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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Diesel This is a forum dedicated to diesel engines and their applicable accessories.


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Old 07-20-2014
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Universal M25 Knock?

OK Diesel engine gurus, I have a puzzle...

I have a 1987 Universal M25 with unknown hours, but the hour meter was installed about 395 hours ago. When I bought the boat, four years ago, it showed 175 hrs. The engine has a tendency to drop oil pressure when warm (drops from 45 to 10 PSI after several hours of running), but never overheats, and oil pressure never goes low enough to set off the alarm.

I recently installed three new injectors, and all seemed well until yesterday.

Yesterday, there was little wind on Narragansett Bay, so we motored for about 4 hours down the West Passage to Wickford. We had a nice lunch, then motored out, and raised the sails, as the wind had piped up... After making our way back up the East Passage, and passing Bristol Harbor inlet, the wind died again, so I fired up the engine again.

We ran the engine for about an hour at 2700 RPM, when suddenly it started knocking. I quickly throttled down, and the knock changed frequency. All the gauges looked normal (Oil @20 PSI, Water temp @180, Voltage @14+) I popped it into neutral, and the knock persisted. I looked over the transom, and noticed smoke (white - possibly grey) out the exhaust. I went below, and opened the engine compartment, but could not locate the source of the knock, so I shut it down.

I checked the oil - warm but the level was OK, looked for leaks - none, looked at the coolant overflow - OK. There was some oil that had escaped from the breather tube, but not a lot, and it really isn't that unusual. Because I could find nothing amiss, I started the engine again, and the knock was gone! We safely motored back to the pumpout, and then the slip (about another hour) without a problem.

What happened? What should I look out for?
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Old 07-20-2014
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Re: Universal M25 Knock?

Get a copy of your engines Repair and Maintenance Manual (aka - "Shop Manual') and verify your injection pump timing.

Grey 'smoke' coming out the exhaust is usually a sign of either 'over-fueling' / unburnt fuel and mostly is caused by incorrect engine timing; but, can be caused by low compression.

If the timing is found to be correct, then look for broken / sticking piston rings or an exhaust valve that is cracked or not making a complete seal with its valve seat ... or is simply 'hanging up' (not making its full 'stroke' ... such can be caused by a pin hole water leak inside the exhaust manifold, etc. 'back draining' into a combustion chamber when the engine is shut down), etc. etc.
Then I would suggest that you do an 'air pressure leak down test' of the combustion chambers (instead of or in addition to a compression test). With the injectors removed, High pressure air is put into each combustion chamber with each piston in turn at bottom dead center, and you look for 'where' the air is leaking out of: - crankcase breather (broken/sticking piston rings), exhaust system (ex. valve problem), air intake ( intake valve problem), into the cooling water (blown head gasket), out of an adjacent combustion chamber (blown head gasket), etc. etc.
Verify your injection timing FIRST, as on most small diesel engines the injection timing should be checked about every 500-750 engine hours as a routine 'maintenance item'.

hope this helps.

Last edited by RichH; 07-20-2014 at 10:42 AM.
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Re: Universal M25 Knock?

If the engine ran OK after you put the new injectors in them & the knock just suddenly appeared afterwards, it is possible something fouled an injector tip, effecting the spray pattern? The fact that the knock just "went away" may indicate a temporary problem with the fuel delivery system?

As Rich mentioned timing is critical and should be checked. You might want to pull the injectors and have them checked by a shop. I have read that water reaching the injector tips can cause damage to them. Perhaps check your water separator for any water accumulation?

I have heard diesels that knock somewhat when cold but as they warm up the knock subsides, but that is different than your description. Let us know what you find.

Paul T
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Old 07-20-2014
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Re: Universal M25 Knock?

If your oil pressure gauge is reading correctly you should have a minimum of 10 psi per 1000 rpm at operating temperature.
If your oil pressure has changed at low rpm and you have an audible knocking noise you may have spun a big end bearing.
If you are able to remove the oil pan do so and look for pieces of bearing.
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Old 07-20-2014
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Re: Universal M25 Knock?

