Originally Posted by LittleRedToyota

unless your engine is pinging, you should run whatever octane the manufacturer suggests (should say in your owner's manual).

running higher octane gasoline will reduce pining, but it can actually increase carbon build up.

higher octane does not mean the gas is "better"...it just means it is harder to burn (which is why it stops pinging).

carbon build-up is caused by incomplete combustion.

fuel harder to burn = more likelihood of incomplete combustion = carbon build-up.

engines are designed to run gasoline with a specific octane rating. if you run an octane level higher than what the engine is designed for, you can get incomplete combustion (because the gasoline is harder the burn than the engine was designed for, so the engine does not completely burn it).

the result, over the long-term, is carbon build-up.

now, once you already have carbon build-up, you may need to switch to (or continue running) higher octane fuel. the reason is that the carbon build up effectively reduces the volume of the combustion chamber, thereby raising the compression ratio (higher compression requires higher octane). also, the carbon deposits retain heat, thereby raising the temperature of the combustion chamber.

however, if you do not have pinging problems and your engine is stock, you should run the recommended octane level, not higher or lower.

Originally Posted by Chuki

IMy guess is either I have bad knock sensor, or carbon buildup.

Originally Posted by LittleRedToyota

my guess is carbon build up--try seafoam (sucked through one of the little vacuum hoses attached to the throttle body).

there have been a number of threads about seafoam on this forum...do a search for it and you'll find all kinds of info.