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Merc 454 losing oil pressure

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  • Merc 454 losing oil pressure

    Trying to get some other views on my problem here
    Motor- 89 Merc 7.4 330hp w/ 279 hours
    Had the boat out a few times this year and the last 2 days I've had this problem.
    Starts up w/ 35-40psi @ idle. Runs great, get up to 2500rpm goes to 45psi.
    Once it gets into operating range it is at about 25-30psi. I have about a 25 minute ride out to when I'm no longer in the no wake zone and I can get into it. About 10-15 minutes of 3200+ RPM's I notice that this engine just slowly starts to lose oil pressure. Over a course of about 20 seconds it drops right to zero. Obviously I shut it down let it cool off etc. Start it back up and PSI is right back to 20+ at idle.
    Check the bilge no oil from either motor is in the bilge. No leaks, no water in the oil. Oil level is exactly where it was before the run.
    Was hoping it was a bad sender, replaced it. Next day Same run as the day before same thing happens right about at the same time.
    I'm off to buy a mechanical gauge hoping that somewhere there is a short to the gauge. There is no ticking, no knocking, motor doesn't heat up.

    Any ideas? This boat is nowhere near as fun on 1 motor as it is on 2. Sorry for the long post just wanted to get all the info out I had.

  • #2
    Re: Merc 454 losing oil pressure

    You are on the right track... verify with a mechanical gauge. If its reading good, I would ground out the sending unit wire to verify gauge operation, etc. Do you have an alarm? Could be the wiring on the alarm?
    1990 Stingray 536ZPX 4.3 Mercruiser - SOLD
    2001 Glastron GX185 4.3 Mercruiser - SOLD
    2012 Yamaha SX190 1.8L Jet Boat
    Tow rig - 2013 Silverado 2500HD Duramax


    • #3
      Re: Merc 454 losing oil pressure

      No alarm on it. I know it sounds crazy and I'm in the works of finding a louder alarm. Twin big blocks with through hulls the only time I ever heard the alarm on it is was when I left the key on without the motors running. I just picked up a mechanical gauge from my friends shop and I plan on putting it in the water tomorrow and seeing what it does. I have read a few things online about a 30# relief spring within the oil pump pickup that can be faulty but other than a few posts on a couple websites I've never seen/heard of one of these going bad.
      Going to pull the dash today and check all the terminals on the back of the gauges and hope that something is faulty.


      • #4
        Re: Merc 454 losing oil pressure

        You have an '89 with only 279 hours? That's just barely broken in. My '92 has 1275 hours on it.

        What oil you using? May be a little thin when it's hot, if it's wrong type. There are a ton of reasons oil pressure drops off:

        1. Pump Relief valve stuck. Have seen them stick before, but usually on a high milage engine that has had poor maintenance / few oil changes, causing them to gunk up. They will open on a cold start with cold, thick oil and stick there.
        2. Bearings
        A. With engine noise/knocks = Rod / main
        B. Without noise = cam bearings
        3. Lifter bores too much clearance
        4. Bad gauge / sender
        5. Not common, but a gunked up oil filter that doesn't have a relief / bypass valve in it can do that.

        The list goes on. I would go after the gauge / sender and change the oil / filter first. If you aren't getting lifter noise, chances are you don't have "zero" oil pressure. With no oil pressure, the lifters will make a racket. Manual for my '92 7.4L states oil pressure at 2,000 RPM is 30-60 PSI. Minimum oil pressure is 4 PSI.
        '92 Chaparral 2550 SX Sport
        454 BBC Mercruiser Bravo 1
        Tow with '04 F150 Larriat SuperCab 4x4 5.4 Triton
        US Coast Guard Aux.


        • #5
          Sign up today
          Re: Merc 454 losing oil pressure

          Yeah its a 89 with 279 hours. Bought it with 250 end of last season already put more hours on it that the previous owner did in about 5 years. Used to work with the guy I bought it off of he barely used the thing (which is what my concern is for bad seals/bearings). Unfortunately I haven't gotten a chance to take it out yet that damn 4 letter word called work is getting in the way. The oil (20-50) is fresh with about 10 hours on it along with the filter, I'm going to change the filter just because maybe something is plugged up inside I can't tell and it's a cheap part. I figure at 3200rpm if it really had 0 PSI then I'd be hearing some serious noise out of top end (and lower I guess). I'll update this weekend with my mechanical gauge and fresh filter and hopefully have good results!