Zero Compression Cylinder 2 – 1998 Mercury 115

[JK Splice]

Hello everyone,
I have a 1998 Mercury 115 ELPTO with zero compression on cylinder #2.

My findings:
All cylinders have matching compression except cylinder #2 which has 0.

Piston #2 is burnt with a hole (explains the 0 compression) and cylinder walls are scored and have lost cross hatch marks. Not sure how this happened, probably detonation.

Cylinder 1 is slightly scored just like cylinder #2 but it still has normal compression matching both cylinders #3, #4. Cylinders #3, #4 are all good.

Overall, I still have to measure the cylinder bore size to see if anything changed but done think so since cylinder #1 is good. I might need to hone cylinder #2 walls to remove the scoring since it looks like the piston itself took majority of the damage.

My questions:
Will honing the scored cylinder wall #2 be enough to repair and add new piston & rings?
Should cylinder #1 which is slightly scored but has normal compression be touched?
I'm no expert any tips help.
Thank you.

 

[Scott Danforth]

no

you will need to bore and hone. and Merc's use blind holes. so just the machining will be about $300 and you have to find a machine shop that does blind hold boring and honing

you may be better off and cheaper to find a different motor or power head

 

[racerone]

Both # 1 and 2 need boring.---------And you need to determine what happened.----Correct the fault.------If not it will just go " crunch " again.

 

[Texasmark]

Hello everyone,
I have a 1998 Mercury 115 ELPTO with zero compression on cylinder #2.

My findings:
All cylinders have matching compression except cylinder #2 which has 0.

Piston #2 is burnt with a hole (explains the 0 compression) and cylinder walls are scored and have lost cross hatch marks. Not sure how this happened, probably detonation.

Cylinder 1 is slightly scored just like cylinder #2 but it still has normal compression matching both cylinders #3, #4. Cylinders #3, #4 are all good.

Overall, I still have to measure the cylinder bore size to see if anything changed but done think so since cylinder #1 is good. I might need to hone cylinder #2 walls to remove the scoring since it looks like the piston itself took majority of the damage.

My questions:
Will honing the scored cylinder wall #2 be enough to repair and add new piston & rings?
Should cylinder #1 which is slightly scored but has normal compression be touched?
I'm no expert any tips help.
Thank you.

I just bought a 115 4 cyl 2 stroke age about 17 years and usage unknown. Compression was 129-129-129-130, tested and marked on the water jacket by the dealer's mechanic whom I trust and he had replaced the water jacket cover that was leaking on the PO and went through pre delivery checks, don't know what else needed attention as the dealer offered "try before you buy" and I didn't accept the offer. Initially, on my first couple of times out (shake down cruise if you will) it had whatever gas was in it and at times I thought I heard pinging that was unrelated to the overrun clutch ticking upon deceleration at high rpms...... have since been working on the right prop for the rig as the one on it was apparently removed by somebody and something else that no way would run this rig replaced it.....that's another story and I think I'm there with a sweet prop.

Anyway, as soon as I was able to get to a non-ethanol dealer I filled up with 91 octane "gasoline" with Sea Foam and Mercury fuel additives.....( Big "over dose" of Sea Foam to clean up carbon deposits I found on top of the pistons.) and after a bit I never heard that noise again. So, my guess is pre-ignition since those engines run 20-25 (range) BTDC timing at WOT. I know it can happen as I had a 90 triple of the same vintage, design, mfgr. and I was diddling with the timing one day while it was running and I got it set too high (on 87 ethanol) and it took off on it's own and pegged the Mercury 7k tach fpr all of a very excited eternity (seemed like) to me. Turning the key off did nothing and the fuel tank was hard plumbed to the engine on that boat. In a panic, what to do, what to do, I grabbed a pair of pliers and pinched the fuel line and got it to stop.

I think looking for another powerhead that you can rely on or another engine isn't all that bad of an idea as you can run up the tab on an OH really fast even with you dong the labor, hoping you put everything back like it belonged and all that....BTDT stuff.

Edit: Thinking about the rest of the problem.......I know that pre-ignition can burn holes in pistons but the scoring......guessing....... with zero compression, the cylinder couldn't draw in a complete fresh charge of fuel mixture meaning that there was nothing to lubricate that cylinder, like wouldn't matter on a 4 stroker with an oil sump. That would explain the secondary problem- cylinder wall to piston skirt scuffing...to me anyway.

