Oil Pressure switch dodgy?

[jeremyshaw]

Ever since I got my boat with its two Yanmar 3JH engines the oil pressure readings have been a bit iffy. Despite having plenty of oil and with no problems with the temperature, the readings wander between low and normal for no apparent reason. I've tried different kinds of oil, and now have blowers to reduce the compartment temperature but nothing has changed.

I've asked a few mechanics about this and they've all said "yes Yanmars do that. As long as you have plenty of oil and the temperature is OK don't worry."

However on a recent passage from Curacao to Bonaire (straight motoring into the wind) one engine showed even lower pressure than the previous range - though I'd just checked the oil and the temperature was fine. Looking at Calder's book I then scared myself by finding that a faulty pressure switch might be the cause and this would result in no oil being circulated round the engine, even though the pump was fine and there was plenty.

So my questions are:
1. Is this a possible cause?
2. If so, is there a way to diagnose it for sure?
3. How hard is it to change a pressure switch? The manual make it look easy but...

As I have 500 miles to plug to windward bound for Trini or Grenada on my next trip I want to make sure I'm not going to wreck my engines!

Any advice, as ever, greatly appreciated.

 

[captainslarty]

A faulty oil pressure SWITCH would not cause the problem, a faulty oil pressure relief valve MAY, but is unlikely.
Is the pressure gauge electronic or pipe fed ?.

Dont know this particular engine at all though..
Joe

 

[sailinglegend420]

Had intermittent high(off the scale) readings on my Yanmar 4JH3E. I cleaned out the damper that the sensor screws into and all was fine.

 

[boatmike]

I am suspicious of both engines having an identical fault at the same time.
Not impossible of course but unlikely. You probably have electrical senders on your pressure gauge. It would not be too difficult to find a direct manual gauge that could fit in the same hole. You might find if you do that the pressure is OK but there is an electrical fault of some kind.

 

[boatmike]

Interesting.... what do you mean by a "damper"?

 

[Trundlebug]

It's certainly not uncommon for an oil pressure switch to be faulty, and this usually results in the oil pressure warning light staying on.

Oil pressure switches are simple on / off diaphragm operated switches. When the oil pressure is sufficient, it presses against the diaphragm and breaks the normally closed switch to turn the warning light off. When the diaphragm fails the warning light stays on.

Pressure senders for gauges are rather different. Mechanical ones very rarely lie and are very reliable.
Electrically operated senders can be faulty if there is an electrical fault (most likely) but as has already been said the cause could also be the pressure relief valve.

I would check the electrics thoroughly, then remove and clean out the sender unit, and the presure relief valve, before retrying. Only buy a new sender if the fault prevails.

 

[boatmike]

Ah! just got my brain engaged! Some oil pressure sensors screw into an adaptor that has a very small hole in it, possibly to "damp" the pulsations of the pump. If indeed the engine oil becomes sludgy, this hole could become blocked. If it does it will cause your problem. worth taking one of the senders out and cleaning it. Could even require a very quick burst on the engine to blow the crap out but don't put your hand over the hole!

 

[lw395]

If you don't trust the sender, swap them between engines, preferably wiring the gauges across as well so that the sender stays with the same gauge.
How low in psi are we talking?
It doesn't sound like the pressure regulator valve is seized totally open, that would always give quite low pressure. If it were seized a bit open, then pressure would be way too high cold at anything above tickover.
It may not be seating properly.
I assume there is no possibility that the oil pump is drawing in air, ie too little oil/ xs heel etc?
Is it generally the case that yanmars are a bit prone to this, due to having not ever so big oil pumps? My 3gm has had lowish pressure for the last 5 years, not getting any worse though.
Cheers,
Chris

 

[savageseadog]

One thing I've come across in a couple of car engines is that there was a gauze filter on the oil pickup in the sump, the pickup for the oil pump that is. If you get a bit of crud it can block the flow especially at low rpm. Don't know if the Yanmar is the same but it's possible.

 

[jeremyshaw]

Thanks for the advice everyone. The gauges are indeed electric, so the fault may be in the sender(s) or possible the pressure relief valve. I think it's less likely there is crud in the pickup as the pressure seems to vary regardless engiine revs. I'll strip down the sender and the relief valve and see if there is any change.