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Old 13-09-2012, 19:48   #1
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Low compression light on yanmar

The low compression light recently comes on whilst my yanmar 3gm30f idles.

There was recently a crack in a fuel hose. Now that I've bled the fuel line I think that the light doesn't come on any more.

Or should I be concerned about something more serious?

Thanks!
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Old 13-09-2012, 19:57   #2
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Low compression? Maybe low oil pressure?
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Old 13-09-2012, 21:39   #3
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Re: Low compression light on yanmar

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Low compression? Maybe low oil pressure?
Never heard of "low compression" light either -- sure you got that right?
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Old 14-09-2012, 02:10   #4
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Oil pressure or rate of charge is low. Before you start engine, with engine on, not cranking, not yet started, do you get a warning buzzer?
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Old 14-09-2012, 05:39   #5
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Ok, now I feel like a fool! Now that I look it does say low pressure as in oil!

The oil was a bit low but not below the minimum mark.

The light only came on when idling the engine after running it at high revs for a while. Never when it first started.

is it just the low oil or is there something sinister?

Sorry again!
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Old 14-09-2012, 06:35   #6
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How old was the oil and how long we're you running hard?
I'll guess that the really hot oil became too thin to maintain pressure at low revs until it cooled down a bit. The lesson here is to reduce speed slowly enough to allow the oil pressure to regain an equilibrium (temp and pressure)
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Old 14-09-2012, 06:37   #7
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Top off oil. If oil is black and not clear, change it (unless it is Royal Purple or a speciality oil that is not clear in the first place).

You may need to slightly increase idle speed.

Apologies don't cut it around here, to all that responded to your call for help, send us plane tickets and we will go sailing!
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Old 14-09-2012, 06:54   #8
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Re: Low compression light on yanmar

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Top off oil. If oil is black and not clear, change it (unless it is Royal Purple or a speciality oil that is not clear in the first place).
I'd be changing oil at every shut down if I followed this!!!
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Old 14-09-2012, 08:59   #9
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Re: Low compression light on yanmar

Check to make sure you are using the correct viscosity oil.
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Old 14-09-2012, 15:09   #10
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Re: Low compression light on yanmar

Quote:
Originally Posted by GaryMayo View Post
Top off oil. If oil is black and not clear, change it (unless it is Royal Purple or a speciality oil that is not clear in the first place).

You may need to slightly increase idle speed.

Apologies don't cut it around here, to all that responded to your call for help, send us plane tickets and we will go sailing!
To clarify what Sailmonkey said, black is the normal colour for oil in a diesel engine once it has a couple of hours usage. It is NOT an indication that it needs changing.

And I agree that y our symptoms show that when the oil is good and hot its viscosity is not sufficient to maintain pressure at idle. That could indeed be due to some dilution of older oil, or minor wear in bearings, or wear in the oil pump itself. IMO, not something to worry about very much, but an oil change might be in order just as a diagnostic tool.

Cheers,

Jim
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Old 14-09-2012, 15:47   #11
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Re: Low compression light on yanmar

Its good practice to keep the oil near the top "Full" mark too, not just above the minimum mark. More oil means there is more fluid to cool the return oil too..

Even blonde sailorchic's keep the oil up to the top mark.. topped mine up today...
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Old 14-09-2012, 16:53   #12
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Re: Low compression light on yanmar

It could always be the sending unit failing too.
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Old 14-09-2012, 18:32   #13
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Re: Low compression light on yanmar

The problem could be the sending unit. It could also be fuel dilution of the lube oil. Or it could be just what happens after a heavy load when you back off to idle.

If you want to track it down, hook up a mechanical pressure gauge so you can see exactly what is happening. 10 psi at hot idle would be normal. Also an oil analysis will rule out fuel dilution.

David
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Old 15-09-2012, 01:06   #14
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Re: Low compression light on yanmar

Low oil level(within limits )woud not cause this. Check you raw water filter to see if you are not getting enough cooling water thru your system. Check the output water in your exhaust to see if it looks normal. If the water flow doesnt look correct see if your raw water pump has shed some blades that are plugging your oil cooler. If everything looks normal then buy a mechanical oil pressure gauge and see if you really have a problem or just a bad gauge sender. As has been said before, use the correct oil and filter to make sure you didnt make a simple mistake.____Grant.
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Old 15-09-2012, 02:21   #15
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Re: Low compression light on yanmar

I had the same thing a couple weeks ago. The buzzer would try to come on at an idle, but when I rev'd it the buzzer would stop. Then a couple days later it wouldn't quit. I broke out the schematic and trouble shoot the sender and gauge. At first I thought it was a bad diode. It turned out to be a short. The wire harness was rubbing a sharp corner by the shifter.

Wrapped it with electrical tape and tie-wrapped it away from the sharp edge. All is well and lesson learned.
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