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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.
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