Mercruiser 5.7 spark plug cylinder 5 looks bad

[fotonews]

Spark plug cylinder 5 looks bad, compression is 109, all others between 138-140.What causes a spark plug to have build up like this. Water intrusion from manifold? valves out of whack? cylinder head gasket?

 

[HT32BSX115]

Spark plug cylinder 5 looks bad, compression is 109, all others between 138-140.What causes a spark plug to have build up like this. Water intrusion from manifold? valves out of whack? cylinder head gasket?

Howdy, Any number of things could cause it, but with all that carbon, and low compression, I would guess at a broken or or severely worn out ring(s)

Either way, no "mechanic-in-a-can" will fix it.....that head is coming off. Then you'll know more. If the engine hasn't ran in a long time, you may also find that a leaking riser gasket allowed water into an open exhaust valve at one time or another.

 

[Bondo]

Ayuh,.... Detonation is my guess,....

#5 just happen to be the worst cylinder of the 8,....

Probably from a lean condition,....

 

[fotonews]

Mr Bondo, why cylinder 8 is the worst, please elaborate... also from my primitive knowledge lean condition means not enough fuel so issue could be with intake valve?

 

[fotonews]

I mean cylinder 5...

 

[Bondo]

Mr Bondo, why cylinder 8 is the worst, please elaborate... also from my primitive knowledge lean condition means not enough fuel so issue could be with intake valve?

Ayuh,..... What induction system is on this 5.7l motor,..??

Carb,..?? Which carb,..??
Efi,..?? Mpi,..??

Boat motors tend to go lean from crud in the fuel system from the tank to the intake,....
Water, 'n sittin' alot contribute to this,...

When fuel can't be properly delivered, it goes lean, sometimes some cylinders more than others,...

It looks like piston pieces built up on yer spark plug, from detonation,...

If ya just fix what's broke, or go for a reman, the boat's side of the fuel system will still be Suspect,.......

 

[fotonews]

5.7 EFI TBI fresh fuel.The piston and cylinder walls look fine although a lot of carbon in combustion chamber. Heads went for resurfacing, will get them back in a couple of days.

 

[Scott Danforth]

Post a picture of the pistons.