A. Carbon Knock: Overview

If a top of engine knock noise is heard when starting a cold engine and decreases after the engine warms up, carbon may have accumulated in the combustion chamber and on the piston tops. Carbon accumulation reduces the clearance between the piston and cylinder head, causing contact and noise during combustion (Figure 3). This noise can be mistaken as HLA noise. Carbon knock can be distinguished from HLA noise by increasing the engine speed. Carbon knock noise will lessen or disappear as engine speed reaches approximately 2500 rpm. If carbon is suspected, perform the following Carbon Cleaning Procedure.