Burned valve or worse 1997 5.7 LX

[AKJohne]

0K158784 Alpha I ~ 570 hours on rebuild (not by me)
Saturday we were wrapping up a week long trip and had good weather, running From Unakwik to Naked Island, about 25 miles, we had good water and was running hard, ~ 3800-3950 RPMS. Typically run 3200 to 3600 sometimes 3800, have recently seen 4800+ with the current prop. In any case I noticed rpm and speed dropping slightly, initially I thought the tension screw on the binnacle maybe needed tightening, although it had never backed off before. Then noticed the temps were up over 180, maybe 195 area. Typically runs just above 160, about 170. ALWAYS... I scan the gauges pretty often but can get complacent. I backed off immediately and pretty quickly the temps returned to normal, we were fishing so stop and drift, start, move, drift. I immediately noticed some hard staring and a miss. Pulled plugs and found this in #3 hole.
I checked comp at all 8 with digit in spark plug hole and noting on #3. These were Denso Platinum plugs and changed all 8 to the spare Delcos I had. We finished our day at more conservative RPMS and back to Valdez the following day. It was another 75 miles back to the harbor all told. Ran pretty good for a 7 cylinder, cruise along at 3200 and 22-25 mph, A uninformed passenger would not know anything was amiss. Noticeable at idle. Today I verified with a actual compression gauge and verified dead hole, the rest of the cylinders OK, one a130, some at 155, cold motor, not great but OK.
A leak down test confirmed intake issue, no air noticeable at exhaust or valve covers.

I am in the process of removing the head today but ran out of gitty up, I am hoping the piston is ok. Once the head is off I will decide if I will rebuild or replace, Vortec or original. (12 bolt manifold).

The nature of this post is the "what caused it"? My long time friend, smarter than me on engines thought the Delco plugs were a little on the light side, (I Rebuilt the weber 4V carb last year and used the original jets). He also thought the toasted plug was a serious case of detonation/preignition.

Timing was set at 12 base and should have been 10... if that was the cause then I learned a valuable lesson. Thunder bolt ignition with knock sensor, no funny alcohol fuels here, I always run 90 octane when I can, but in the harbor it is 87 only.

Any ideas here? You guys have always been helpful, I rarely contribute to this forum but always appreciate the help I receive. Thanks!

 

[Bondo]

Ayuh,.... Listen to yer Bud,.... Classic case of detonation,....

That's piston stuck on that plug,....

Probably a combination of hot timin', 'n a lean condition,....

 

[Scott Danforth]

that plug insulator is also heat cracked. so it got hot

most likely running lean.

once the head is off, have the head tested and take a good look at the piston. as Bondo pointed out.....bits of it will be missing.

being a 12 bolt intake, that means you have a 1995 motor (vortec heads came out in 1996)

I would upgrade to vortec heads if it was me.

 

[AKJohne]

I did not get a chance to pull the head this evening. Hoping for the best and expecting the worst.

Big thing is WHY, so I can avoid it from happening again.
I need to check the knock sensor, I was under the impression if it was inoperative it would hold back the advance....? I was looking at my manuals and do recall merc had 5 or 6 different ignition modules. The motor did not come in this boat,, which is a heavy cabin boat, I think it was on a Alumaweld ski-boat, now thinking it may have the wrong advance curve for the application. I will have to identify that and ask for more help/advice.
If the piston is OK, I will rebuild the heads so I can use it the rest of our short season. If the motor needs to come out I will plan on some Vortec heads, I am under the impression Vortecs have smaller chambers and should be coupled with the right shaped piston... Any advice here?


