Momentary loss of oil pressure

[dwaynec]

A couple times this year, when I started the starbd engine (8v71n) in our boat, it would build oil pressure to about 20lbs and then all of a sudden drop to zero and then start the climb back up again. My guages are not electric, but are plumbed from the engine via copper tubing.

When it happened the first time, I immediately shutdown the engine when the pressure dropped and went to the engine compartment expecting to see a lake of oil in the bilge because of a burst oil line or some other problem. Nope, everything was fine, and the oil level was still up to full. I then restarted the engine and watched as the pressure climbed and never dropped. This has happened twice.

I talked to a friend on the dock who has a lot more experience than I and he said that he thought it was probably an air bubble in the oil line going to the guage, and to keep and eye on it. I have checked the line all the way from the engine to the guage at the helm and can see no sign of a leak anywhere.

Any ideas?

Dwayne

[REBrueckner]

I agree with the air bubble in the oil gauge line idea. But I always assumed the oil pressure from the engine end compressed the air in the line at the gauge end anyway, so I'm not sure. It could happen,I guess, without any leak. If it is a leak, it could be in the oil gauge line..have you checked the visible portions? Check under hidden runs where oil might run(which might be really difficult to detect a tiny leak).

At the engine, any actual loss of oil pressure would probably be audible via noisy valves, but I personally don't know what that would sound like nor how audible over engine noise it actually is. I assume your oil level is normal!!

To check you gauge, you can either put another manual gauge on the engine and at engine start have one person read the engine gauge, one the helm gauge. Another way, put on an electric oil pressure gauge sender on the engine, run a pair of tempy wires to the helm so you can read the electric alongside the manual gauge....compare the two readings.

Alternate approaches: Remove the existing tubing/flex line whatever and run a whole new one to the gauge...You can also switch pressure lines between your port and stbd engines...see if the OTHER gauge drops occasionally...

Please post when do do ID the cause, it's an interesting issue.

[MikeP]

Normally, when installing a direct reading oil pressure gauge, there is no requirement to bleed the line. In fact, if you disconnect the line at the gauge, you will usually find that there is no oil there at all. THe oil will compress the air in the line and result in a pressure reading that will be the same as the oil pressure.

Re the temporary drop in pressure...Nothing comes immediately to my mind other than a marginal oil level which, under certain engine attitudes, might allow the pickup to uncover. But it sounds as though that can't be the issue. If it was an electrical gauge, I'd definitely look hard at it, but a mechanical gauge basically can't malfunction in this way without obvious symptoms (like oil dripping out).

Do your low oil pressure alarms work? They should sound if you turn on the key and do so after cranking/starting until pressure builds up. If they are functioning properly, does the oil pressure alarm sound when this pressure drop occurs? When it happens does the pressure immediately return to normal? In other words, does the needle drop and then immediately pop back up?

[dwaynec]

Unfortunately my alarms are not functioning right now. I have been trying to figure out what is wrong with them, but I have no manual to show how it was originally wired and so it is a trial and error thing right now. Hopefully I will have this problem corrected by spring. My Crozier system is the kind with four lights, one each for oil and temp for each engine. My voltmeter went for a swim this last summer, and now I am hoping that Santa will be real nice to me and get me a replacement so I can do a better job of tracing this stuff.

I am pretty sure it is not an oil level issue as I do a pre-flight each time I start the engines, and the level is always very close to the full level with the engines cold. With cost of getting these engines rebuilt or replaced (Would be a real bear on this boat) I could value my time at several thousand dollars per hour, and it would still be a lot more cost effective to check the oil and coolant level before starting the engine.

Thanks for the replies.
Dwayne

[jim rosenthal]

It sounds like this is a momentary oil starvation issue, and therefore potentially a serious one. The oil pumps in those engines are gear driven- either they work or they don't. They don't typically wear out unless some kind of garbage goes through them that trashes the pump.
Could something have momentarily blocked the oil pump pickup? Most of these pickups swing from the bottom of the oil pump. Could the pickup foot have swung up above the oil level for a moment?
I do not think it is a bubble in the line either. There could be a piece of crud in the line that prevents the pressure from being transmitted up to the gauge. Or the gauge itself (the Bourdon tube in it) could be getting ready to let go, which will be interesting when oil comes out of the back of the gauge. It might be worth taking the gauge out and flushing the line with clean oil...just a thought.
I guess it comes down to: is the gauge acting crazy or do you really have a momentary loss of oil pressure? What I would do: if the gauge is old, replace the gauge and make sure the line is okay. Or use the one from the other engine. If it happens again, then I guess unfortunately it is the real thing.

[Avenger]

My admittedly old, 71 series Detroit manual shows a number of relief and bypass valves in the lubrication system. While I wouldn't discount the possibility of an air issue I would also consider that maybe one of these valves has failed, is sticking, or is causing the symptom by doing its job. In particular, there is a 15 PSI differential bypass valve on the full flow filter (again, old manual, YMMV) If there was a failure in the FF filter circuit i.e. a blocked line, it would open causing a pressure drop until the rest of the system charged with oil. If you have a good manual for your engine and can figure out the oil flow path, and exactly where the gauge is tapped in it might give you an idea of what to look for as well. First, I would try to tee a simple mechanical pressure gauge into the system to verify that your gauge is accurate, but you'll probably find that it is.

[Trojan]

An air bubble or air in the line will not cause the gauge to drop. The more air there is in the line to the guage slower the guage will react. If you want faster reaction time of your gauge, just blead it. It sounds like you might have the pickup for the guage near a main bearing or something similar. That when the engine first starts the void between these two items is maybe large in respect to oil vs. air. The air gets in the system from setting or oil draining back from sitting ahead of the pump. Maybe the air blows out this void letting the existing oil pressure to the gauge return to zero. Once the air is expelled and the new oil reaches this void pressure builds again. It does not take much of an air loss to drop this guage. If your alarm is ahead of this air, it probably will not see this drop. Just a thought.

Bill