Spark Plug Exploded!?...

[horsebru]

1990 Merc 260, 5.7L V8<br /><br />Cruising at <3000 RPM last week. There was a loud noise like a gunshot going off, immediately followed by a loud tap tap sound - very metallic. <br /><br />I immediately stopped the engine and found that the head on the #2 plug was missing - still haven't found it. <br /><br />The engine still starts fine, oil pressure and coolant are perfect - no emulsion. Just that taptaptap noise.<br /><br />Does anyone know what the cause of this could be? Do spark plugs spontaneously commonly explode like this? <br /><br />What happens to an engine when a spark plug just stops working like this?

 

[Bondo]

Re: Spark Plug Exploded!?...

Did it say Champion on it before it Blew ?????

 

[trog100]

Re: Spark Plug Exploded!?...

what do u mean by the "head".. the internals or the outer ceramic insulator.. <br /><br />spark plug internal insulators and the center electrodes sometimes kinda disappear.. they usually do it quietly thow and the bits go straight out the exhaust valve..<br /><br />no.. spark plugs dont spontaneously explode like yours did.. and the tapatap noise means some damage has occured to your upper cylinder/valves and possibly piston..<br /><br />none of this would affect your coolant or engine oil by the way..<br /><br />trog100

 

[Don S]

Re: Spark Plug Exploded!?...

Did you do a compression test on that cylinder? Does the engine still run smooth, without a miss. Did you pull the valve cover and check the springs and pushrod on that cylinder?

 

[achris]

Re: Spark Plug Exploded!?...

Did you put a new spark plug in?

 

[olbuddyjack]

Re: Spark Plug Exploded!?...

The "tapping" noise could be compression leaking through the missing plug body. Put another plug in and tell us what happens.

 

[horsebru]

Re: Spark Plug Exploded!?...

I had a compression test done on the engine 2 days prior to plug blowing out.<br /><br />Compression was from 105-125 psi on all 8 cylinders.<br /><br />Everything in the center of the spark plug, is missing, including the ceramic insulator and all internals. The only remainder is the metal 'ring' with the threads.<br /><br />I have replaced all the plugs, but want to make sure I won't damage the engine by restarting it. That is why I am asking what could have caused the problem...<br /><br />Thanks for all the replies.

 

[olbuddyjack]

Re: Spark Plug Exploded!?...

My guess would be that the plug was cracked during removal or installation for the compression test. The broken plug finally blew apart during operation.<br />I would say that any damage is done and it wouldnt matter if you started it. You might want to wait and see if anybody else has a different opinion before you try it.

 

[vipzach]

Re: Spark Plug Exploded!?...

you probly had a damaged plug that finally gave when the piston came up on the compression stroke and hence the gun shot sound. I would guess that nothing went into the cylinder. If it is not hard on your boat pull the head off and check, there could be an easier way...maybe someone else has a suggestion. That is about the wildest spark plug story I have ever heard!

 

[Bondo]

Re: Spark Plug Exploded!?...

Again,.....<br />

Did the plug say Champion on it before it Blew ?????

I'm sure the any Debris was Blown Out from the compression.........<br />I'd load it up with a Fresh set of AC MR43Ts,+ Go Boating............

 

[trog100]

Re: Spark Plug Exploded!?...

yes the plug must have been cracked and blew out under fireing hence the loud bang.. bung some new plugs in and off u go.. never seen one fail like that before..<br /><br />was it a champion.. he he <br /><br />trog100

 

[Don S]

Re: Spark Plug Exploded!?...

It doesn't make any difference what the compression was a few days ago, I'm talking about a compression test after the plug blew out, maybe something broke that took out the plug and is causing the noise.........<br />Where the plugs CHAMPIONS

 

[horsebru]

Re: Spark Plug Exploded!?...

The plugs were AC's. I replaced them with NGK's. It seems there is a bit of a consensus that it should be safe to start the engine. I'll do a compression test and fire it up to see if the noise is gone. It sounded like a hammer rhythmically hitting an anvil.<br />There is still no trace of any part of the plug innards - only the ring with threads. The thing came out incredibly clean. There is not even a trace of epoxy etc on the metal where the ceramic was. <br />I even looked for a hole in the hull...<br />Thanks to all.

 

[sue5878]

Re: Spark Plug Exploded!?...

Don S<br /><br />Is there some truth in the old saying "Just because you've got a head like a spark plug, doesn't mean you're a Champion"<br /><br />Neil

 

[outboardguy]

Re: Spark Plug Exploded!?...

I'd say that the guy that did the compression test did not tighten the spark plug enough.When the spark plug is tight the heat is dissapated to the head, but when the spark plug is loose the heat stays in the metal at the base of the plug until it expands enough to shoot the porcelin across the room.I've experienced this exact situation on a few boats and each time the metal left in the head came out with very little effort.I would have to agree that there is probably not too much damage due to debris from the spark plug as it all got blown out,but a compression test is in order.

 

[horsebru]

Re: Spark Plug Exploded!?...

Compression test was ok. The engine runs well now. Thanks to everyone for the help.

 

[olbuddyjack]

Re: Spark Plug Exploded!?...

horsebru,<br />Thanks for the update! I'm glad to see everything came out on the good side.