Engine Tick/Knock/Rattle Noise (Reprogram VCM)

This bulletin cancels and supersedes Corporate Bulletin Number #77-61-21. Please discard #77-61-21 (Section 6 -- Engine).

Some owners may comment about a light tick or knock noise that resembles either a valve lifter tick/rattle or a piston or piston pin slap. The condition will be most noticeable on cold start. This noise can be present upon engine decel after initial starting flare and may not be consistently repeatable.

On the initial cold start under certain low temperature conditions, the engine RPM flare may cause the piston to tip while at top dead center. This tipping allows the small carbon deposits above the top ring land to contact the inside of the cylinder bore making a light tapping noise. While this is not detrimental to the performance, durability or ability of the engine to meet emissions standards, it is an undesirable audible noise condition.

A new engine calibration has been developed for the 1998 and 1999 model year vehicles built before August of 1998. A new calibration for the 1996 and 1997 model year vehicles is being developed and this bulletin will be re-issued as soon as that information is available. The new calibration reduces the idle flare in terms of maximum RPM and length of time the flare occurs allowing the seconds necessary for thermal expansion of the piston in the bore.

There are no physical parts to be installed. Reprogram the VCM with the appropriate calibration from the table below.

The calibrations mentioned above are available on 1998 Techline CD 13 or newer.

For vehicles repaired under warranty, use: