Pulled out the motor for the rebulid and crack it open and find this. Tech says it looks like its been running super rich? How is that possible on a TPI car with O2 sensors? It sould shorten the injector pulse too correct the rich condition no? The car has ALWAYS had a rotten egg smell under load and it has NO cats, could this be related. Even with 94 Octane in 20 degree weather the motor detonates CONSTANTLY, WTF? The tech says it almost looks like a carb car running rich? Any ideas guys? I had a custom chip burned by TPIS but I'm not sure if that will fix the problem? The previous owner installed a Hypertech chip, could that be part of the problem. This car has never run right since the day I bought it.

 

Definately carbon build up. It also looks like a build up of burnt oil. Check valve guides for wear and replace seals.

 

Aftermarket chips are junk, get it professionally tuned.

 

oh look...ANOTHER hypertech chip w/ detonation issues...
yep, looks like carbon/oil burn build-up

 

I think that the carbon buildup can cause detonation problems. Maybe the carbon gets so hot that it causes the fuel to burn too early during the compression stroke? Pre-ignition?

The pistons will clean up with a little elbow grease. I used a soft brass brush and solvent on mine. A pain in the butt. Takes about 20 min/piston.

 

I've also heard that high octane fuel will buildup carbon on the piston tops faster than a lower octane. Engines with high compression and/or lots of timing need high octane fuel.

Maybe you should clean all the carbon off, set the base timing at stock (6 deg?), and try running 89 octane fuel. If it doesn't knock, you don't need the higher octane.

 

yes, carbon can induce spark, but usually it would be caused by more build-up than you had. - I do agree on the octane. If you're not running enough compression to support the higher octane, it won't help you any. - yes, it could be the cause of your carbon build up...after all, if you aren't burning it all, it ends up somewhere...

 

I'd be a little more concerned about the hole in the piston in that second photo. That's not a typical detonation hole.

 

Holly crap! Is that a hole? I thought that it was a drop of coolant..

 

I just hit it with some contrast in photo shop. It looks like a hole that's filled with coolant.....

 

Those pistons appear to have some sort of depression worn into them, maybe from all the pounding, so they need to be replaced anyway. The depression looks like some sort of dish or something. Everyone knows that GM would have never put pistons like that in any of their engines....

You'd gain probably about 0.3 compression going with flat tops. Mind the quench.

 

Coupla things here.....

1. If an engine isn't running right it will build up deposits in the chambers/piston faces. Period. Any time you have bad combustion- too rich, too lean, timing way off, outright misfire, you've got unburt crud left over. It slowly builds up on all the parts. That looks more like oil to me, but who knows. Probably a combination of oil and unburt fuel (too rich). Agreed, valve guides & seals should be checked- you can lose a TON of oil through leaky guides.

2. You've almost certainly had some detonation. How do I know? Becuase on some pistons there are clean spots. The detonation blew the carbon off of them like a little explosion in the chamber. It knocks it loose from the piston face. Detonation usually starts near the exhaust valve, away from the plug, near the quench area- and that's roughly where the clean spots are on your pistons.

Did the carbon contribute to the detonation? Probably. But is it the root cause? Maybe not. Bad running contributes to carbon buildup which then causes detonation. OR MAYBE.... it would have had detonation even with clean chambers if the tuning is wrong.