E, isn't 2700 rpm too high for that engine?
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Re: Universal M25 Knock?

With those symptoms and oil press. I'd bet it's time to lift her up and re the bearings before it does in the crank journals.
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Re: Universal M25 Knock?

Two kinds of knock, ignition & bearing. Ignition, kind of like a ping in a gas engine, short & sharp. Bearing, kind of low frequency, longer duration, awful sound. I think if it was a bearing it would be constant & not "go away"? Suggest you try the easiest things first. I don't know the specs for that engine, but 10 lbs at operating speed seems a bit on the low side?

Paul T

Last edited by dabnis; 07-20-2014 at 02:44 PM.
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Re: Universal M25 Knock?

Quote:
Originally Posted by RichH View Post
Get a copy of your engines Repair and Maintenance Manual (aka - "Shop Manual') and verify your injection pump timing.

Grey 'smoke' coming out the exhaust is usually a sign of either 'over-fueling' / unburnt fuel and mostly is caused by incorrect engine timing; but, can be caused by low compression.

If the timing is found to be correct, then look for broken / sticking piston rings or an exhaust valve that is cracked or not making a complete seal with its valve seat ... or is simply 'hanging up' (not making its full 'stroke' ... such can be caused by a pin hole water leak inside the exhaust manifold, etc. 'back draining' into a combustion chamber when the engine is shut down), etc. etc.
Then I would suggest that you do an 'air pressure leak down test' of the combustion chambers (instead of or in addition to a compression test). With the injectors removed, High pressure air is put into each combustion chamber with each piston in turn at bottom dead center, and you look for 'where' the air is leaking out of: - crankcase breather (broken/sticking piston rings), exhaust system (ex. valve problem), air intake ( intake valve problem), into the cooling water (blown head gasket), out of an adjacent combustion chamber (blown head gasket), etc. etc.
Verify your injection timing FIRST, as on most small diesel engines the injection timing should be checked about every 500-750 engine hours as a routine 'maintenance item'.

hope this helps.
Got the manual, and know how to adjust the valves, but the timing marks are IMPOSSIBLE to check while the engine is in situ, unless I use an inspection camera... and I don't have one.

This also would not fit with the fact that it seems to have, at least temporarily, fixed itself...

Quote:
E, isn't 2700 rpm too high for that engine?
Naw, the M25 rev range is from 1000 to 3200RPM. (my idle is around 800RPM).

But the point is valid, that I was pushing the motor shortly before this symptom manifested itself.

Quote:
Two kinds of knock, ignition & bearing. Ignition, kind of like a ping in a gas engine, short & sharp. Bearing, kind of low frequency, longer duration, awful sound. I think if it was a bearing it would be constant & not "go away"? Suggest you try the easiest things first. I don't know the specs for that engine, but 10 lbs at operating speed seems a bit on the low side?
This is a diesel, so I think that I can rule out ignition problems. I agree that a bearing would not fix itself.

I think that the end of the season may be time to finally pull the motor (not a job that I look forward to). I also have a PSS shaft seal, which means that the boat will have to be hauled.

Yes the decreasing oil pressure has been a long running mystery. It starts out at 50 PSI, the gradually drops, but never so low as to sound the alarm. It has done this for the past 2 years, possibly longer.

My suspicion is that I may have an exhaust valve which either stuck open, or was opening prematurely (dirt?)...
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Re: Universal M25 Knock?

E, I think there's an issue with oil filters. something about a ck valve in the filter can? Not sure what it was. someone change oil.. had no pressure because of the wrong filter.
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Old 07-21-2014
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Re: Universal M25 Knock?

Good guess Denise,

Been there, done that three years ago, when I first noticed the oil pressure strangeness.

The check valve on the M25 is above the oil filter on the starboard side. IIRC, it is a 19MM hex head, which you can back out to clean the passage, and the ball bearing.
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