Erratic diesel rpm - jump start on my search please? - SailNet Community
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post #1 of 7 Old 06-23-2016 Thread Starter
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Erratic diesel rpm - jump start on my search please?

friend of mine has a 1980's 1GM diesel that is well cared for and in good shape. he tells me that he had run boat under power for around an hour, at around 26-2800 RPM, with no signs of issues. At the end of that period, the engine RPM started to fluctuate up and down by around 200 RPM but otherwise seemed to be running fine. He has not pulled filters or done any other diagnostics (not particularly mechanical). Can anyone identify common causes for this erratic running and useful search terms for this board? slight air leak is my first thought. The boat has had just nominal use since launch a couple of weeks ago. Thanks for all help, Mike

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post #2 of 7 Old 06-23-2016
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Re: Erratic diesel rpm - jump start on my search please?

Filters first, noting the contents of the bowl for water as you should always do. Bleed well so start up is not an issue. Air leak maybe but not until establishing clean filters.
Get to know the motor, mine has a fine screen on top of the fuel pump I check It on most filter changes it could be a problem if never cleaned.
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post #3 of 7 Old 06-23-2016
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Re: Erratic diesel rpm - jump start on my search please?

Considerations

1. Check rpm with a 'real' tachometer, preferably a hand held laser device and a 'mark' on the front pulley sheave. Reason: tachometers on diesels are electric/electronic devices, and they and their wiring/connectors are subject to corrosion. That corrosion can variably change the voltage, etc. as the engine heats up and which 'dries out' the corrosion yielding (usually) less 'conductivity'.

2. Alternator variable loads .... leads to variable engine horsepower output.
If the alternator is fully performing its 'output' ... more HP required. If the (smart) regulator has various stages (bulk, acceptance, float voltages) for battery charging, expect engine rpm to be 'changeable' during the electronic control changes. An 'unstable' (beginning to fail) alternator regulator can also contribute to variable engine loads; also, changes in amps draw for onboard equipment such as refrigerator, lighting, electric head, electric autopilot, etc. etc.

3. Intermittant Fuel starvation due to:
Filter is periodically 'unloading' debris; Intermittent air leak due to loose or incorrectly chosen 'couplers', loose hose clamps, pin holes in tubing, etc. etc. And ....small bacterial plugs in the tubing that compress during high flow requirements and which then relax (less compression) during lower (rpm) demand. The relaxation of 'the strings' at lower fuel demand/rpm allows greater flow with less 'back pressure'.
A partly clogged tank vent can act in the same manner.

3. presence of water in the fuel .... small 'slugs' of water collecting (usually) in the bottom of the tubing, hosing, etc. being periodically 'entrained' into the fuel.

4. worn-out or 'sticking' engine 'governor'
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Last edited by RichH; 06-23-2016 at 03:56 PM.
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post #4 of 7 Old 06-23-2016
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Re: Erratic diesel rpm - jump start on my search please?

My way of clicking almost everything fuel related with my little diesels is to "simply" disconnect the fuel return line at the tank, or some convenient place, put it into a clear clean glass jar. Start the engine, high RPM in gear if you like. If it runs clear and clean then just about all the common problems are eliminated.

If the flow is low there is a clog. You'll simply need to know what the flow normally looks like. Mine looks like what might come out of a soda straw if you are bored at a bar.
If there a bubbles, even just one, you have an air leak.
If there is water or crud find a new hobby not involving things with moving parts.
If there is no fuel flow....well you either have much more work to do or simply need to buy some fuel.

None of those thing? You can probably move on the non-fuel problems. However with small diesels it is almost always a fuel issue.

If there has ever been water thru the injector pump it is probably ruined. Likewise if there has been water in the engine oil...the mayonnaise problem...the injector pump is ruined as it is by far the most rust sensitive part of a diesel. It is built like a pocket watch inside.
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post #5 of 7 Old 06-23-2016
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Re: Erratic diesel rpm - jump start on my search please?

Your friend's 30 year old boat not well cared for, best advice is never free basic maintenance has to be done by the owner or some who knows how.

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post #6 of 7 Old 06-24-2016 Thread Starter
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Re: Erratic diesel rpm - jump start on my search please?

Thanks for the specific suggestions. Some of those I hadn't initially thought of, like the blocked fuel tank vent, and sticking governor. Most likely it will be something pretty minor, but with older boats you never quite know. Mike

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Re: Erratic diesel rpm - jump start on my search please?

Quote:
Originally Posted by aloof View Post
My way of clicking almost everything fuel related with my little diesels is to "simply" disconnect the fuel return line at the tank, or some convenient place, put it into a clear clean glass jar. Start the engine, high RPM in gear if you like. If it runs clear and clean then just about all the common problems are eliminated.

If the flow is low there is a clog. You'll simply need to know what the flow normally looks like. Mine looks like what might come out of a soda straw if you are bored at a bar.
If there a bubbles, even just one, you have an air leak.
If there is water or crud find a new hobby not involving things with moving parts.
If there is no fuel flow....well you either have much more work to do or simply need to buy some fuel.

None of those thing? You can probably move on the non-fuel problems. However with small diesels it is almost always a fuel issue.

If there has ever been water thru the injector pump it is probably ruined. Likewise if there has been water in the engine oil...the mayonnaise problem...the injector pump is ruined as it is by far the most rust sensitive part of a diesel. It is built like a pocket watch inside.
Good post by aloof !!!!!!

A good (and continual) way to do this and without having to 'guess': is to place vacuum gages on 'downstream side' of every damn 'filter' and then monitor them on a routine basis ..... and then when 'crap' happens simply check and compare against your written log of indicated pressures with the current reading.

A drop in vacuum from 'normal' indicates an air leak, or a filter that is 'bypassing', or has a broken 'pleat' in its filter media, empty tank, etc.
An increase in vacuum indicates a filter that is 'plugging' with debris
An increase in vacuum across all filters indicates a plugged dip tube or blocked tank vent; OR, that the 'first' filter downstream from the tank in the series is blocking. etc. etc. etc. etc.

How to set up the parameters to know when to change filters .....
With 'brand new' filters, run the engine at near wide open throttle and full load .... and very S-L-O-W-L-Y close the tank valve until the engine begins to 'stumble' and lose rpm; then quickly read EACH of the gages for that maximum vacuum value. This will indicate the maximum VACUUM possible in the system before engine 'shut down'.
Take those values and multiply by .9 (or .8) .... that will establish 'new' maximum 'operational' vacuum gage values for Wide Open Throttle Operation ... the vacuum on any gage in the fuel system that indicates that one or ALL the filters HAVE to be changed AASAP (as you only have about 10% (or 20%) of filter 'dirt capacity' remaining before complete engine 'shut down').
With gages installed, you don't have to agonize over 'what the hell is going wrong' and can quickly locate, identify, and then remedy 'the problem'.

;-)
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