Mercruiser 454 knocking bad at idle speed

[BamaFrazier]

Port engine (Mercruiser 454) purrs like a kitten when not in gear. But, when the transmission is engaged it knocks like crazy at idle speed. Seems fine at higher velocity, though.

At first I thought maybe this darn E10 gas had fauled my carburator. I have a 1990 Sea Ray Sundancer 350, so have the plastic gas tanks. But, surely it's a transmission problem, right? I mean, it only knocks when the transmission is enganged.

I had had a problem with the engine running hot for months until I finally found a soft spot in the hardwalled hose feeding the freshwater system that cools the heat exchanger. Could running the engine too hot have screwed something up?

I checked, and I have plenty of transmission fluid. What am I missing?

 

[FireIsland1]

join the crowd. there are pleanty of threads about trans knocks. if it goes away at off idle rpm, then forget about it . mine's been llike that for 6 years no change for the worse

 

[Air O'Nautical]

Good news!

It is most likely an ignition problem.
Do a standard tune-up and it will be fine.
(plugs, wires, cap+rotor ect.)

Many of us have had this problem with several different boats.
I know it sounds soo.. much like the tranny is the problem...its not.
Searching this site will find many threads of the problem.

Happy days,
Mark.

 

[BamaFrazier]

Thanks, guys. That's what I wanted to hear. I'll probably go ahead and do the plugs, wires, etc., anyway. But, it's good to know that it's not a major problem that should keep me in the dock.

Cheers!

 

[carterchapman]

+1 to the ignition advice Gary.

I had the exact issue on my port engine/tranny last fall. New plugs and it's gone...

 

[Redhook98]

Yep, go through your ignition.... When your idle RPM is low (i.e. 550-700) and you have any issue at all with your ignition system, you will feel it (and hear it) in your tranny. The reason is simple. Without a perfect ignition system firing like it should, each revolution is not perfect. The sound you hear is the backlash in the damper plate and gears of the transmission. Does it do any harm? A little, over time..... The two fixes are to either make sure your ignition system is 100% (or close), or bump your idle speed to 800. I make sure my igniton is the best it can be, and I bumped my idle to 750-800.

 

[sporters78]

Also check the coils

 

[Redhook98]

Also check the coils

:thumbsup: +1 I had one in my AJ. Who would have guessed a newer boat like that with low hours would have a coil going bad? Worth checking out!

 

[Carver370]

What transmissions do you have? There are a few that notoriously knock when put in gear at low rpm's. Its the nature of the beast.

 

[390x]

I've never seen a gas/twin screw boat with inboards that didn't have at least one knocker out of the pair. A lot of times one engine is just a hair faster than the other and it "drags" the slow motor causing the knock (backlash). If you're feeling really ambititious, you can rip the tranny apart and shim everything tighter and it'll probably go away.....assuming that it isn't being caused by your engine being out of tune.

Assuming everything is in tune, turn the idle up just a little bit and pretend you never heard it.

 

[Hans]

Port engine (Mercruiser 454) purrs like a kitten when not in gear. But, when the transmission is engaged it knocks like crazy at idle speed. Seems fine at higher velocity, though.

At first I thought maybe this darn E10 gas had fauled my carburator. I have a 1990 Sea Ray Sundancer 350, so have the plastic gas tanks. But, surely it's a transmission problem, right? I mean, it only knocks when the transmission is enganged.

I had had a problem with the engine running hot for months until I finally found a soft spot in the hardwalled hose feeding the freshwater system that cools the heat exchanger. Could running the engine too hot have screwed something up?

I checked, and I have plenty of transmission fluid. What am I missing?

Not sure if it's the same knocking but read this very long thread. Still dealing with it. http://clubsearay.com/forum/showthread.php?t=33567&highlight=hans

 

[BamaFrazier]

Guys,

Just wanted to say a hearty "THANK YOU" to all of you for the quick (and very helpful) advice. I replaced all the plugs in the port engine, and she is purring like a kitten now. Knocks are completely gone. Three cheers for Club Sea Ray! I toasted you guys with a few cold brews this weekend. ;-)

Cheers,

GF