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  1. #1
    Join Date
    Jun 2017
    Location
    N. Central Texas
    Posts
    19

    Default Guage reading low oil pressure

    Kinda got a head scratcher:
    I've rebuilt a 70's era GM 4cy. Merc. 120. Its in front of a 90's era Mercruiser outdrive.

    The engine is low hours, it still has the ghost (barely visible pattern) of the factory crosshatch pattern in the cylinders. The rebuild went perfect: New lifters, crankshaft bearings, pistons, etc. The bearing tollerances are text book perfect. The engine was so low time, I did use the original factory oil pump that looked fine. Compression is good enough it will idle down below 650, and after a couple hours, the oil is still yellow.

    The engine couldn't start or run any better. I put new (electric)Farina guages in and re-wired it. It's all perfect, except the oil pressure reads low in the 20-ish psi instead of being in the 30's. It has (Valvoline) straight 30 weight in it like the book calls for. I put a new sending unit on the block, but the reading didn't change.

    The oil flow & such looks fine when I removed the valve cover. At a low idle- 700, its quiet,
    and oil is flying out of the pushrod end of the rocker arms like you'd expect to see.


    I haven't put a different mechanical guage on it, but was wondering if the wire going from the sender to the guage would let it read low if it was too long, or the wrong size. I re-wired the dash with plain stranded 16 guage. I think its getting power just fine, the new alt. is reading 14 volts on the dash guage.


    Thanks for any input.
    I don't think the engine is in danger. If it only had a dash light for oil pressure monitoring-
    it wouldn't light up unless the pressure was below 5 or 10 pounds.
    Its just that as well as it went together,
    and as simple of a engine as it is anyway, I think the pressure should read higher.
    Last edited by Winger Ed.; 12-02-2017 at 10:23 PM.
    OK folks. Enough of this idle chit-chat.
    EVERYONE! Back to your oars.
    The Captain wants to waterski!

  2. #2
    Join Date
    Dec 2015
    Location
    maplevalley WA
    Posts
    3,659

    Default Re: Guage reading low oil pressure

    Is that at idle speed or high rpm? Do you hav room to pull the pan and put in a new oil pump? Normally low pressure is due to crank journals if they were in spec (did you plastiguage) then your oil pump is likely the issue.
    It takes a nickle to go first class!
    If it aint broke dont fix it!!

  3. #3
    Join Date
    Jun 2017
    Location
    N. Central Texas
    Posts
    19

    Default Re: Guage reading low oil pressure

    Quote Originally Posted by kimcrwbr1 View Post
    Is that at idle speed or high rpm? .
    Thanks. That was quick, I'm not even done will all my editing yet.

    The pressure is reading 19-20 at idle.
    With moving, retiring, etc., I haven't run it much yet. About 5 gallons of fuel or so.
    Some in the dock, some on the lake. As rpm increases, the pressue does climb.

    At 2,000 its gets to about 25, then on up to 30 at 2,500-3,000 rpm.
    The engine is so fresh, I've only run it up to 3,500 for a few seconds and it
    did get just over 30 psi oil pressure.

    Its a Alum. hull with some room under the engine to drop the pan,
    not quite enough to get it out from under there though
    To change the oil pump, at the front of the engine--- it'd be like wresteling with the devil.
    Last edited by Winger Ed.; 12-02-2017 at 10:42 PM.
    OK folks. Enough of this idle chit-chat.
    EVERYONE! Back to your oars.
    The Captain wants to waterski!

  4. #4
    Join Date
    Dec 2015
    Location
    maplevalley WA
    Posts
    3,659

    Default Re: Guage reading low oil pressure

    Take and short the oil sender wire to ground with the key on engine off and the gauge should peg high pressure. That will tell you if the wiring is good. I would think you should see around 60psi above 2500 rpm?
    It takes a nickle to go first class!
    If it aint broke dont fix it!!

