue causing the bad running ? Maybe I’ve picked it up wrong. I ran ac delco 993 plugs in mine for the first year. They as far as I know, aren’t iridium. They are platinum minimum spec for these engines. Ran ok but didn’t last long. Changed to the alternative and preferred spec ngk 5599 proper iridium plugs a while later. Ran so much better I thought. But A little more expensive.
I was recently made aware that there is a huge black market fake version of both these plugs on the market. Ngk even released a document on how to spit a fake version. That’s how prominent the counterfeit market was with these. Just a word of warning perhaps. If they seem too cheap…they probably are for a reason.

 

[Searay205]

The AC 41-993 are Iridium not Platinum. Box indicated, AC deIco website indicated, rebate validated. I am 99% sure they are genuine since the amazon store was AC Delco. I checked gap very carefully and all had the little pad of alloy on the ground strap. The ngk IX did not have the alloy pad on ground strap and isn't supposed to, reason for short life. Iregardless it was the wrong plug. Yes the plug was causing the misfire on #3. That said I wasn't getting a code I found the miss with my Rinda. Again I had another characteristic I didn't like a flat spot, bog under max load. 99% percent of folks would never feel as they don't drive a boat like I do, lol. It was also eliminated. I originally thought it was my cheap Sams club gas causing the bog.

On a side note the ac deco and Iridium ITR 4A-15 visually looked identical, I ordered and received the NGK before the AC but then decided to look at my owners manual and order what it suggested the AC Delco. I returned the NGK.

On a side note I got Quicksilver 20W40 semi-synthetic from Walmart for $32 a gallon. Everyone else was $53? If you looking to do some maintenance another good deal.