I have a 1987 35.5’ C & C MKIII. It has a 3GM30 Under the hood. love this very resilient engine. It’s not the first one I have owned. My former Hunter 36 had the same engine. Ok so the C & C is new to us. The engine starts and purrs like a jewel. Except for at very low base idle there is a somewhat loud chattering noise heard and felt in the cockpit. The moment you raise the idle even a couple hundred rpm’s the noise disappears. I’m a former automotive technician turned automotive engineer retired. So I’ve always diagnosed and repaired my mechanical toys And still do. At first I thought it was valve lash but now I think not. It’s currently winter in the Great Lakes area where our boat is hibernating. I live 700 miles south. So I can’t get my hands on it until spring launch. And I have not put my stethoscope on it either-yet. So I’m thinking maybe there is some part of the assembly where the transmission connects to the engine that has a worn part or maybe an adjustment to lessen lash. If anyone has experienced this on a 3GM30 I would really appreciate your comments here.
3GM30 noise unknown?
- Thread starter Jimongee
- Start date
I do not know the specifics of your engine & transmission. I do know I have heard a similar noise before my engine & transmission were rebuilt. In my case it was the transmission rebuild and the install of a new Damper Plate that resolved the issue.
The springs were old and sloppy. Here is an image of a damper plate.
The springs were old and sloppy. Here is an image of a damper plate.
Thank you jssailem. From your photo of the dampener plate and my past automotive career…now I have confidence of what it is so I’m not going to worry about it for now. I will just maintain the engine idle a slight bit above base idle…until I have the chance to disassemble and replace. Of course I’ll bet once I get back to the boat with my mechanics stethoscope ….I will have 100% confirmation that you are correct. Thanks again
As jssailem I have the same engine. My tranny is a Kanzaki KM3A (2.61/1). Engine ran perfectly in neutral without any abnormal noise or knocking. No problem in reverse either. Severe knocking in forward gear until RPM reached +/- 1300. Engine alignment was perfect and cutlass bearing was new. Removed tranny and had it rebuilt. Old parts were given to me so I could definitely see that the clutch was worn out, allowing the tranny to slip until RPM reached above 1400. That tranny had lasted 11 years from Fall of 2007 to Fall of 2018 (about 20 000 nm cruised in those years). Rebuilt tranny installed in Spring of 2019 and has been running perfect since (about 5 000 nm). Good luck rebuilding yours.
