My new mystery - Spark Plug Blown Out

MNbowrider72

MNbowrider72

New Member
Apr 7, 2019
27
Boat Info
1994 180 BR Signature
Engines
5.0 Mercruiser w/Alpha One Drive
Yesterday, I just finished replacing the fuel/water separator on my Merc 5.0 Alpha One ('94 180BR Signature). A previous owner had upgraded from a 4.3 and it's a tight fit. The separator seemed to solve 90% of the fuel delivery issue, as I just had it out on the water this afternoon and was achieving mid 40s mph top speed when previously it was only mid-30's with occasional spikes into the 40's and frequent bogging.

Feeling victorious, I was lapping the lake at full throttle when I hard a fairly loud "pop". I slowed to an idle and looked inside while my son idled back to the dock, as we were close. Upon further inspection, I could only tell that a spark plug wire had fallen off. I did not notice any loud internal noise that would indicate a thrown rod, or something catastrophic, only a loud engine. It seems that the core of the plug shot out, or was knocked off, with some force, and that the sound was air escaping the cylinder. The motor is fairly new, oil clean/full and maintained pressure. To add to the mystery, I found a seemingly unrelated nut, and hose clamp in the bilge -as seen in the attached pic. I had cleaned the bilge spotless when I did the fuel/water separator yesterday, so I know they weren't there before. The plug location is very sheltered and not near any moving things that could knock it off.

I am at a loss, and of course it started to rain while I was diagnosing... so here I am online. Any ideas where to start??
 

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The hose clamp and nut could have been left over from the repower even thou you said you cleaned the bilge spotless. They are not related to the spark plug failure. Go over the engine room with a fine tooth comb to make sure no clamps or fasteners are missing.

That being said doing "laps" around the lake may be fun but it very stressful on the engine. What rpm was the engine turning at full throttle? Marine engines should not be run at full throttle for more than a minute or two for diagnostic reasons only. As far as the spark plug goes it is possible it just failed or was defective. It does happen. I would install a good quality complete set of plugs and not overthink it. Keep the rpms at no more than 80% of rated rpm or you will run the risk of engine damage.
 
hottoddie said:
The hose clamp and nut could have been left over from the repower even thou you said you cleaned the bilge spotless. They are not related to the spark plug failure. Go over the engine room with a fine tooth comb to make sure no clamps or fasteners are missing.

That being said doing "laps" around the lake may be fun but it very stressful on the engine. What rpm was the engine turning at full throttle? Marine engines should not be run at full throttle for more than a minute or two for diagnostic reasons only. As far as the spark plug goes it is possible it just failed or was defective. It does happen. I would install a good quality complete set of plugs and not overthink it. Keep the rpms at no more than 80% of rated rpm or you will run the risk of engine damage.
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I am thinking of getting one of those remote scope cameras for my phone, as the area near the transom is very hard to see. I wasn't watching the rpms and probably should have been -was more focused on speed and feel to gage any improvement in fuel delivery. It was a small lake, so really it wasn't more than a minute or two. Really hoping they are completely unrelated. Will start with that plug first, run it with the muffs, and if it just broke off, then I will hopefully be fine and move on to finding the clamp/nut issue. Thanks for the input!
 
You didn't do any harm by running it WOT. In fact, you can do that for quite some time. The plug would've popped out eventually no matter what. Weird things happen, sometime - that's all. Change all of your plugs while you're at it.

Based on the grunge on them, that clamp and nut have been in the bilge for a long time - wherever they were hiding, you just missed them when you cleaned. Can't hurt to look, though. But in order for that clamp to have come off by itself, there would have to be a hose dangling free - and that doesn't happen without it being obvious.
 
MNbowrider72 said:
I am thinking of getting one of those remote scope cameras for my phone, as the area near the transom is very hard to see. I wasn't watching the rpms and probably should have been -was more focused on speed and feel to gage any improvement in fuel delivery. It was a small lake, so really it wasn't more than a minute or two. Really hoping they are completely unrelated. Will start with that plug first, run it with the muffs, and if it just broke off, then I will hopefully be fine and move on to finding the clamp/nut issue. Thanks for the input!
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I would definitely take a look inside that cylinder. The electrode is bent.. I'm really curious what the spark plug collar looks like. I'll speculate it is possible that a valve floated at full rpms, got smacked by the piston and hit the plug. A photo of the spark plug collar would be really helpful. It is also possible that it just blew out but when that happens it doesn't bend the electrode.
 
PlayDate said:
I would definitely take a look inside that cylinder. The electrode is bent.. I'm really curious what the spark plug collar looks like. I'll speculate it is possible that a valve floated at full rpms, got smacked by the piston and hit the plug. A photo of the spark plug collar would be really helpful. It is also possible that it just blew out but when that happens it doesn't bend the electrode.
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Good points regarding the plug electrode. While you are there doing the plug, I would do a compression test to rule out any issues with that. Keep us posted
 
PlayDate said:
I would definitely take a look inside that cylinder. The electrode is bent.. I'm really curious what the spark plug collar looks like. I'll speculate it is possible that a valve floated at full rpms, got smacked by the piston and hit the plug. A photo of the spark plug collar would be really helpful. It is also possible that it just blew out but when that happens it doesn't bend the electrode.
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Good point. I am hoping the electrode bent on impact with the hull. Dreading the thought of damage inside the cylinder. Been looking for a good reason to buy one of those scopes, I know I will use it for other things. Will update when I get a look inside the cylinder. Thanks!
 
