Jim Grant's Tech Tips

'99 GMC K2500, Cold Start Knock

Q: First thing in the AM the engine in my '99 GMC K2500 has a cold start knock (piston slap during deceleration or coming off fast idle) which lasts for approximately one minute depending on the outside air temperature. I first saw an article that was from CLICK and CLACK and in the newspaper. I called the GMC Service 800 number.. They said to have the problem confirmed by a GMC Dealer and they would add 100K miles to my warranty. The question: What is the real cure for this problem and how will it be resolved?.

A: Oh, yes there is a buzz in the trade regarding the piston slap/knock noise problem that GM is having. The other manufacturers (Nissan Ford to name a few) that have been through this problem are just grinning like the Cheshire cat over this issue. What is the real cure/fix? Depends on if it is a piston problem, cylinder problem or both. To answer that requires, removal, disassembly and inspection of the engine. In today's engine the focus is on light weight and reduced friction. On the inside of engines this philosophy is being pushed to the limit and the limit is biting back. The dilemma is that the piston, when cold, has too much room to move inside the cylinder of the engine. With extra room the piston slaps the side of the cylinder wall, which results in a knocking noise, until the piston and cylinder warm-up and reduce the room available for dancing. How big will this problem become for GM? It's hard to say. There are a lot of GM engines with this piston design. GM is stepping up and saying they'll provide an extended (if that's the right wording) warranty on their engines because it's not a problem. The quandary is, it takes time/mileage or a disassembled engine before the real answer is known.