Low Oil Pressure

[txarmydude]

I am getting low oil pressure on my 5.7L Mercruiser. I have taken it to the lake twice since I got it running and each time the oil pressure has shown low. At idle, the boat runs around 10 PSI oil pressure, and it sort of "bounces around" with the idle. At 4000 RPMs it is about 30 PSI, and anything in between idle and 4000 is relative between 10-30 PSI.

I just had this motor put in this boat. It is a 1977, and was severely neglected. All the electronics were original equipment. The boat also used to have a Mercruiser 888 (Ford 302) in it. I have fixed many of the electrical problems, but there are some quirky things I am still trying to get worked out. The boat runs GREAT. The last time I had it at the lake, I ran it over 4000 RPMs for a couple of hours at a time with NO issues.

The oil and filter are brand new and it doesn't leak a drop. Should I be looking for a sending unit (and where on the motor should I be looking to find it)? It it possible that the gauge itself is bad? Thanks.

 

[Bondo]

Re: Low Oil Pressure

It it possible that the gauge itself is bad?

Ayuh,... Tee in a mechanical gauge, 'n check it...

 

[txarmydude]

Re: Low Oil Pressure

I have been unable to borrow an oil pressure gauge. I will try to borrow one from the auto parts store this weekend. The one that they sell is like 60 bucks, and I can order one from harbor freight for 25. Anyone have the part number for the sending unit on a 260 Mercruiser 5.7L??? Is it also referred to as and oil pressure SWITCH??? Thanks for the help!

 

[txarmydude]

Re: Low Oil Pressure

Oh, yeah... Is the sending unit one of those parts that MUST be marine, or can I use a cheaper one from a 350 Chevy???

 

[krisnowicki]

Re: Low Oil Pressure

oil PSI knows not if it is in a boat or car. Get an el chepo and T it in. 10psi at idle is ...ok not good but ok. 15 or higher would be ideal. But I think older cars warning lights used to turn on at either 4 or 7

 

[projectcop]

Re: Low Oil Pressure

A small block chevy can sucessfuly run on 5 pounds of oil pressure per 1000 rpm's. Not the best thing for it, of course; but it will do it for hundreds of hours.
If you wind up having to swap out the oil pump, do NOT do the old "hot rod" thing and put a high pressure spring in a high volume pump. If you do this, you will have to put a 10 quart pan on it to keep from sucking the pan dry.
I say this after having spent 10 years working in an automotive machine shop in San Francisco btw....

The location for the oil pressure sender should be either near the base of the distributor or at the front of the engine near the right side cylinder head.

 

[Rocky_Road]

Re: Low Oil Pressure

I have been unable to borrow an oil pressure gauge. I will try to borrow one from the auto parts store this weekend. The one that they sell is like 60 bucks, and I can order one from harbor freight for 25. Anyone have the part number for the sending unit on a 260 Mercruiser 5.7L??? Is it also referred to as and oil pressure SWITCH??? Thanks for the help!

An oil pressure "SWITCH" is only used for the engine alarm circuit, and not the same as the oil pressure sending unit.

Happy boating!

 

[StevNimrod]

Re: Low Oil Pressure

it sort of "bounces around" with the idle.

A mechanical gauge will probably tell the story either way. I had a similar problem and it ended up being a short in the wire right at the sender. Still get a mechanical gauge though, and see how the readings compare. If you can't find one complete for a reasonable price, try to find a simple mechanical pressure gauge and build a hose to connect the gauge to the open port in the tee you'll need.

These can be had from automotive parts stores (these senders were used in old AMCs and Jeeps) but they are typically more expensive than your boatyard (hope Merc and the aftermarket doesn't figure that out).

 

[Scorpion8]

Re: Low Oil Pressure

Check the connector at the sending unit, or on the back of the oil pressure gauge for loose connections.

 

[txarmydude]

Re: Low Oil Pressure

I took the sending unit out last night and it rattles. The new one does not rattle. Hopefully I can get a gauge from someone today to mechanically test it...

 

[txarmydude]

Re: Low Oil Pressure

I installed the new sending unit today, but I heard a nasty rumor after I had already completely finished....

Is it true that you are not supposed to put teflon tape on the sending unit??? Cuz if so, I might have to revisit that thing tomorrow...

Can anyone confirm or deny???

 

[Gettinther]

Re: Low Oil Pressure

I installed the new sending unit today, but I heard a nasty rumor after I had already completely finished....

Is it true that you are not supposed to put teflon tape on the sending unit??? Cuz if so, I might have to revisit that thing tomorrow...

Can anyone confirm or deny???

yes
Teflon tape can insulate sending unit from making a good ground connection causing poor gauge performance

 

[txarmydude]

Re: Low Oil Pressure

So, do you put anything on it to keep it from leaking? Or do you just tighten the heck out of it?

 

[Gettinther]

Re: Low Oil Pressure

So, do you put anything on it to keep it from leaking? Or do you just tighten the heck out of it?

Personally I either just tighten it if its in a brass fitting or put locktite thread sealer not thread locker on it
Check ground with an ohm meter should have 0 resistance across the casing not from the wire connection

Brass fittings usually seal pretty good

 

[txarmydude]

Re: Low Oil Pressure

Thanks for everyone's help! I replaced the sending unit and Oil pressure is perfect from idle thru WOT!