Thanks,

 

[Scott Danforth]

You were running lean and maybe borderline detonating. Then you were running hot which makes detonation worse

Service the fuel system, service the cooling system

Advance curve is fine as the motor doesnt know what its bolted in

 

[AKJohne]

Cooling system is fine, fuel system is fine, I believe it got hot because of detonation. Maybe broken oil ring that lowered fuel octane... possible lean and why just that cylinder? I ran 75 miles to get back after #3 lost compression.
Arent the module's/advance curve specific to application? Ski boat, cruiser etc

Busy, but will get the head off soon

 

[dubs283]

possible lean and why just that cylinder?.....Arent the module's/advance curve specific to application? Ski boat, cruiser etc

possibly a valve/piston/head gasket issue with just #3. won't know for sure til you pull the head. water intrusion to the combustion chamber can cause increased combustion chamber temps as well

timing advance curves are relative to the engine, not the boat. mercruiser has no idea what kind of boat the engines they produce go into, nor do they care...it's not until the boat manufacturer files paperwork for warranty does mercruiser know what boat the engine is in

 

[Rick Stephens]

Easy to have a 1409 on the lean side and not know it. Go for years if it isn't run hard for long enough, or had a little timing advance. Add in the extra advance, run lower octane fuel, push the throttle a teensy bit more and bang, ya gots a hole in a piston.

I'd go through the carb once again. Pay extra special attention to float height. Then either up the primary jets or go to next size up (smaller diameter) rods.

 

[AKJohne]

Head is off and I may have gotten off lucky as it appears the piston is ok. Interesting note is the the head bolts were not as tight as I would expect them to be. Pulled both heads while I was at it.
I( want to do a leak down on the cylinders so I can evaluate the rings, will check all of them out so I can compare.

Then will get the head repaired and may have both heads looked at.
I will get a larger set of mains and do a WOT plug reading and verify ignition/knock sensor is functioning as it should.

Pistons appear to be .040 over and no glaring issues with the cylinders, not much cross-hatch visible, and maybe expected at 551 hours. Once its back together will do a comp test again and see how it runs. Looking at the motor, I need to plan to rebuild or repower before too long as I am putting on over 100 hours a season.
Thoughts?

 

[AKJohne]

"Good head" and cylinder/piston

 

[Rick Stephens]

Head is off and I may have gotten off lucky as it appears the piston is ok. Interesting note is the the head bolts were not as tight as I would expect them to be. Pulled both heads while I was at it.
I( want to do a leak down on the cylinders so I can evaluate the rings, will check all of them out so I can compare.

Then will get the head repaired and may have both heads looked at.
I will get a larger set of mains and do a WOT plug reading and verify ignition/knock sensor is functioning as it should.

Pistons appear to be .040 over and no glaring issues with the cylinders, not much cross-hatch visible, and maybe expected at 551 hours. Once its back together will do a comp test again and see how it runs. Looking at the motor, I need to plan to rebuild or repower before too long as I am putting on over 100 hours a season.
Thoughts?

Have to complete your assembly/rebuild before doing a leak down. That should always be done before tear down or not at all. Once you have heads done and reinstalled a compression test is all you need to determine ring quality. Or you can pull pistons and rings and measure end gap.

Do not start with a WOT run plug reading. Your problems weren't at WOT. Much more likely you were only opening primaries. You need to have someone watch for you and open the throttle until primaries are all the way open but before secondaries and do your test there. Read the 1409 calibration manual and get very familiar with how the carb is calibrated. Once you understand al the pieces you will know how to proceed with a testing process.

I always start with idle. Then I go to wide open primaries. Once that is done I work on mid range and throttling up/acceleration behaviors. (needles, springs, throttle pump) Then I recheck primaries. Last is secondaries as they re dependent on having the primaries properly calibrated.

 

[AKJohne]

Have done both leak down and compression test b4 teardown. Will make a plate to check rings, I could wait until its back together, if I have rings issues would like to know now. Carb is a Weber 750, essentially a edlebrock 1410. Weber makes them for ed
Secondaries start to open at 4200, start flowing fuel at about 4400. If I recall correctly, checked that last time out.
I have downloaded the ed tuning guides and will wrap my head around that. Good advice on checking b4 secondaries open. More experience with delorto mc carbs. Principal is the same. Takes air flow to make things happen in a carb. I replaced the needle's with nos oem quicksilver needles when I rebuilt the carb, new springs included. Merc used a 3 step needle at that time. No longer available. Mikes carb did not have a cross reference for a 2 step. I think I will focus on the mains.... and quit doing long runs at high rpm....! The halibut were calling me!

