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-   -   HIgh RPM in neutral (http://www.sailnet.com/forums/pacific-seacraft/29072-high-rpm-neutral.html)

Philip Littman 02-12-2007 10:06 PM

HIgh RPM in neutral
 
I have a PSC 34, l998 with Yanmar 3jh2 engine. At full throttle in neutral the Rpms are 4000. This has been checked with a another meter and the reading is accurate. The book says the maximum rpms at the flywheel should be
3600. The mechanics are not sure what is going on. The engine seems to run fine otherwise. What is going on?
Phil Littman PSC34 #317

camaraderie 02-12-2007 10:09 PM

Has it been checked with a laser counter gun or just a meter?

hellosailor 02-12-2007 10:12 PM

Fire the mechanic and, as Cam suggests, make sure an actual tachometer is used. Diesel engines can run away if they suck in too much fuel oil past bad rings and other problems, somehow either a governor is out of whack, or there is too much fuel and air getting into the engine.

A mechanic shouldn't be baffled by that. the guy pretending to be a mechanic, owes you a refund and a beer.

gc1111 02-13-2007 09:24 AM

First make sure your RPM readings are correct. With diesels, unless you have a mechanical takeoff, there can be lots of errors. Most tachs operate by measuring the frequency of the AC generated in the alternator. This requires calibration by an external reference. Best option is an optical tach. These operate by putting a reflective patch on the flywheel, shining a light on it and counting the returning light pulses. I got one off EBay for about $40.

If your RPM measurements are correct, fire the mechanic. You most likely have a problem with the govenor.

RickBowman 02-13-2007 10:15 AM

3JH2 Tachometer
 
Quote:

Originally Posted by gc1111
First make sure your RPM readings are correct. With diesels, unless you have a mechanical takeoff, there can be lots of errors. Most tachs operate by measuring the frequency of the AC generated in the alternator. This requires calibration by an external reference. Best option is an optical tach. These operate by putting a reflective patch on the flywheel, shining a light on it and counting the returning light pulses. I got one off EBay for about $40.

If your RPM measurements are correct, fire the mechanic. You most likely have a problem with the govenor.


The 3JH2 Yanmar tachometer uses a magentic pickup sender that converts the rotary motion into an electrical signal by counting the number of teeth (127) of the ring gear that is connected to the flywheel. PP 9-42 Yanmar Service Manual.

hellosailor 02-13-2007 01:44 PM

Rick, it could be that the boat was permanently deformed by sailing in the Bermuda triangle. A simple residual 10% time/space compression distortion would leave the boat, um, 30.6 feet long now, with the engine operating 10% too fast simply because the ring gear had now been permanently reduced in size by 10%.

That kinda stuff happens all the time, right? <VBG>

So, using a new tape measure that's never been on the boat before, first we measure the boat...<G>....

Edwright105 02-13-2007 05:22 PM

Ok, I am a marine engineer so I thought I would weigh in... First off I don't really think that running your engine at wide open throttle in neutral is a very good idea.. Diesel engines do not respond very well to this kind of treatment. Diesel engines run best under loaded conditions (rated full load is optimum but not realistic that you will run at that load all the time). Likely the rpm of 3600 that you are reading about is under loaded conditions... if not you could have a slight problem with your governor, a difference of 400 rpm is however not anything you need to worry to much about. As for having bad rings, and blow by I don't think that is very likely because once a diesel starts firing on an external fuel source ie. lube oil blowing by the piston rings... which is very unlikely, the engine will usually run away and you will have a very big problem on your hands (a catastrophic failure) In any case a difference of 400 rpm's is not a huge deal, don't sweat it
take care

RickBowman 02-13-2007 06:58 PM

Loaded RPM
 
Quote:

Originally Posted by Edwright105
Ok, I am a marine engineer so I thought I would weigh in... First off I don't really think that running your engine at wide open throttle in neutral is a very good idea.. Diesel engines do not respond very well to this kind of treatment. Diesel engines run best under loaded conditions (rated full load is optimum but not realistic that you will run at that load all the time). Likely the rpm of 3600 that you are reading about is under loaded conditions... if not you could have a slight problem with your governor, a difference of 400 rpm is however not anything you need to worry to much about. As for having bad rings, and blow by I don't think that is very likely because once a diesel starts firing on an external fuel source ie. lube oil blowing by the piston rings... which is very unlikely, the engine will usually run away and you will have a very big problem on your hands (a catastrophic failure) In any case a difference of 400 rpm's is not a huge deal, don't sweat it
take care


Ed,
I picture this engine being run at open throttle as only a test, and it was discovered that it was actually able to reach 4000 RPM by using two different tachometers. But just suppose that the tachometer sensor is faulty? Cam was right in suggesting eliminating a possible error with the tachometer setup and using an independant electronic rpm tester. The 3600 RPM is a one hour rated output at the flywheel. The continuous rated output for the 3JH2 is 3400 RPM at the flywheel. If I remember correctly on this particular diesel the adjustment for the high speed setting at the govener is wired off by the factory to proper specs. The low speed idle adjustment was left as end user adjustable. So, if 3400 RPM is the factory continuous rated output, and this engine is purported to be able to run at 4000 RPM wouldn't that make it as 17% above redline condition in continuous operation mode?

RickBowman 02-13-2007 07:11 PM

HelloSailor
 
Quote:

Originally Posted by hellosailor
Rick, it could be that the boat was permanently deformed by sailing in the Bermuda triangle. A simple residual 10% time/space compression distortion would leave the boat, um, 30.6 feet long now, with the engine operating 10% too fast simply because the ring gear had now been permanently reduced in size by 10%.

That kinda stuff happens all the time, right? <VBG>

So, using a new tape measure that's never been on the boat before, first we measure the boat...<G>....

Is that possible deformation anything to do with the time/space/continum? Have we watched way to much star track or played space invaders to often or what? :D

Edwright105 02-13-2007 07:25 PM

Your right Rick, if 3400 RPM is rated then yeah that is high, I am still wondering what it will rev up to when loaded? I would be willing to bet that this problem lies with a misadjusted governor, if it's an electronic governor, I would be questioning the rpm signal going to the governor... you may have a faulty sensor there. Unfortunately one of the only ways to diagnose an electronic problem is to plug it into a computer which you will be charged for, the computer will then give the technician an error code and tell them exactly what's wrong.


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