I've noticed some oil getting into one of my cylinders for quite some time but I have never been able to figure out what is going on. I've performed leak down tests and everything looks alright. However, I finally think I have found the culprit. Today, I pulled out a spark plug and took a peak down through the tube. If I am correct (please, someone correct me if I am wrong), directly through this tube I can see the top of one of the pistons. However, today something a bit different stood out... I saw what looked like a 1 inch crack running down the center!

Has anyone seen this on an MCS before? If so, how much did it cost for the fix? Oh... and if this is NOT the piston I am looking at, what is it?

 

A 1" long crack in the center of the piston?

Could be, but I'd think it would show in lower compression and also in the leakdown test.

Are you running an undersized pulley and or a tune?

The pistons I've seen that have failed have done so in the ring land, usually on the side of the piston over the pin.

Fixing it requires replacing the piston, and depending on your mileage, usually a total engine rebuild.

 

Yep... 17% reduction but no tune. I've got JCW injectors on there as well to try to avoid leaning out too much. Not sure how much it has helped.

The leakdown test we did was some time ago and maybe the crack has enlarged since then. The car has only 50k miles. I am definitely seeing a crack, just not positive of what I am looking at.

 

There's a good chance your only seeing a crack or difference in texture on carbon buildup on the piston, if the piston were cracked that much, there should be more serious signs.

 

Good point. I know I see a crack, but I can't tell if its metal that is cracked or the buildup on top that is cracked. I am definitely getting some oil in that cylinder, so this is what led me to believe that the crack is in the piston and not just on buildup. But yea... if it were a cracked piston I would think things would be worse.

 

My bet is if it was truely cracked, the car would not be running....
Since you are running a 17%, jcw380's and it sounds like notune, i bet you are rich some of the time, and are seeing a bit of carbon...seeing that kind a dammage with such low miles unless the car was abused would be kinda amazing....
Maybe you have a sticky/worn ring that is letting the oil in...
Done a compression check?!

 

Not lately. Now, I can't get the thing to stop misfiring. Gonna try some new plugs next and see if that solves the misfire. If it does, I'll take it somewhere to get some testing done to figure out wtf is going on.

Is piston ring replacement a big job?

 

When you take off the head, you can inspect it....
A valve issue is MUCH more of a common reason for a misfire...
what type of history does the motor have? Any major events, or regular issues leading up to this? Bought new or used? Any idea what octane fuel was usually run?
To crack pistion, i would expect a major event, detonation wise happened....and likly dammaged the head...

 

I'm noticing right now that no matter what a plug looks like when I put it in, I can run the car once and it will come out black. My only guess is that the oil in the cylinder is mucking it up and causing this which in turn is causing the misfires... I really don't know for certain though.

I did have one major issue a long while back that caused the spark plug to be ejected and actually stripped the threads on the head itself. This was repaired several years ago and I do not believe the two are related though this is the same cylinder.

I bought the car new and have always run 91 through the car.

 

Maybe you can have it walnut shell blasted just enough to remove any carbon and gunk and then determine if it's a real crack or not. If there's a chance it's a real crack, I certainly wouldn't be driving the car or running the motor, though. If it really is a crack and it lets go, you're looking at a lot of money to repair. But if all you have to do is replace the piston (or all four pistons), that's a lot less you'd have to spend.