Lazy Daze said:
You didn't do any harm by running it WOT. In fact, you can do that for quite some time. The plug would've popped out eventually no matter what. Weird things happen, sometime - that's all. Change all of your plugs while you're at it.

Based on the grunge on them, that clamp and nut have been in the bilge for a long time - wherever they were hiding, you just missed them when you cleaned. Can't hurt to look, though. But in order for that clamp to have come off by itself, there would have to be a hose dangling free - and that doesn't happen without it being obvious.
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Lazy Daze said:
You didn't do any harm by running it WOT. In fact, you can do that for quite some time. The plug would've popped out eventually no matter what. Weird things happen, sometime - that's all. Change all of your plugs while you're at it.

Based on the grunge on them, that clamp and nut have been in the bilge for a long time - wherever they were hiding, you just missed them when you cleaned. Can't hurt to look, though. But in order for that clamp to have come off by itself, there would have to be a hose dangling free - and that doesn't happen without it being obvious.
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Good point on the grunge. I was beginning to wonder how it got inside the threads of the nut when the motor and all hoses are like new and spotless. It's possible I missed them. The fuel/water separator was in such a tough spot that I had to fashion two hooks from coat hangers... one to lower the filter behind a motor mount, and the other to pick it back up on the other side and fish it around the fuel pump, all while standing on my head as there is a vertical support for the sundeck on both sides of the motor with a modest cut out for access. Of course, I dropped the old one in the bilge and spilled fuel, so I put an old towel down there to mop it up and fished it out with the coat hanger. Just my luck, they used a different filter mount during the changeover, and I had the wrong filter. Turned out, I needed a Johnson brand and not the standard Mercruiser.
 
MNbowrider72 said:
Good point on the grunge. I was beginning to wonder how it got inside the threads of the nut when the motor and all hoses are like new and spotless. It's possible I missed them. The fuel/water separator was in such a tough spot that I had to fashion two hooks from coat hangers... one to lower the filter behind a motor mount, and the other to pick it back up on the other side and fish it around the fuel pump, all while standing on my head as there is a vertical support for the sundeck on both sides of the motor with a modest cut out for access. Of course, I dropped the old one in the bilge and spilled fuel, so I put an old towel down there to mop it up and fished it out with the coat hanger. Just my luck, they used a different filter mount during the changeover, and I had the wrong filter. Turned out, I needed a Johnson brand and not the standard Mercruiser.
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Boat ownership would be boring if it wasn't for those "surprises"!

Regarding the vertical supports... in most boats, there's only a few L-brackets holding those on. After you do it once or twice, it only takes about 5 minutes to remove them. Just an FYI...

I have a feeling the electrode got bent upon the plugs expulsion - either on the way out the hole at a funky angle or, as you said, impacting something. Otherwise, tip of the electrode would likely show an impact. But certainly can't hurt to check things out.
 
The scopes are relatively inexpensive. I would also do a compression test before restarting the engine. One other thing to consider is if you had any water enter the cylinder at high rpms. It has to go some place and the hydraulic pressure of compression would easily blow the plug out.

How about a picture of the spark plug collar?
 
Thanks all for the input. Here is the outcome...

The spark plug collar came out of the head just fine and there is no missing material from the plug which could be in the cylinder. I purchased a Bosch FIX7669 endoscope from O'Reilly's and am satisfied with it. I was able to probe the cylinder. The bore is smooth, no damage to piston, and the edges of the valves look fine. I've determined that the bent electrode was not due to contact with the piston, or valves, but rather due to the arc that the plug wire likely made as the plug was expelled. It would have swung the electrode right into the motor mount, or sundeck support, at the right angle to product that bend and cause the plug to pop out of the wire.

Regarding the hose clamp and nut found in the bilge, I probed around and saw nothing missing. I think Lazy Daze was right. Though I thought I had cleaned the bilge spotless, the grunge on them says they had to be down there for some time -as everything actually on the motor is spotless. It was just a confusing coincidence.

I replaced the plug, put on the muffs, and the engine purrs just fine. Though the sundeck supports are actually permanent, fiberglass reinforced wood, I should have no problem adding L brackets next season for easier access. I will use the endoscope for plenty of things, so I'll call that an investment. All the tips were much appreciated and I'm glad to have found Club Sea Ray.
 

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Glad to hear it was an easy fix. Now... are you tempting fate by not replacing the others? ;)
 
Lazy Daze said:
Glad to hear it was an easy fix. Now... are you tempting fate by not replacing the others? ;)
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Right?! Been in a huge time crunch with a business move... just a few more putts around the lake and she'll be hibernating. I'll have the extra plugs and socket onboard though, lol.
 
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