 

[Faztbullet]

The pics you posted show piston/cylinders that are damaged are #3 and #4 not 1 and 2.

 

[racerone]

Another good catch.-------Yes it is 3 and 4 that are damaged.------I believe that engine does not get fuel to #3 and 4 like the top 2 cylinders at low RPM.-----It only runs / idles on the top 2 cylinders.--------Someone may have tampered with carburetors.----Or motor was operated with plugged / partially plugged carburetors.-------Find and correct the issue or it will go " crunch " again.

 

[JK Splice]

no

you will need to bore and hone. and Merc's use blind holes. so just the machining will be about $300 and you have to find a machine shop that does blind hold boring and honing

you may be better off and cheaper to find a different motor or power head

Thank you, i will look into that.

 

[JK Splice]

Both # 1 and 2 need boring.---------And you need to determine what happened.----Correct the fault.------If not it will just go " crunch " again.

I thought so, and yes i need to determine if it was a timing issue or if the oil pump failed. Thank you

 

[JK Splice]

I just bought a 115 4 cyl 2 stroke age about 17 years and usage unknown. Compression was 129-129-129-130, tested and marked on the water jacket by the dealer's mechanic whom I trust and he had replaced the water jacket cover that was leaking on the PO and went through pre delivery checks, don't know what else needed attention as the dealer offered "try before you buy" and I didn't accept the offer. Initially, on my first couple of times out (shake down cruise if you will) it had whatever gas was in it and at times I thought I heard pinging that was unrelated to the overrun clutch ticking upon deceleration at high rpms...... have since been working on the right prop for the rig as the one on it was apparently removed by somebody and something else that no way would run this rig replaced it.....that's another story and I think I'm there with a sweet prop.

Anyway, as soon as I was able to get to a non-ethanol dealer I filled up with 91 octane "gasoline" with Sea Foam and Mercury fuel additives.....( Big "over dose" of Sea Foam to clean up carbon deposits I found on top of the pistons.) and after a bit I never heard that noise again. So, my guess is pre-ignition since those engines run 20-25 (range) BTDC timing at WOT. I know it can happen as I had a 90 triple of the same vintage, design, mfgr. and I was diddling with the timing one day while it was running and I got it set too high (on 87 ethanol) and it took off on it's own and pegged the Mercury 7k tach fpr all of a very excited eternity (seemed like) to me. Turning the key off did nothing and the fuel tank was hard plumbed to the engine on that boat. In a panic, what to do, what to do, I grabbed a pair of pliers and pinched the fuel line and got it to stop.

I think looking for another powerhead that you can rely on or another engine isn't all that bad of an idea as you can run up the tab on an OH really fast even with you dong the labor, hoping you put everything back like it belonged and all that....BTDT stuff.

Edit: Thinking about the rest of the problem.......I know that pre-ignition can burn holes in pistons but the scoring......guessing....... with zero compression, the cylinder couldn't draw in a complete fresh charge of fuel mixture meaning that there was nothing to lubricate that cylinder, like wouldn't matter on a 4 stroker with an oil sump. That would explain the secondary problem- cylinder wall to piston skirt scuffing...to me anyway.

Texasmark interesting story you have there. I can only imagine having to pinch the fuel line to get it to stop. I agree pre-ignition sounds like what happened here. Thanks for the help.

 

[Texasmark]

................. if the oil pump failed.

Looking at the oil pump drive gear on your crankshaft pic. that is said to be Phenolic (yes-no you guys that have been there) must be the pump or oil supply to the pump, not the drive. The pump gear is metal.....had mine out on another engine. Haven't had a pump apart so I don't know what's to fail internally there.

 

[JK Splice]

The pics you posted show piston/cylinders that are damaged are #3 and #4 not 1 and 2.

Yeah, i wasn't sure if i had the correct order. So i just labelled them accordingly for image referencing purposes. I looked through the service manual and i didnt really see where they specify which cylinder is which. Thanks

 

[racerone]

The pump drive gear is around the crankshaft.-----It is NOT metal.-----It be a plastic type material.----The gear on the pump itself is in fact metal.