Owners of 2016-2019 Cadillac ATS, CT6, CTS or 2017-2019 XT5 vehicles may experience a rattle, tick or knock-type noise. The noise may be accompanied by DTC P0300, P0302 or P0305 on the active fuel management system (AFM) cylinders. You may find that this noise is hard to isolate to one area of the engine. The condition may be caused by spongy stationary hydraulic lash adjusters (SHLA lifters) or damaged switching roller finger followers (SRFF) or a left bank timing chain tensioner gasket not sealing. The condition may be present all of the time, only under load, intermittent or when the engine is started after a hot soak.
Perform injector diagnosis to ensure that there are no leaking injectors causing noise due to excessive fuel in one cylinder. Replace any out-of-specification injectors and re-evaluate the noise.
Perform cylinder cancellation for each cylinder as this may help to pinpoint the source of the rattle noise.
Remove the intake manifold and visually inspect for excessive carbon on the intake valve stems and valve tip. If carbon is present, clean the valves or replace the cylinder head as needed. Re-evaluate the concern.
Remove the cam cover of the affected bank or both if unable to isolate to inspect for soft, spongy or damaged SHLAs or rockers on all cylinders. Push down on each rocker end at the SHLA to test for a spongy or soft feel. Check for any visible damage to non-AFM rocker clips. If soft/spongy or damaged SHLAs or rockers are found on either intake or exhaust side of any cylinder, it will be necessary to replace all the affected cylinder SHLAs or rockers and re-test.
If the replacement SHLAs are still spongy, it will be necessary to remove the affected cylinder head and inspect the head gasket for debris in the oil passages of the gasket. If debris is found, inspect the camshaft caps for wear or discoloration due to lack of oil. If wear or discoloration is found, replace the cylinder head with camshafts. If no wear or discoloration is found, clean the oil passages and replace the cylinder head gasket. Change the engine oil and filter and re-evaluate the noise.
Without starting the engine, crank the engine over and ensure that the valves are opening and closing. If they are not operating correctly, inspect the SRFF for any damage.
Inspect the SRFF and rocker assemblies for any damage or dislodged condition. Camshaft carrier assembly removal will be necessary to fully inspect the rockers and SRFFs. If any one of the SRFFs is damaged, it will be necessary to replace all four on the affected cylinder.
Inspect the left bank timing chain tensioner gasket for sealing integrity. Replace the tensioner and gasket if the gasket shows improper sealing.
If the noise remains, raise the vehicle. With the engine running, with ears or a stethoscope, listen to the right and left side of the engine. Once the noise is isolated, or if unable to isolate, remove the piston and rod assemblies from the affected bank(s) to inspect for a loose rod bushing in the rod or a loose wrist pin in the bushing.
Check for visible damage to non-AFM rocker clips.
Inspect the cylinder head gasket oil passages for debris.
Inspect the timing chain tensioner gasket for signs of improper sealing.
