Only one melted piston?

Hey folks.

A friend of mine has a 2000 Acura Integra GSR. It has the B18C engine
(1.8L 4cyl VTEC). Last summer he installed a turbocharger on the
engine. He knew the engine was kind of tired (it smoked a little bit
on acceleration), so he didn't boost a whole lot (3psi, iirc). He
also ran it a little rich to keep from burning it up. Well,
eventually it started to smoke a lot, and he pulled the engine when it
got real bad. (He's building a brand new one for the car as we type).

What is interesting, is that 3 of the 4 cylinders look relatively
normal. There is some black carbon buildup on the valves and piston
heads, likely from running rich. However, cylinder #3 is spartan
clean- everything in it looks like shiny new metal. And of course,
#3 is the cylinder where all the smoke was coming from- a small
section of the piston (about 1") is melted... on the intake side.

Any thoughts as to why just one piston would melt, and why that one
cylinder would be clean? We were thinking maybe a failed injector,
but the car wasn't missing or detonating at all. It ran with full
smooth power all the way up until the engine was pulled. Also, if the
piston was going to melt, why on the intake side and not the exhaust
side?

Thanks for any insight on the mystery.

-ph

phaeton wrote:
> Hey folks.
>
> A friend of mine has a 2000 Acura Integra GSR. It has the B18C engine
> (1.8L 4cyl VTEC). Last summer he installed a turbocharger on the
> engine. He knew the engine was kind of tired (it smoked a little bit
> on acceleration), so he didn't boost a whole lot (3psi, iirc). He
> also ran it a little rich to keep from burning it up. Well,
> eventually it started to smoke a lot, and he pulled the engine when it
> got real bad. (He's building a brand new one for the car as we type).
>
> What is interesting, is that 3 of the 4 cylinders look relatively
> normal. There is some black carbon buildup on the valves and piston
> heads, likely from running rich. However, cylinder #3 is spartan
> clean- everything in it looks like shiny new metal. And of course,
> #3 is the cylinder where all the smoke was coming from- a small
> section of the piston (about 1") is melted... on the intake side.
>
> Any thoughts as to why just one piston would melt, and why that one
> cylinder would be clean? We were thinking maybe a failed injector,
> but the car wasn't missing or detonating at all. It ran with full
> smooth power all the way up until the engine was pulled. Also, if the
> piston was going to melt, why on the intake side and not the exhaust
> side?
>
> Thanks for any insight on the mystery.
>
> -ph

If one cylinder is running lean (clogged injector, etc) the other
cylinders sharing the same O2 sensor will trim rich.

phaeton wrote:

> Any thoughts as to why just one piston would melt, and why that one
> cylinder would be clean? We were thinking maybe a failed injector,
> but the car wasn't missing or detonating at all. It ran with full
> smooth power all the way up until the engine was pulled. Also, if the
> piston was going to melt, why on the intake side and not the exhaust
> side?
>

If that cylinder were burning a little more oil than the others, that
could cause it to go into detonation before the other 3 (engine oil is
very low octane and causes detonation). Heavy detonation will knock all
the carbon off, and can also melt parts.

Just a SWAG on my part.