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Old 24-08-2009, 23:05   #1
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Low Oil Pressure Alarm

My alarm has gone off twice now but I'm still not sure why.

1st time was when I just got the boat and I had started the engine about 3-4 different times and ran it for a short time at low rpm before shutting it down, all because of variable winds around some islands and a tide deadline. I checked the oil and it was low, so i added about 1L, started it up and after about 2 mins (I know i shouldn't run it for 2 mins w/ the alarm on) the alarm turned off. Check the oil again and it was overfilled, about 1L over.

Yesterday (3 months, 60hrs later) with the same kind of start stop senario the alarm turned on again. I had just changed the oil with Castrol GTX 15-40 about 10 hours prior, but I changed it today anyways before start up and everything is good again, no alarm.

Each time the alarm comes on when I'm re-starting and just never turns off however; after shut down and restart it still comes on. Also, each time i had been running approx 1200 rpm.

Does anyone know what might be causing the alarm to trigger?

Its a 1979 Volvo MD11C, it also emits white smoke, not sure if there's any relation. Perhaps low compression? Worn crankshaft bearings, and low rpm?

Any suggestions would be great, I'm really trying to get another year out of her.
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Old 25-08-2009, 12:33   #2
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What does the sensor do? Triggered by pressure Low / High / by the Temp ?

We have an MD7, a similar engine just smaller. Our alarm is triggered by too low pressure. If yours is similar (look up the manual) then the alarm buzzing the first time was OK - you had too little oil (how it came back and was 1 L too much later I will be following this thread to know). Do you have a spare sensor? If you do - try it - sometimes they go nuts (ours did - but it rather became completely silent, which does not mean yours will behave the same).

Make sure the grade of the oil is right, the oil is for a Diesel engine and that it is exact level. And make sure the filter is OK and permits flow (note that some aftermarket filters are not exactly Volvo specification - there are filters with a cut-in valve that will open/stop the flow at pre-set psi and if this valve is not exactly Volvo specs the engine will buzz the alarm).

You will get not just 1 year from this engine, if you make no harm - the old Volvo are good beasts!

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Old 25-08-2009, 14:04   #3
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It’s an alarm for low oil pressure, and the filter was a Volvo filter. I'm starting to think the worst about this engine, it will probably need a total rebuild or re-power.

There's blow-by occurring for sure, as its noticeable in the engine room. Also, it takes several cranks for the engine to start and I think this might be because of lack of compression. It takes several cranks for the cylinders to build up enough compression to start, I've read that this will also lead to low oil pressure at low rpm's.

I wonder if I run my engine at higher rpm's if I can get by for a little longer; you know delay the inevitable.
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Old 25-08-2009, 14:52   #4
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There are a few reasons why oil pressure can drop.

As oil gets older it gets thinner.
As it heats up, the same thing happens.
As an engine gets older the internal clearances increase causing oil to flow faster thus dropping the pressure.
The oil pump could be creating less pressure as it ages.

For a yacht diesel, thirty years is getting old. They typically corrode out before they wear out.

An oil analysis will tell you how close you are to needing to replace the engine. It is possible that you are already there.
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Old 25-08-2009, 18:36   #5
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Diesels hate slow running except just after start before the oil pressure builds up. A happy diesel is worked regularly and pretty hard (not flat out though), or else they better be 'winterized' - water replaced with anti-freeze (which doubles as anti-rust), air intake blocked, oil and oil filter replaced.

Have seen more diesel problems from unuse than from abuse.

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Old 25-08-2009, 20:22   #6
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I would suspect

I would be suspecting the sending unit. They can and do go bad. If you can, put a real guage on there, so you know what is really going on. Most of the sending units are set to go off at such low oil pressure they are more than likely usless anyway.
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Old 25-08-2009, 22:32   #7
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My guess is that the problem is in the electrical circuit and not the engine itself. Quite possibly the oil pressure sending switch, the wire leading to it or a grounding issue somewhere. To activate a low oil pressure alarms due to low oil level you'd have to be almost completely out of oil - a quart or two low won't do it unless that's more than 75% of the sump's capacity.

High oil pressure is over-rated. Diesel engines can run thousands of hours with relatively low oil pressure. I've seen over-the-road truck-tractors with tens of thousands of hours routinely run at 30 psi - about half their specified pressure.

With the advances in bearing metallurgy, huge improvements in lubricating oil technology and immensely effective filters, failures of conventional reciprocating engine's lower ends are rare. By lower ends, I mean main and connecting rod bearings, cylinder walls and pistons, camshafts, etc. Add to that the extremely mild conditions most recreational marine diesel engines operate under (no dust; hardly ever at full throttle, full load; constant RPMs and owners fretting over them constantly - can you imagine how long your car would last if you just put it in gear at one speed? Only occasionally accelerating/decelerating? No long idle periods, stuck in heavy traffic? Your car would last forever!). The hardest thing on a marine diesel engine is the environment under which it operates - high humidity, with the corrosive salt air. Engineers figured out how to make diesel engines reliable 60 years ago. The differences between engines now and then are negligible.

Unlikely you actually have a low oil pressure problem.
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Old 26-08-2009, 04:19   #8
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Quote:
Originally Posted by nhschneider View Post
My guess is that the problem is in the electrical circuit and not the engine itself. Quite possibly the oil pressure sending switch, the wire leading to it or a grounding issue somewhere...
Excerpted from:
Testing Engine Gauges & Senders:
Basic Engine Gauge Theory and Testing

Oil Pressure Gauges:

1. Disconnect Dark Blue Sender Wire from Gauge, and turn power ON.
The Pressure Gauge should read ZERO Scale (Low Pressure).

2. Short the Gauge Sender Terminal to Ground.
The Gauge should deflect to FULL Scale (>High Pressure).

3. Measure Resistance from Dark Blue Sender Wire to Ground.
American Press. Sender: 240 Ohms (Zero Pressure) 33.5 Ohms (Full Pressure @ 80 psi)
European Press. Sender: 10 Ohms (Zero Press.) & 180 Ohms (Full Press. @ 150 Bar)

4. Disconnect Dk Blue Wire from Sender on Engine Block.

5. Test continuity of Dk Blue Wire.
Zero Ohms from Block End to Gauge End - If not, repair or replace Wire.

6. Measure Resistance from Sender Terminal (on Block) to Ground on Block.
Should read as per (3) above - If not, replace Sender.
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Old 14-09-2009, 05:44   #9
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The same thing happened to me

I had an identical setup (1979 MD11C) and it did the same thing twice in a two week period before the oil pump gave up the ghost while in a channel full of tow boats. I had to run the engine in gear at fast idle for ten minutes and it did not seize but it made a grinding sound when restarting.

One thing that I found on the internet is to check the oil dipstick/ strainer assembly for leaks. Apparently the seal on the strainer is essential to oil pressure. In my case the diagnosis was bad oil pump and I opted to re-power rather than rebuild. This was expensive but worth it for me.

Good luck and may I suggest a tow service policy. It really paid off for me.
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