loud transmission noise at idle/low rpms then goes away when underway

Tom wieland

Tom wieland

New Member
Sep 2, 2019
24
Boat Info
1996 370 Searay Sundancer
Engines
454 Mercruisers
launched my 370 Searay Sundancer on Saturday and had a loud noise coming from the transmission at low idle/rpms then as I get underway, the noise goes a way. hope it just needs a tune up not transmission work. Checked fluid and was full.
 
Sound like it need a tune up... a slight miss at idle will make them rattle.
 
If one or more cylinders is not firing it will cause the damper plate to "rattle". As rpms increase and the engine smooths out the "rattle" goes away. Time to check out the ignition system including the distributor cap and plugs wires. A compression check wouldn't hurt either just to be sure the engine is sound.
 
Have mechanic checking it out next week. will do tune up regardless as has been several years. He also says could also be not enough transmission oil pressure. so I guess there's a transmission oil pressure pump?
 
A rattling damper plate has nothing to do with transmission oil pressure. If the only problem is the noise and the transmission functions properly the oil pressure is fine. Good chance the tune up will make a difference and make sure the idle speed is set correctly or maybe even 50 rpm above spec.
 
he also said there's a transmission fluid filter that may be causing the problem. I told him to do it all. both engines. Dont want to be caught out in the bay... Boat tow may not want to come close enough to tow now due to Covit-19 and mandatory distancing ..
 
had engine tuned up, plugs, wires, distributor, still makes loud noise, but engine does run better and idles at lower rpms . next he will remove transmission filter to replace and look for pieces.
 
check your compression, make sure all cylinders are firing and your timing is set right (you'll need to put it in service mode to set timing if you have EFI). A misfire makes the transmission sound like it's coming apart at dead idle, in gear or out.
 
Tom wieland said:
had engine tuned up, plugs, wires, distributor, still makes loud noise, but engine does run better and idles at lower rpms . next he will remove transmission filter to replace and look for pieces.
Click to expand...

What is the idle rpm in and out of gear? I prefer idle out of gear to be set slightly higher than spec at around 800 which usually nets around 650 in gear. If then you have a damper plate rattle it could just be worn damper springs which will make noise but no damage.
 
Tom wieland said:
...still makes loud noise, but engine does run better and idles at lower rpms.
Click to expand...

It should be idling at the specified RPMs. Idling at below spec RPMs shouldn’t be a goal even though it does tell you that the engine is running well.

Low idle can cause the rattle as well.

Your mechanic should know this if us jamokes do.

;)
 
just a follow up. mechanic did total tune up . When I first put it in gear , it still made noise. but as I moved the boat around in the slip, the noise completely went away?? Still going to have him replace the transmission fluid and fluid filter, but guess I get to keep my $10,000 it would have cost to do the transmission.
 
We had the same issue on our 1991 420DA with gas engines. I bumped RPMs about 25-50 and never heard the noise again
 
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