We've finally gotten an answer on the cold-start piston knock involving 4.8, 5.3 and 6.0-liter V8's in '99 to '02 GM cars and trucks. General Motors has previously released a Technical Service Bulletin explaining the condition that occurs after approximately 12,000 miles and a normal accumulation of carbon above the top piston ring.
I spoke with Sam Winegarden, GM program manager and chief engineer for small- and big-block engines. Sam explains that following extensive testing, General Motors is confident the start-up noise will have no adverse effect on performance or durability of the engines involved.
As opposed to the larger pistons of earlier design engines, today's technical refinement requires the use of smaller pistons to reduce weight and reciprocating mass in order to aid in efficiency and durability. This reduction in size also reduces the stability of the piston in the cylinder bore, which can promote piston noise.
Preliminary testing on these engines showed no sign of a problem, but, as in many other situations, the symptoms came to the surface after the vehicles were subject to normal driving conditions.
The manufacturer's stand on the issue is that it doesn't warrant a service bulletin or recall promoting a repair. We were also assured the problem has been corrected on the '03-model line with a piston-design modification along with the addition of a polymer coating.
My response was from the viewpoint of a consumer who purchased a brand-new vehicle that developed a notable engine knock before the first major service. The complaint was not only about the annoyance of the condition, but also the effect on resale and depreciation.
The answer was that General Motors would be dealing with the consumer on a case-by-case basis, and complaints have been resolved by means of warranty extension and parts replacement.
The owners of involved vehicles exhibiting the cold-start piston knock should have the service manager of the GM dealership, which has been evaluating the noise, contact the manufacturer's zone representative. Hopefully you can come to satisfactory terms.
