Mechanical oil pressure gauges go bad ?

1993 vintage Murphy, barely reading 5 lbs in the engine room but the VDO electric gauges for that engine reading 25 to 30 lbs.

And yet the direct reading one ought to be the most accurate, so it's a bit unnerving to see such low pressure all of a sudden on this gauge (this at the dock, 600 rpm, Detroit 12v71TA with 280 hours since overhaul)

I guess my question involves wondering how common is it for a mechanical oil pressure gauge to fail ?

I assume you have two engines? If so swop the gauges and see what happens.

Capt.Bill11 said:

I assume you have two engines? If so swop the gauges and see what happens.

Click to expand...

Good advice, the same as I was just going to give. Had to do that just a few weeks ago myself, one gauge reading 40-50 lbs as always, the other reading 10 all of a sudden. I created a little wire jumper with a couple alligator clips, the low gauge read 40-50 once I jumped it over to the other engine. Turned out to be a bad sending unit, surprisingly cheap (for boats), about $40 to replace, but then I have pretty common 454's.

Make sure to swap out the whole assembly from engine to gauge.

caltexflanc said:

Make sure to swap out the whole assembly from engine to gauge.

Click to expand...

As a first step why not just do the gauge? It might tell you right up front that the gauge is the issue.

kthoennes said:

Good advice, the same as I was just going to give

Click to expand...

Good grief....I was looking for an answer to the question, not Captain Obvious observations. Yes, of course I could swap gauges but before I go to that trouble just wondering how common direct oil line gauge failures are.

Caladan said:

Good grief....I was looking for an answer to the question, not Captain Obvious observations. Yes, of course I could swap gauges but before I go to that trouble just wondering how common direct oil line gauge failures are.

Click to expand...

I had one fail this year

I have capillary tubes to my lower helm gages. They seem reliable and accurate but they're starting to weep.

Before swapping anything, take a look at it component by component. I'd consider removing the sending unit and looking for an obstruction in the oil line. Clean as much as possible and reinstall to retest.

Next, run a new capillary line temporarily by whatever route is easiest to test the other existing components. Run it from the existing sender to the gage. Fill it with oil by starting the engine before connecting to the gage. Any difference?

Then start swapping gage connections to see where the problem lies.

Mechanical gauges do fail. pressure pulsations in the line wear them out. Replaced many over the years. Try another gauge.

Caladan said:

Good grief....I was looking for an answer to the question, not Captain Obvious observations. Yes, of course I could swap gauges but before I go to that trouble just wondering how common direct oil line gauge failures are.

Click to expand...

Gee, now I'm sorry I bothered. Feel free to fumble along on your own.

I guess my question involves wondering how common is it for a mechanical oil pressure gauge to fail ?

Everything can fail , but with mechanicals check the tubing has not been crushed.

BIGFOOT , lives in some bilges.

Ski in NC said:

Mechanical gauges do fail. pressure pulsations in the line wear them out. Replaced many over the years. Try another gauge.

Click to expand...

Thanks. Based on your experience I was inspired to switch port and starboard gages and the port engine that was reading 5 lbs is now reading 32 lbs at idle. So, yep....failed gage.

The interesting part is once removed, I see the working gage is obviously newer (body and internal bracket look much cleaner compared to the other gage, and has pristine bright white model no. sticker), so apparently this isn't the first failure of one of those gages.

The worrisome part is now that I know the model number I can access the data sheet on the website, and see that the two electrical wires on each gage are suppose to activate a solenoid or whatever to automatically shut down the engine when the pressure is low enough. This didn't happen...if 5 lbs isn't low enough, what is ??

Obviously the pressure wasn't really that low, but the gage thought it was, so it should have shut down the engine. So now I need to figure out why it didn't.

If curious, click on "diagrams" at link below to see what I mean. Mine is Murphy A20P-150.

20P / 25P Series | Murphy by Enovation Controls

================

(on edit)

Further contemplation of the diagram sheet seems to indicate that perhaps it is only the PABS models that will actually set off an alarm and shut down the engine automatically at preset low pressure.

Then again it looks like the standard P model could be configured for auto engine shutdown, just not pre alarm. My A20P gages do have two wires on each gage....if not for auto shutdown then what would those wires be for ? What do you guys think ?




(I probably should mention, before anyone suggests looking at wiring diagrams, there aren't any except for select systems like the helm toggle wiring, bilge pump system and halon fire system. Pleas to Viking for more wiring schematics have fallen on deaf ears in the past)

"This didn't happen...if 5 lbs isn't low enough, what is ??"

First you need to set the low pressure point at which you wish the alarm to operate.

Then you need to have operating an electric alarm , horn , bell whatever.

To secure the engine automatically requires either an electric stop solenoid , or if a mechanical cable pull stop the Murphy pull spring system installed and ARMED.

25 + years of operation doesnt sound bad , just check to see if the auto secure System is on the boat , the gauge alone wont do it.

Tho an alarm if the pressure at cruise drops is easy and cheap to install.

"Pleas to Viking for more wiring schematics have fallen on deaf ears in the past) "

There might be some Viking folks on a Viking boat board.

Otherwise offer to PURCHASE a set of schematics , you may have better results.

I have seen gauges get plugged with something. If you can crank the engine with the gauge out to get a little oil to flush the passage. Have fuel off so it wont start. You don't need more oil in Detroit bilges.

Often that is for setting off an alarm, not auto shutdown. It's the same alarm that sounds on most diesels when you turn on the ignition before you turn the engine over. So if wired for auto shutdown , no starta da engine. It varies, though, some boats have a separate oil pressure switch from the gauge system.

bayview said:

I have seen gauges get plugged with something. If you can crank the engine with the gauge out to get a little oil to flush the passage. Have fuel off so it wont start. You don't need more oil in Detroit bilges.

