Brand new spark plug exploded and left me stranded. (AC MR43T)

[Nextelbuddy]

So I just replaced all the spark plugs in my mercruiser 5.7 I use the recommended Quicksilver AC MR43T spark plugs....

I I was running wide open throttle 4800 RPMs for probably 3 minutes The boat had been running great all day and then all of a sudden I heard a loud gunshot and then a really bad noise probably the compression stroke of that one cylinder because the spark plug was missing.

II thought maybe I was just dumb and I didn't thread in the spark plug tight enough and it loosened up and popped out but I looked up into the cylinder head and I found the broken part of the spark plug and the electrode porcelain part exploded off leaving the threaded part of the spark plug still in the cylinder head so it almost looks like the spark plug just was a manufacturing defect possibly?

now I'm nervous to run these spark plugs now and want to change them out to something else.

would engine detonation cause the spark plug to blow out like that leaving the threaded part still into the cylinder head?

Is is there another recommended spark plug that I can run for my engine?

 

[Nextelbuddy]

I just pulled the piece that was left in the cylinder head out and it looks fine it literally looks like the spark plug wasn't assembled or sealed properly from the factory it's such a clean pop out.

 

[alldodge]

Manufacturers defects happen all the time, but I'm not sure that is what this is
I would pull the other plugs to verify, but the plug looks like it was getting real hot, maybe lean

 

[Nextelbuddy]

Manufacturers defects happen all the time, but I'm not sure that is what this is
I would pull the other plugs to verify, but the plug looks like it was getting real hot, maybe lean

Well that certainly may be possible I guess. So let's say I pull all of the other plugs and they have the same rainbow color which usually signifies high heat... that would indicate a lean condition across the whole system and since this is a carburetor system with a pretty simple fuel system set up that will mean that I would start checking fuel pressure to make sure the fuel pressure is proper?

Now say i pull the other plugs and none of them show that same color discoloration and it was only isolated to that one cylinder then I wonder what could cause a link condition on one cylinder in a carbureted engine

 

[alldodge]

Being new plugs right now we don't know, just saying if it was me I would pull some others to verify. If they all look the same, and the electrodes are not white but tan colored then mix is good.

If there black then it s rich (but don't know how much running happened after plug blew)

Just saying need to compare, then decide if it was a issue or a one off issue

 

[Rick Stephens]

Agree with AD. Plug should not be discolored. Warning sign..

 

[Scott Danforth]

If the plug was loose, it will run hot. If the plug gets too hot, the porcelain lets loose.

That plug looks like it got really hot

 

[Nextelbuddy]

If the plug was loose, it will run hot. If the plug gets too hot, the porcelain lets loose.

That plug looks like it got really hot

The threaded plug that was left in the head, I was able to remove it with my fingers so you may be on to something. I did tighten each plug with my rachet and 5/8 socket but it's possible that plug came loose with hot cold expansion and then ran hot as you say.

 

[Scott Danforth]

if a plug is not tight, it cant transfer heat to the head. if you can thread them out with your fingers, they were not tight.

if you dont have a fancy tapered thin-wall plug socket, take one and your 4-1/2" grinder and a standard 5/8" plug socket and make one. you have to make sure the socket clears the marine exhaust castings.

 

[Nextelbuddy]

just found another review on amazon (same listing i got mine from originally) and another user back in 2019 also had the same issue where the porcelain blew out of the base.

really have me thinking i should just go ahead and also give the NGK BR6FS a try. that's what was in the engine before i switched to the MR43Ts.

i hear ya loud and clear on the heating up of a plug being a major clue... but i've had several different forced induction vehicles making 800+ HP on the track and off and never once had spark plug blow out. the engine went before the spark plugs did. (yes i'm aware, marine engines have different LOAD responsibilities and are held at higher RPMS under load for longer compared to automotive engines that have transmissions that help bring down engine RPMs at higher speeds.)

it really is amazing these engines can run for long periods of time at 4500+ rpms under heavy load.


i bought 8 new MR43T just to have on hand and will check the other 7 today, make sure they are all torqued to spec again and throw in a replacement MR43T. will cross my fingers.

https://www.amazon.com/product-revie...ews-filter-bar

They're just spark plugs right? Wrong!
I put these in my 4.3LX boat engine and had one burn out on #1 cylinder in the first couple hours..
(identical to the other guy that gave one star).
Hmmm that was weird. So I put another one in and no issues.
****Then last weekend, I was gently cruising back into the dock when it Sounded at first like I blew a head gasket.
Then I realized that the PORCELAIN blew out of the metal threaded base on the #4 Cylinder.!!!!!!
(Due to a weak crimping process around the porcelain)
OMG I have never seen that happen. You don't see that everyday!!
Not even on my 850Hp engines!
So 2 out of 6 plugs bad.
Guess what I'm gonna do to the other remaining four spark plugs?
Yup, throw them in the trash where they belong. because my cutoff return date was Oct. 11th. (ordered on Sept.9th)

 

[MyLord]

I've heard that amazon and other on line sites , unfortunately have a lot of fake plugs usually fake ngk , maybe they started ac delco fakes also ..... maybe just wondering out loud.

 

[Nextelbuddy]

checked the other pugs, they were tight, installed new MR43T plug gapped at .035 like the others. Engine started and sounded good, idled fine revved fine. will test on the water with caution this weekend.

 

[nola mike]

checked the other pugs, they were tight, installed new MR43T plug gapped at .035 like the others. Engine started and sounded good, idled fine revved fine. will test on the water with caution this weekend.

What did they look like? I'd be very concerned that you're running hot because of a lean condition or timing issue rather than a loose or faulty plug.

 

[Nextelbuddy]

What did they look like? I'd be very concerned that you're running hot because of a lean condition or timing issue rather than a loose or faulty plug.

i only pulled the other 3 plugs for cylinders 8, 4 and 2 and they did not have the discoloration that cylinder 6 did which was the one that exploded. the tips had brown color which seem to indicate a good air/fuel mixture.

i pull the other 4 for cylinders 1, 3, 5, 7... just checked them to make sure they were tight. was losing light fast after work.

i feel pretty confident that if cylinders 8, 4 and 2 look good and cylinder 6 exploded and the base rhat was left in the cylinder head was able to be removed by hand.. that indicates it worked its way loose and heated up due to poor heat dissipation in the cylinder head.

as for timing, i do want to get a timing gun and check the base timing of the engine so that is in the works to get done.