  5. #5
    Join Date
    Jun 2017
    Location
    N. Central Texas
    Posts
    19

    Default Re: Guage reading low oil pressure

    Quote Originally Posted by kimcrwbr1 View Post
    I would think you should see around 60psi above 2500 rpm?
    I'll ground the gage, and with a dead ground, I'd be real surprised if it doesn't peg out.

    Typically, these older GM engines had lower oil pressure than say- Ford.
    And, I guess that's partly why, on average, they didn't live quite
    as long as some of the other manufacturer's engines did.

    I expected it to be about 35-40 with a factory pump.
    As good as the head & block looked, no noticable wear on the starter brushes,
    it seemed about in as good of shape as a car engine with about 15,000 miles on it.
    I used the oil pump again because it looked so new inside, and years ago,
    the boat ran great. I rebuilt it only because it hadn't been done
    since the mid 90's, it was a easy project to do (no machine shop services needed),
    and had sat up for almost 20 years.


    .
    Last edited by Winger Ed.; 12-02-2017 at 11:35 PM.
    OK folks. Enough of this idle chit-chat.
    EVERYONE! Back to your oars.
    The Captain wants to waterski!

  6. #6
    Join Date
    Sep 2003
    Location
    New Tripoli, PA, USA!
    Posts
    15,292

    Default Re: Guage reading low oil pressure

    Before you do anything e;se, put a reliable mechanical gage on it and see what's happening. I don't trust electrical gages with their 'miles of wiring' and possible bad connections.

    Jeff

  7. #7
    Join Date
    Dec 2006
    Location
    Chaumont Bay, N.y., USA
    Posts
    5,238

    Default Re: Guage reading low oil pressure

    The pressure is reading 19-20 at idle.
    With moving, retiring, etc., I haven't run it much yet. About 5 gallons of fuel or so.
    Some in the dock, some on the lake. As rpm increases, the pressue does climb.

    At 2,000 its gets to about 25, then on up to 30 at 2,500-3,000 rpm.
    The engine is so fresh, I've only run it up to 3,500 for a few seconds and it
    did get just over 30 psi oil pressure.
    Ayuh,.... That's well within the standard spec of 10 psi per 1,000 rpms,.....
    Any Grease is Better,... Than No grease at All....

  8. #8
    Join Date
    Jun 2017
    Location
    N. Central Texas
    Posts
    19

    Default Re: Guage reading low oil pressure

    Thanks guys.

    No harder than it gets used, I didn't think the engine was in mortal danger.
    2,500 rpm is about as fast as I ever spin it, and 20-ish psi oil is OK for that,
    especially with the modern generation oils.

    I just wondered if anyone else had low pressure readings from a long sender wire.
    Electric guages are basically a glorified voltage or amp meter.
    I know there is a voltage drop on long extension cords with a heavy load being pulled.
    This sender wire is about 12' long, and I'm not sure if the wire size mattered.

    Its a Franken boat to start with. The hull is a 1968 Cherokee 21', it originally had a
    white Mercruiser that you had to lean over the back to latch down the outdrive
    for reverse-- no hydraulics, just 2 shock absorbers. The engine is a 1977, from the
    bellhousing back its the remains of a salvaged 1993 Alpha One Gen II.
    All the old boat had back in the day was a tach. and a voltmeter. On restoration,
    I put full instruments in the dash, and it seemed like the oil pressure should read higher.



    .
    Last edited by Winger Ed.; 12-03-2017 at 05:48 PM.
    OK folks. Enough of this idle chit-chat.
    EVERYONE! Back to your oars.
    The Captain wants to waterski!

  9. #9
    Join Date
    Sep 2003
    Location
    New Tripoli, PA, USA!
    Posts
    15,292

    Default Re: Guage reading low oil pressure

    As I said, install a temporary mechanical gage and see what's happening!

    Jeff

  10. #10
    Join Date
    Nov 2012
    Location
    South of Boston
    Posts
    63

    Default Re: Guage reading low oil pressure

    Did you use break in oil? It's usually lower viscosity. After break in make sure you use oil with zinc additive to protect the flat tappet camshaft and lifters.

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