Click to expand...

Might even want to follow that with a brake cleaner flush (inside the sender port). I've flushed the offending particulate matter out successfully a couple of times through the years.


Forky
1983 Present 42 Sundeck
Twin Lehman 135's

FF said:

Otherwise offer to PURCHASE a set of schematics , you may have better results.

Click to expand...

I have done exactly that, including setting no limit on how much I would pay. I even have a contact that works at Viking (allbeit in relatively low office position) that has had no luck other than finding 2000 year 65C schematics (which are completely different).

1993 schematics would have been all on paper rather then electronic, and have heard excuses ranging from "they needed the room and just threw them away" to "lost in hurricane Sandy" !

Keeping in mind these excuses I'm hearing third hand so the grain of salt that remains, keeps my hopes up that something will turn up eventually.

Caladan said:

Good grief....I was looking for an answer to the question, not Captain Obvious observations.

Click to expand...

LOL Thanks, Captain Obvious.

Ski in NC said:

Mechanical gauges do fail. pressure pulsations in the line wear them out. Replaced many over the years. Try another gauge.

Click to expand...

X2. take the bad one apart and you will see why. Gears and springs. It may also have a snubber (restricting orifice) that may be clogged.

High Wire said:

X2. take the bad one apart and you will see why. Gears and springs. It may also have a snubber (restricting orifice) that may be clogged.

Click to expand...

http://www.fwmurphy.com/uploaded/documents/pdfs/94116.pdf

Just noticed the "pulsation dampener" which is removeable for cleaning...maybe I won't need a new one after all.

Caladan said:

http://www.fwmurphy.com/uploaded/documents/pdfs/94116.pdf

Just noticed the "pulsation dampener" which is removeable for cleaning...maybe I won't need a new one after all.

Click to expand...

Yes, they clog with dirty fluids like engine oil, seawater, holding tank contents, etc.

High Wire said:

Yes, they clog with dirty fluids like engine oil, seawater, holding tank contents, etc.

Click to expand...

Cleaned it with carb cleaner. The pulsation dampened screw hole was clogged as well. Alas, no cigar...after reinstallation it did improve a bit(reading 15 psi when it was really over 30 according to VDO gauges at helms) for a few minutes but then went back to 5 psi and stayed there.

I just had a failure of a mechanical pressure gauge...the internal mechanism failed and the oil was pumped out in the bilge through the adjustment grommet port on the side.

There wasn't a shut-off valve before the pressure gauge so oil couldn't be refilled.

I don't think this is a common failure, but if you have a pressure gauge I recommend that you instal a shut-off valve.

"I don't think this is a common failure, but if you have a pressure gauge I recommend that you install a shut-off valve."


These failures are very rare , a set of Vice Grips to crush the feed line might work as Plan B.

I agree that these mechanical oil pressure gauges are extremely reliable, especially if they've gone 25 years, or in my case both went 40 years with no issues. Mine started leaking at a slightly-loosened connection but I cut and re-flaired the copper pipe anyway.

Regarding to Searios's comment about a small valve, well I would think there is a much, much higher rate of leaks developing from valves. Generally if a valve is not actuated regularly, then after several years when you do close/open it you get leaks.

"The worrisome part is now that I know the model number I can access the data sheet on the website, and see that the two electrical wires on each gage are suppose to activate a solenoid or whatever to automatically shut down the engine when the pressure is low enough. This didn't happen...if 5 lbs isn't low enough, what is ??"

ALL the Murphy gauges are not "switch gauges" that can be ordered with adjustable hi and/or low set points to be used in a shutdown or alarm system.

Some simply will sound like any oil pressure alarm system when there is little or no oil pressure.

For folks that cruise with friends as crew might want to discover and install a switch gauge package.

Colored markings painted on a gauge are fine for a crew that will monitor the gauges underway (hopefully often) .

The instant notification if a switch sounding a horn or bell , is sure to be noticed.

Folks that want a simple no cost system might consider an old aircraft technique.

Rotate all the critical gauges so the needle is parallel to the dash when warmed up and at cruise.

A drooping needle is easy to spot as the gauge does not have to be read and understood , it simply needs to be noticed, and reported.

Regarding to [B said:

Searios[/B]'s comment about a small valve, well I would think there is a much, much higher rate of leaks developing from valves. Generally if a valve is not actuated regularly, then after several years when you do close/open it you get leaks.

Click to expand...

That's correct. But a normal dip stick check should reveal if oil is disappearing as an early warning. A complete failure means the engine is dead as oil cannot be refilled, unless there is a shutoff valve... I'll prefer the valve any time as I can refill, limp home and get it fixed...

I will probably install an in-line pressure switch too..

I had an electric oil pressure switch screwed into an Onan marine generator. The generator sat under a leaky hatch. Over many years, rain rusted it and one day it popped apart and when the gen ran it pumped out oil into the bilge. they are all mechanical gauges that measure oil pressure, some with built in electronics that convert the signal to electronic, except the new trend in trucks is to contrive the pressure number from a computer calculation, like Dodge does. I think that's a crazy thing to do. They do the same thing for a MPG readout, contrive it from a fuel map, they dont actually measure it, its just fake.

FF; said:

Rotate all the critical gauges so the needle is parallel to the dash when warmed up and at cruise.

A drooping needle is easy to spot as the gauge does not have to be read and understood , it simply needs to be noticed, and reported.

Click to expand...

I like this idea. But would it be better to have the needle vertical at 12:00?