Pressure got the drop on me

[jso]

My 1987 Yanmar 3gm30f oil pressure switch squeals loud when I return to the slip and drop the rpm to 2k or less. Maine S. suggests that these senders go bad so I got the one he suggests but have not installed it yet because I am so worried that there might be a real problem.

I bought a VDO oil pressure gauge and sender to "know" what is going on. When I took the boat out for a test run the pressure pegs mostly at 20 psi at 2800 rpm but rises to 3000 the rpms drop, especially closer to idle. I hope the gauge is just reading opposite the real oil pressure but do not see how that could happen.

The gauge came with a harness that snaps into a slot that cannot be installed wrong with 8 positions (7 wires). I only need three - red (switched 12v), green (signal from sender) and black (ground). Not much room for me to reverse anything.

The only place I fudged a little is in connecting the ground to my DC neg bus bar instead of directly to the engine. It is it possible that could reverse the pressure readings? (It is difficult to get the wire to the engine plus I am shy about removing an engine bolt to connect the ground wire). Help?

JSO

 

[kloudie1]

Low Oil Prressure

Say that again a little slower..?? are ya saying that the maximum oil pressure you see is 20 psi at 2800 rpm? but it rises as you go to 3000 RPM? or is it that the minimum that the gauge can read is 20 and it stays there until you get to 2800 RPM? Need a little clarification.
The book says minimum 7 psi at 850 rpm idle and at 3600, you should be seeing 42-57 psi on the gauge..The alarm is supposed to trigger at around 1.5 PSI..
Sounds like you may have a problem.. On that engine, my primary suspect would be the oil pressure regulator valve. If that thing gets a speck of dirt or whatever jammed in it, you will see low oil pressure readings.. That valve lives in the center threaded stud that the oil filter screws onto. You might be able to see the little ball in the valve after removing the filter. It should be free to move if pushed on with something like a nail. If that looks clean and free then the next thing to troubleshoot is the oil pump itself and the engine main and rod bearings.. these are expensive to work ..
Have you changed the filter recently? Did it begin to sound the alarm after a filter change? Have a new filter handy when you open to inspect the oil pressure control valve after verifying that the valve is clear, install a new filter before the test run... I hope you find the pressure control valve stuck or a piece of trash in it.

 

[jso]

Re: Low Oil Prressure

Claude,

Sorry for the confusion. I just installed the gauge yesterday. When I increase the throttle the gauge reads lower pressure. Once warmed up it stays at 20psi until I reduce throttle. As I reduce throttle the gauge reads higher pressure.

The alarm has been gradually increasing for the last six months or so. I changed the oil and filter in February, there was no effect on the alarm. I hope it is a bad (brand new) gauge but thought these kind of readings might mean something if someone has seen these symptoms before.

John

 

[kloudie1]

Wow! Thanks for the clarification.. that is strange unless it is a gauge designed for a positive ground?.. It might be time to get a little cheapo mechanical gauge and temporarily install it.. I could maybe imagine this happening if the oil pump was sucking air somehow. There is a connection and a plug where that could happen. I think the plug is a big hex headed plug with a copper washer under it. It covers the hole where the oil pick-up tubeis screwed into the front of the oil pan. If you have a plug on the front of the engine that is weeping a little oil, that guy may be a problem. It could also be an air leak inside the pan where the pickup itself is threaded into the pan. If you see a weepy or drippy plug on the lower front of the engine, ya may want to pull it and replace the brass washer and retighten.. That is a strange one.
For the electric gauge, it may be telling you that you have a bad ground connection between the neg buss and the engine.. the sender is referencing the engine block ground and the gauge referencing the buss may be causing the wonky readings? I dunno, but I'd look at the buss ground connections on both ends.

 

[Maine Sail]

Some VDO's and other gauges work incorrectly when used with a US 33-240 Ohm, sender.... You may require a VDO sender but there are others out there that will work.... Most US type gauges Vee-Three, Faria, Teleflex, Datcon, Stewart Warner, Moeller all use 33-240 Ohm range senders. VDO is generally 10-180 Ohm unless otherwise specified...

 

[jso]

VDO Sender

Some VDO's and other gauges work incorrectly when used with a US 33-240 Ohm, sender.... You may require a VDO sender but there are others out there that will work.... Most US type gauges Vee-Three, Faria, Teleflex, Datcon, Stewart Warner, Moeller all use 33-240 Ohm range senders. VDO is generally 10-180 Ohm unless otherwise specified...

Maine,

I have the VDO Gauge and the VDO sender. I still have the original switch (making the noise at low rpm). There is a second port below that where I installed the VDO sender going to the VDO gauge.

