2000 GMC Passenger Vehicle

ENGINE KNOCK IN COLD START(IT SOUNDS LIKE A DIESEL ENGINE TRUCK), THE DEALER SAYS IT'S CARBON BUILD UP,BUT I DID SOME RESEARCH AND FOUND OUT THAT CARBON BUILD UP IS PROBABLY NOT THE CAUSE. THE CUASE IS MORE LIKELY TO BE A PISTON SLAP,BECAUSE WHEN THEY DESIGNED THIS ENGINE THEY MADE THE PISTON SKIRTS TOO SHORT CREATING A GAP BETWEEN THE CYLINDER WALL AND THE PISTON, CAUSING THE PISTON TO RATTLE INSIDE THE CYLINDER WALL WHEN YOU FIRST START THE ENGINE,NOW BECAUSE THE PISTONS ARE MADE OUT OF ALUMINUM THIS MATERIAL EXPANDS WITH HEAT AND CONTRACTS WITH THE COLD,SO WHEN YOU START THE ENGINE YOU CAN HEAR THE KNOCK,BUT AS SOON AS THE ENGINE HEATS UP THE ALUMINUM PISTON EXPANDS AND SEALS THE GAP AND THAT IS WHAT STOPS THE KNOCK. THE DEALER HAS A SERVICE BULLETIN WHERE IT SAYS THAT IT IS A COLD START KNOCK DUE TO CARBON THAT HAS FORMED ON THE PISTON, AND THAT THE KNOCK STARTS IN THE FIRST 12,000-15,000 MI AND THAT IT IS NOT DETREMENTAL TO THE ENGINE. THE PROBLEM IS THAT THIS IS MY SECOND ENGINE AND I ONLY HAVE 30,000 MI ON IT AND IT KNOCKS REALLY BAD IN THE MORNINGS WHEN I START IT UP, AND IN THE AFTERNOONS AFTER I GET OFF OF WORK AT 4 P.M. THE SERVICE REP FOR THE DEALER WHERE I BOUGHT THE TRUCK TOLD ME THAT HTHE ONLY THING THAT THEY COULD DO IS DECARBON THE ENGINE, BUT THAT THE KNOCK WAS GOING TO COME BACK. THE FIRST ENGINE THAT I HAD ON THIS TRUCK STARTED KNOCKING AT 29,000 MI AND IT GOT TO THE POINT WHERE EVEN IF THEY DECARBONED THE ENGINE THEY COULD NOT STOP THE ENGINE FROM KNOCKING,SO THEY ENDED UP HAVING TO OPEN THE ENGINE TO SEE WHAT THE PROBLEM WAS AND IT TURNED OUT THAT THE BLOCK WAS CRACKED IN ONE OF THE CYLINDERS, AND THAT'S WHY THEY ENDED UP HAVING TO REPLACE THE ENGINE. WHEN THEY REPLACED THE ENGINE THEY REPLACED IT WITH ONE OF THE SAME DESIGN AND THEY TOLD ME THAT THIS ENGINE WAS NOT GOING TO HAVE THAT PROBLEM. I FOUND OUT THAT THEY ALREADY REDISINED THE ENGINE THIS YEAR AND IT DOES NOT HAVE THIS PROBLEM.