Oil Pressure Sender failure modes?

[nola mike]

TL;DR:
What would cause an oil pressure sender to fluctuate between reading correctly and reading zero?

Been chasing an oil pressure gauge/issue all summer. Mercruiser, standard analog sender/gauge. It will run fine for long stretches of time, steady at 60 psi, then drop to "zero" (actually more like 5-10, which is where the gauge reads engine off). Anywhere from a second or 2 to much longer. Goes very quickly between readings, so seemed electrical. Ran it at 3500 rpm reading zero for 10 miles a few days ago, so pretty sure at this point that it's not actually low pressure. No pattern on what triggered it or cured it, but becoming more common. Gauge checks out, all connections are squeaky clean, nothing else at the helm malfunctions when the pressure drops. Bench testing the sender checked out (I didn't have anything handy to pressurize it but readings fluctuated a bit when I blew into it). Finally swapped out the sender with a spare. So far so good, but we'll see. My concern was an intermittent clog/air leak at the pickup. Question is do senders fail in that pattern? If so, is it typically an electrical problem or something obstructing the sender hole? Doesn't really matter, just curious.

 

[DeepCMark58A]

Put a pressure gauge where the sensor is, rig up a t with pipe fittings. See what is really happening.

 

[nola mike]

Put a pressure gauge where the sensor is, rig up a t with pipe fittings. See what is really happening.

Intermittent failure in a difficult location. Not really possible to T in there. Like I said, if this was a real pressure then I wouldn't have been able to take the kids skiing this am lol.

 

[Chris1956]

Gee, oil pressure sensors on MerCruisers often fail. The one on my '88 was a can-type. it looked the same as the automotive one on autos of that time.

Ohm out the wiring, clean the connectors and if that doesn't get it, replace it.

 

[Pmt133]

Mine uses a VDO type sender... stays at 45 all the time lol. They are notorious for failing.

 

[nola mike]

Gee, oil pressure sensors on MerCruisers often fail. The one on my '88 was a can-type. it looked the same as the automotive one on autos of that time.

Ohm out the wiring, clean the connectors and if that doesn't get it, replace it.

Yeah, I already did (with a 40 year old spare that I had), and it is an auto can type. I'm just curious as to how they fail moreso than how to troubleshoot the system. Of course intermittent electrical issues are tough.

 

[Grub54891]

Clean all th connections including the large bullet connection at the engine, and under the dash. Also you can do a jiggle test when it’s not working and see if it comes back to life for a bit. That will narrow down where the issue is.

 

[nola mike]

Clean all th connections including the large bullet connection at the engine, and under the dash. Also you can do a jiggle test when it’s not working and see if it comes back to life for a bit. That will narrow down where the issue is.

Already did all that, no changes. Have put about 50 miles on the "new" sender including some good chop with no recurrence so far...

 

[StewartL]

Yep, senders can fail exactly like that usually electrical, not mechanical. Internal resistance gets flaky, causing the gauge to bounce or drop out. Since your gauge and wiring check out and the new sender seems to fix it, that’s likely the culprit. A clogged pickup or air leak would show slower, more consistent pressure drops, not instant zero. Senders just wear out over time, especially with heat and vibration. If the problem comes back, double-check the ground, but odds are the old sender was just dying.

 

[Scott06]

TL;DR:
What would cause an oil pressure sender to fluctuate between reading correctly and reading zero?

Been chasing an oil pressure gauge/issue all summer. Mercruiser, standard analog sender/gauge. It will run fine for long stretches of time, steady at 60 psi, then drop to "zero" (actually more like 5-10, which is where the gauge reads engine off). Anywhere from a second or 2 to much longer. Goes very quickly between readings, so seemed electrical. Ran it at 3500 rpm reading zero for 10 miles a few days ago, so pretty sure at this point that it's not actually low pressure. No pattern on what triggered it or cured it, but becoming more common. Gauge checks out, all connections are squeaky clean, nothing else at the helm malfunctions when the pressure drops. Bench testing the sender checked out (I didn't have anything handy to pressurize it but readings fluctuated a bit when I blew into it). Finally swapped out the sender with a spare. So far so good, but we'll see. My concern was an intermittent clog/air leak at the pickup. Question is do senders fail in that pattern? If so, is it typically an electrical problem or something obstructing the sender hole? Doesn't really matter, just curious.