Could kloudie be right that not connecting the ground wire directly to the engine could make the gauge work in reverse? I have the gauge ground wire connected to a shiny three months old bus bar which takes all of my DC neg to the engine. The gauge ground is the last thing on the bus bar before the cable to the engine.

 

[jso]

kloudie, I will try to find the mechanical gauge. The Harbor Freight here does not have them. Can I trust a cheapo gauge just because it is mechanical?

There is no drippy or weepy oil anywhere in the pan area.

 

[Bill Roosa]

I'd try, just for the sake of it, putting on the new sender and seeing what happens. It is easy and might just solve the problem. KISS
A cheap and fast substitute for a gauge is your finger. Kinda messy but you KNOW if there is pressure at idle. Have a helper start the motor and stay out of the way of rotating parts.

 

[kloudie1]

jso.. a little cheapie gauge won't give you the best accuracy but it will immediately give you info on whether the electric one is speaking close to the truth. Something from AutoZone or O'Reilly or Wally World should be useful and not expensive. Kinda grasping at straws .. I haven't experience with oil pressure that decreases with rpm unless the pump would be entraining air at higher flow.. or something like a potato chip bag is in the oil pan and being sucked up against the pick-up screen.. OK I would guess that it would be hard to get a chip bag in there.. so I'd think a restriction there would be improbable.. Do look for a wet hex plug on the front of the lower engine. I would think if the plug were leaking air with the engine running, it would leak a very little oil when shut down as the oil in the lines above the pump and in the pump itself tries to get back to the sump..
EDIT.. the cheapie gauge I am talking about is $10-$20, try to find one with a max reading of around 80 .. as Bill noted , it is not impossible for that electronic sender or the gauge itself to be bad out of the box.

 

[jso]

kloudie - I got the mechanical gauge. It showed pressure as 40 psi. That was just for 10 or 15 minutes at the slip. I could not take it out to sea with that mess attached to the engine. The VDO gauge still showed pressure at 20 psi and dropped slightly with more throttle.

I also move the ground from the bus bar to the engine. No change for the new gauge. Bad vdo gauge, bad vdo sender or something in my wiring of the connections.All I can do is try to get an exchange for the gauge and sender I guess.

John

 

[kloudie1]

That is about what the pressure should be.. The mechanical gauge did not drop at higher rpm as the electric did?.. suspect electrical sender having a problem with engine vibration (defective). (Maybe a loose sender rheostat arm?). Good on checking for a ground problem.. What did the mechanical gauge read at the point where the alarm quit squeaking?

 

[jso]

That is about what the pressure should be.. The mechanical gauge did not drop at higher rpm as the electric did?.. suspect electrical sender having a problem with engine vibration (defective). (Maybe a loose sender rheostat arm?). Good on checking for a ground problem.. What did the mechanical gauge read at the point where the alarm quit squeaking?

----------------------------------------------

The mechanical gauge did not drop at higher rpm as the new electrical one did. It acted sane at all rpms. However, the mechanical gauge was too bulky so I could not leave it installed at sea. I only used it in the slip both in gear and in neutral so the engine never reached operating temperatures.

I also installed a new alarm sender. When I went out for about three hours yesterday and returned to the slip there was no alarm at idle (850 rpm). The old alarm sender had started squealing when I dropped below 2000 rpm.

So, the original problem is solved (Maine Sail's thesis that these senders go bad all the time proves correct) but I still want to get the new oil pressure gauge working. I have no response yet from VDO on replacing the gauge and sender.

 

[Morgan donor]

JSO - this all sounded familiar as a similar ailment affects old British sports cars and their Smiths gauges. Almost always the sender. Plumb up a T fitting so you can run a mechanical gauge w/o a hose or capillary tube. Then you can easily check. But an engine without oil pressure will pretty much stop turning before you have time to get the engine hatch off. Learn to listen to the noise the engine makes and the way it smells when its happily running. You can sense a marginal situation (be it oil or cooling) by the changes. When ever I run the engine for a long duration I open the hatch, look smell and listen. Some guys use an infrared temp gauge.

Jaguar oil pressure senders are $150 and many new ones don't work. I have a mechanical gauge with a long tube. Been working for 10+ years. This electrical stuff will never catch on...ha ha ha !

 

[jso]

Thanks Morgan.

The new alarm sender makes the noise it should when I turn the key and keeps quiet when the fully warmed up engine gets down to 850 rpm (old one squealed at 2000 after the engine ran for 45 minutes).

I put 9 hours on the engine going out to Catalina Island and back yesterday and today and everything worked well and the engine sounded great ..... but, but, but, I still want to know what the oil pressure is all the time so when something changes I get an early warning.

John

 

[kloudie1]

Good work, JSO. Sounds like you fixed the initial problem only to discover that the new gauge/sender is shot.. Good thing is the real problem (bad switch) was fairly cheap.. and the engine is fine!