There is a bulletin regarding the potential of carbon knock. It involves piston replacement with new design pistons. I have seen some of these have a carbon knock reoccur, it all depends on how you drive it. If I were to make a suggestion, and you want to address it, I would have the pistons and combustion chambers cleaned with a top engine cleaner. If done right, it may make a noticeable change in the concern. If you are consuming some oil, it would possibly help with that also.
Here is the bulletin for your reference.
#03-06-01-025: Engine Cold/Knock/Tick Noise (Replace Pistons) - (Sep 2, 2003)
Subject: Engine Cold/Knock/Tick Noise (Replace Pistons)
Models: 2000-2002 Cadillac Eldorado
2000-2003 Cadillac DeVille, Seville
with 4.6L Engine (VINs Y, 9 -- RPOs LD8, L37)
Built Prior to the Following VIN Breakpoints:
Important: Implementation of this service bulletin by "GM of Canada" dealers requires prior District Service Manager approval.
Some customers may comment about a tick/knock noise, sometimes described as an upper engine ticking noise or a deep low knock that sounds similar to a main bearing knock. This noise is more often heard during engine start up after a long cold soak condition and may or may not diminish as the engine reaches normal operating temperature. The knock may appear to be the loudest at the exhaust manifolds on either side or both. This noise does not change when disconnecting spark plug wires, or individually disabling the fuel injectors. As a general rule, the source of this noise cannot be determined.
The source of this noise may be carbon deposits in the engine combustion chambers.
Notice: Use the correct fastener in the correct location. Replacement fasteners must be the correct part number for that application. Fasteners requiring replacement or fasteners requiring the use of thread locking compound or sealant are identified in the service procedure. Do not use paints, lubricants, or corrosion inhibitors on fasteners or fastener joint surfaces unless specified. These coatings affect fastener torque and joint clamping force and may damage the fastener. Use the correct tightening sequence and specifications when installing fasteners in order to avoid damage to parts and systems.
Install service piston, P/N 89017438, with rings, P/N 89017413. Follow installation instructions in the Service Manual.
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Monday, March 15th, 2010 AT 10:59 PM