 

[dubs283]

valve looks tuliped, piston looks washed. probably from water entering combustion chamber

i would check the exhaust manifolds and risers for leaks

 

[AKJohne]

I was curious about the piston as well, manifold, riser and elbow gaskets were still doing the job, I replaced the elbows this spring and use oem gaskets, actually pleased to see the exhaust manifold to head connection was in fine shape and the head gasket was not blown out either.

 

[AKJohne]

Gaskets & such, why is the top of the piston so clean?

 

[dubs283]

appears to be witness marks of some seepage past the head gasket at the cooling ports

you mentioned the head bolts did not seem to be as tight as you'd expect, perhaps this was the source of water intrusion

 

[AKJohne]

I dont think there was any water intrusion into the cylinder, I think the piston was washed with fuel when running the 75 miles back to the harbor. No compression at this time. Heads are at the machine shop for new parts as needed and getting refreshed.

We made a plate and did a leak down test on all the cylinders and #3 was not the worst, not great numbers but OK for a couple more hours. Thinking rebuild replace in the next 200 hours, that will depend on future leak down/compression test. Also attached pic of the compression test before removing heads. All in all I think I got off very lucky. lessons learned...

 

[AKJohne]

Cleaning prepping parts and waiting to get the heads back.

Thinking about the loose plug on the #3 cylinder, did the plug loosen after it got hot? or did the loose plug somehow contribute to the heat in that cylinder? all other plugs were tight, actually very tight.... I have hard time thinking one just loosened up on it's own.

Next question, does the Mercruiser knock sensor system actually work? I can diagnose the component's according to the book and will do that, but wondering if this system actually "hears" detonation? I think with some help I can do some experimenting to see if I can notice a reduction in timing, I know Tbolt ignitions can be hard to see with a timing light at any RPM other than base.

Thoughts,....? Thanks,

 

[Scott06]

Cleaning prepping parts and waiting to get the heads back.

Thinking about the loose plug on the #3 cylinder, did the plug loosen after it got hot? or did the loose plug somehow contribute to the heat in that cylinder? all other plugs were tight, actually very tight.... I have hard time thinking one just loosened up on it's own.

Next question, does the Mercruiser knock sensor system actually work? I can diagnose the component's according to the book and will do that, but wondering if this system actually "hears" detonation? I think with some help I can do some experimenting to see if I can notice a reduction in timing, I know Tbolt ignitions can be hard to see with a timing light at any RPM other than base.

Thoughts,....? Thanks,

Yes the knock sensors do work and i think it is basically a gm system or at least the sensor and module are. If you have a timing tape on the balancer you can see the total advance at any rpm. There are probably more elegant ways but i think you can wrap the side of the block with wrench while running to see if it retards the timing. Im sure if you goggle it you see some ways and probably some bs too…

mostly it looks to me like you were too lean. You may want to stab in a air fuel ratio gauge using an adapter like this https://www.hardin-marine.com/p-104396-o2-spacer-stock-mercury-riser.aspx

I recently started using one to tune my tri-power GTO carbs. I was totally blown away how much more information you get with an air fuel ratio meter then reading plugs

 

[AKJohne]

Yes the knock sensors do work and i think it is basically a gm system or at least the sensor and module are. If you have a timing tape on the balancer you can see the total advance at any rpm. There are probably more elegant ways but i think you can wrap the side of the block with wrench while running to see if it retards the timing. Im sure if you goggle it you see some ways and probably some bs too…

mostly it looks to me like you were too lean. You may want to stab in a air fuel ratio gauge using an adapter like this https://www.hardin-marine.com/p-104396-o2-spacer-stock-mercury-riser.aspx

I recently started using one to tune my tri-power GTO carbs. I was totally blown away how much more information you get with an air fuel ratio meter then reading plugs

Good info on the o2 riser, I didn't know they existed for marine, thanks for posting/sharing that....!