I would suspect you have a ground issue between gauge and the block maybe at cannon plug. Not sure if power supply to gauge or voltage drop to gauge will do it...

as mentioned put a mechanical gauge on a T run it so you know there is oil pressure in the engine.

 

[nola mike]

I would suspect you have a ground issue between gauge and the block maybe at cannon plug. Not sure if power supply to gauge or voltage drop to gauge will do it...

as mentioned put a mechanical gauge on a T run it so you know there is oil pressure in the engine.

Nope. ALL connections everywhere were redone when the engine was out this spring. Cannon plug was disconnected and cleaned (again... done in the spring). None of the other gauges are acting strangely (same 12v supply). As mentioned, not really possible to t into my sender the way things are set up. it would have to be a total loss of power to have it drop to zero. I'm going to cut open the old sender and see what it looks like. A couple of hours of skiing this am with no wonkiness still.

 

[dingbat]

I'm going to cut open the old sender and see what it looks like. A couple of hours of skiing this am with no wonkiness still.

 

[Horigan]

A pressure switch is very different than a pressure sensor/sender. A pressure sender has a strain gauge, piezo sensor, or some other means of measuring strain on a diaphragm to convert into an electrical resistance. A switch just opens or closes a set of contacts.

 

[Scott06]

I'm going to cut open the old sender and see what it looks like. A couple of hours of skiing this am with no wonkiness still.

you must be bored or out of beer... looks like the thing works now so leave it alone.

Seriously though I converted my factory gauge on my 65 GTO (was 0-60 psi) bc my engine makes about 80 psi and gauge was always pegged above idle. I was able to find a 0-80 or 90 psi sender that had same ohm range as original one. So 60 psi on gauge is actually 80 ... I think I gotta drop to 30 or 40 wt oil...

 

[nola mike]

you must be bored or out of beer... looks like the thing works now so leave it alone.

Seriously though I converted my factory gauge on my 65 GTO (was 0-60 psi) bc my engine makes about 80 psi and gauge was always pegged above idle. I was able to find a 0-80 or 90 psi sender that had same ohm range as original one. So 60 psi on gauge is actually 80 ... I think I gotta drop to 30 or 40 wt oil...

yeah bored. just curious as to what exactly was causing it to do that. it caused me much angst when the op suddenly dropped while on plane with a crew, but only intermittently

 

[Scott06]

yeah bored. just curious as to what exactly was causing it to do that. it caused me much angst when the op suddenly dropped while on plane with a crew, but only intermittently

yeah always watching OP and temp. Had an over heat last summer due to weeds on outdrive... had been towing my daughter skiing then picked her up took off through some weeds and it went from zero to alarm going off in an instant

 

[nola mike]

yeah always watching OP and temp. Had an over heat last summer due to weeds on outdrive... had been towing my daughter skiing then picked her up took off through some weeds and it went from zero to alarm going off in an instant

I have no alarms on the boat, and have been toying with installing some for a long time. if the alarm went off and the pressure dropped, then I know it's the real thing

 

[Scott06]

I have no alarms on the boat, and have been toying with installing some for a long time. if the alarm went off and the pressure dropped, then I know it's the real thing

Well yes and no... I think the factory OP switch is tripped at 4 or 7 psi .... so at lower rpms yes it may save an engine but higher rpms ... if it was low level and you had 15 psi at 3k rpm not sure it would help

We a grew up with older hardware that had no trim gauge, dog clutch drives, outboards you could start in gear ....

 

[Scott Danforth]

I have no alarms on the boat, and have been toying with installing some for a long time. if the alarm went off and the pressure dropped, then I know it's the real thing

easy to add.

 

[nola mike]

easy to add.

I am so underwater with easy projects I can't tell you. Busy hurricane-proofing today. I finally got the boat working well enough that I actually cleaned it yesterday for the first time this year...Next easy boat project is a momentary switch for the fuel pump. And finish the other side of the trailer before the end of the season. And install a remote for the boat lift. And get the "free" jet ski working. And find the leak on my sunfish. And install a garboard drain on the skiff and drill into the hull to free the water in the hull. These are the "fun" boat related projects, not the "need to do" in life projects. Ugh. Now I'm depressed.

Well yes and no... I think the factory OP switch is tripped at 4 or 7 psi .... so at lower rpms yes it may save an engine but higher rpms ... if it was low level and you had 15 psi at 3k rpm not sure it would help

Excellent point.