Common Rail and No-load idle?

[jetlag]

I have an '07 44DB with 500 QSC's. When I run up the engines with the transmissions in neutral they start shaking at about 1500 rpm. It sounds like a misfire without the noise. They run great under load and make about 2650 rpm on the WOT. I wanted to check out the top speed governor but there is no way I'll bring either of them up over 1700 rpm as it actually gets violent. I feel it on the bridge and can't imagine what it must look like on the motor mounts below. Is this due to a light weight flywheel or something. I used to check out my Cummins 370's like this time with no issues. Comments, suggestions? Thanks in advance.

Jet Lag

 

[bmac]

When I was winterizing this year I ran the engines up in RPM's with the transmissions in neutral but I didn't have that experience. I think I ran them up to about 1800 RPM and took a look down below and everything was relatively smooth. I'm sure one of the resident diesle experts will give you some ideas.

 

[tobnpr]

Both engines? Same symptoms?

 

[jetlag]

both engines, identical. I read somewhere a long time ago that this could happen under a no-load situation but I can't confirm that now. If anybody had the QSC500's I would appreciate hearing what their engines do. Thanks and Merry Christmas

 

[jetlag]

Hey Festivus,
they are smooth up to around 1600 and below. Did you ever get my response regarding the engine zincs? Not that I was much help...but I did try. Thanks again, Smez

 

[Robski97]

I had a bad injector on one motor. During the " Test " process they ran the motors very high in the rpm band..... Noisey yes, lots of shake YES.... normal?? dont know but ive heard them called king Kongs vibrator... Both motors ran the same under the high RPM test they did. The injector even passed there test...

Rob

 

[bmac]

Frank

First, this isn't Festivus (same boat though). Yes I got your response on the zincs, nope, not much help but thanks anyway. See my response above on my experience with running the engines (QSC 500's) up in neutral. I only did this to warm the ngines up faster prior to changing the oil and winterizing the cooling system. I'm pretty sure I let them both sit at about 1800 rpm for a while and I did run down below to check the engine room. I didn't observe any shaking or backfiring...and I'm not sure what would cause that to happen if it doesn't with the boat in gear:huh:. Sorry, I know this doesn't help, if you're lucky one of the resident diesel experts will weigh in here.

Hey Festivus,
they are smooth up to around 1600 and below. Did you ever get my response regarding the engine zincs? Not that I was much help...but I did try. Thanks again, Smez

 

[Carver370]

I don't know if those boats have underwater exhaust but bringing the rpm up in neutral on a 460 that I run shakes the boat pretty bad, but I can tell its not coming from the engines.

 

[bmac]

They do have underwater exhausts but they are fairly far astern of the engines. You can definitely feel the rumble as engine rpm increases while sitting at the dock in neutral.

 

[fwebster]

Part of the flywheel or swinging weight is in the transmission. The transmission in gear adds the additional swinging weight to the rotating assy for the engine. This is particularly true with in-line engines.....not so much so with opposed or v-8's. Without the transmission load and weight on the engine it is pretty much like running your lawnmower without a blade on it..................its gonna shake.

This one goes in the category of "don't fix what ain't broke".

 

[jetlag]

Thanks to all and BMAC(Brian) that was a mental slip up. As I was typing I recalled the post from Festivus. Sorry for the mix-up but I think he's pretty good company
Also to everyone else, what I'm describing or attempting to, is an intermittent shake...not a rumble or constant...it's more like a cutout every second or two with smooth running in between. I think Frank (hope it was Frank) hit the nail on the head. May be the lessened flywheel weight.

Thanks to all and Merry Christmas/Happy New Year. Above all, safe and fun boating,

FSmez
Jet Lag
44DB

 

[jetlag]

Thanks to all for the help. Brian, sorry about the Festivus. That name was on my mind from a previous post. Not a bad one to be confused with though! Also, my exhaust are not the issue. This is very clearly an engine running smooth with an erratic cut-out every second to 3 seconds or so. I think Frank is spot on with the flywheel angle.

Thanks again,

Jet Lag

 

[jitts3]

Is this the first time attempting this test with this boat? It would not be a mechanical failure due to two facts. Bolt engines would not have the identical failure at the same time. Also the engines achieve more RPM at WOT under a load. Now more parts are spinning. (Engine and trans) The engines are fine.
I am not farmilliar with marine diesels, but very knowledgeable with on highway diesels. There may be a computer controled rev limiter that comes on in neutral?
Are your engines Mechaical or ECM (Computer) Controled?

 

[jetlag]

Actually, I've done this several times with these engines. Same results so it is not a new thing or a failure. Also, the engines are governed at 2650 or so and I never got them over 1800. I know that there is nothing failed or broken, I was just looking for someone that could tell me that they have witnessed the same behavior on theirs. They are Cummins QSC500's, common rail engines.

Thanks for the input,
Jet Lag

 

[sbmar.com]

Jetlag,

Not being there and "seeing" what is actually happening makes this a tough call from far away.. But if I were to guess, I'd say it's related to the underwater exhaust design when sitting at the dock (not moving forward)..

Next time you are out, try traveling at, say 9K's, and then pull one engine out of gear. Adjust the steering and the other engine and try and maintain an even 7-9K's.. Weather needs to be nice so all is safe and you have 100% control of all around you... Once you are comfortable, try WOT no load and see what happens..

Just a guess, but I'll bet your issue goes away..


Tony