Idle RPM Vibration

[ccvalentino]

Hello,

I’m just over 400 hours on engine in my 2005 Catalina 350.

Recently, the vibrations and rattles at idle have become more pronounced. I had my trusted mechanic out last week to check things over. He said everything was ok, but my idle RPMs were a little low. I went down to check on the boat today and it’s still got what I would describe as a fair amount of vibration/rattle. Once I’m up over about 1100RPMs it all clears up and everything smooths out. My mechanic said it could be that the motor mounts are worn or the damper plate (as others have experienced).

This might have brewing for a while, but it seems like it came on all of the sudden.

Thoughts appreciated.

Thanks,
Chris

[KenKrawford]

Chris, I had a somewhat similar problem with engine vibration a couple of years ago. It turned out that one of the nuts on the top of a motor mount had vibrated loose. Tightening it solved the problem. It's not a bad idea to put some nail polish above each of the 4 top motor mount nuts to verify that there's been no movement. I don't have any experience with damper plate issues (fingers crossed) so can't comment on that issue.

[ccvalentino]

Thanks Ken — I’ll take a look. Great idea with the nail polish. It’s hard to describe vibrations, but it just feels like something is lose…

Chris

[wolfe10]

Yes, checking engine mount fastener torque should be done at oil change intervals.

Engine mount to stringer and the top nut. If very loose, a good idea to re-check shaft (actually flange) alignment.

Also, if you have a lot of unexplained dust in the engine room, may be time for a new dampener plate--several good posts on this.

[ElizabethG]

Brett,

Any idea what the torque specs are?

Bill
C350, #193
Atlanta (Home) / Charleston, SC (Boat)

[wolfe10]

Bill,

No sorry, I don't. Just pull them up to snug (to engine stringer). More on nut on top (appropriate for diameter of threads).

[russp]

Hi Chris,

Sounds like typical damper plate issue. I had exactly what you describe, kept my idle above 1000 rpm for a few years to get reasonably smooth idle but didn't like crashing it into gear. Now I keep the idle down but pretty much as soon as I get out of gear I increase throttle speed back to a very smooth idle out of gear, like you, around 1100 rpm.

I have replaced the damper plate and it is a lot better, and at present much of the "rattle" is from the swinging back flow plate in the exhaust muffler. Eventually I will get around to replacing that too.

Regards

[ccvalentino]

I did not know that there was a back flow plate in the muffler -- in addition to the vibration, the exhaust has been a little "louder" than a few years ago. Is the back plate inside the muffler?

Chris