mercruiser 3.0l piston hitting spark plug?

[virmagicus]

I just joined iboats I recently purchased a 1987 Rinker V170 with the 3.0L mercruiser for $600. The boat is in really great shape. The previous owner said that it sat for about 2 1/2 years. The outdrive was off and he had all the parts to fix. Bellow kit that included a gimbal bearing. I installed everything with success. I also rebuilt the mercarb and fresh tuneup...oil, plugs etc. I could not get the boat to idle and stay running good under 1200 rpm on land with muffs. I also had a hard time with it wanting to stall returning the shifter in neutral after being in forward. I dropped her in the water a couple of days ago to make some carb adjustments while cruising. Ran great for 1 1/2 hours in forward but still would stall when shifting in neutral. Yesterday I attempted to make another adjustment...still with no luck. As I was coming in to dock I heard a lot of hard tapping all of a sudden. I popped the valve cover and everything looked ok. Next I pulled the plugs..One plugs electrode was pushed in tight and the other was cocked sideways. It seems the piston was hitting the spark plug. Any clues as to of what would cause this? Thanks!

 

[watrski4ever]

They must be the wrong plugs for that engine.

 

[virmagicus]

They must be the wrong plugs for that engine.

Purchased the recommended plugs MR43T

 

[alldodge]

I just joined iboats 😃 I recently purchased a 1987 Rinker V170 with the 3.0L mercruiser for $600. The boat is in really great shape. The previous owner said that it sat for about 2 1/2 years. The outdrive was off and he had all the parts to fix. Bellow kit that included a gimbal bearing. I installed everything with success. I also rebuilt the mercarb and fresh tuneup...oil, plugs etc. I could not get the boat to idle and stay running good under 1200 rpm on land with muffs. I also had a hard time with it wanting to stall returning the shifter in neutral after being in forward. I dropped her in the water a couple of days ago to make some carb adjustments while cruising. Ran great for 1 1/2 hours in forward but still would stall when shifting in neutral. Yesterday I attempted to make another adjustment...still with no luck. As I was coming in to dock I heard a lot of hard tapping all of a sudden. I popped the valve cover and everything looked ok. Next I pulled the plugs..One plugs electrode was pushed in tight and the other was cocked sideways. It seems the piston was hitting the spark plug. Any clues as to of what would cause this? Thanks!

Agree with Waterskir , what kind and plug number are you installing? Could use an engine serial number
If you have the 115HP you should use AC-MR43T or NGK BR6FS
If you have the 135HP you should use AC-MR43LTS or NGK BPR6EFS

and :welcome: to iboats

 

[jester31]

Stop everything, and put the right spark plugs in! You probably have the long reach spark plugs in there by mistake!

Are you sure they're not the AC-MR43TS long reach?

 

[virmagicus]

I'm positive they are the correct plugs. I ran them for an 1 1/2 hours. Two of them are fine and still have the .035 gap.

 

[alldodge]

I'm positive they are the correct plugs. I ran them for an 1 1/2 hours. Two of them are fine and still have the .035 gap.

In that case you may have some bigger issues with carbon build up, or a connection rod or piston coming loose. I'm ruling out detonation because you said they were bent over and not disintegrating

 

[virmagicus]

In that case you may have some bigger issues with carbon build up, or a connection rod or piston coming loose. I'm ruling out detonation because you said they were bent over and not disintegrating

Thanks. As far as the carbon buildup...would that just be on top of the pistons that would be the problem?

 

[bfjou812]

Did you try rotating the motor with the spark plugs out? Which 2 plugs were bent? I'm wondering if there was some water that entered and the motor tried to hydro lock? I would think if a rod or wrist pin were loose there would be a loud knocking getting louder as the motor ran along with some vibration.

 

[virmagicus]

There was a loud tapping. It sounded like it was coming from the top end though.

 

[Bt Doctur]

how long are the plug threads? more than 1/2 inch and you got the wrong ones.

 

[virmagicus]

I'll check and post back. Anyone think this is possible from pre combustion? One of the plugs shows the electrode is pushed to one side?

 

[Bondo]

I'll check and post back. Anyone think this is possible from pre combustion? One of the plugs shows the electrode is pushed to one side?

Ayuh,.... Welcome Aboard,..... It's very possible it was Detonatin' long before the pistons came apart, 'n hit the plugs,....

Either way, I'm thinkin' pullin' the motor is the next step,...
It sounds like Serious internal damages,....

 

[achris]

Boat sat for 2 1/2 years. Did you drain the fuel tank, all the lines, carb and fuel pump and fill it with fresh petrol? If not, you most likely have exactly what Bondo suggested, detonation... and a dead engine.

 

[alldodge]

I'll check and post back. Anyone think this is possible from pre combustion? One of the plugs shows the electrode is pushed to one side?

I don't see that happening