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  1. #1

    Question 1990 OMC 4.3 Oil Pressure Drop and Stall

    Hi All,

    I'm a newbie to this forum and to boat ownership. Like a lot of folks, I know automotive well, but am trying to get to grips with the differences in boating.

    I recently bought a 1990 Four Winns 190 Freedom with the GM V6 based OMC 4.3 in it. I've done a partial engine rebuild (lifters, con rod bearings, gaskets etc.). Also put all new ignition (coil, distributor, wires and plugs) in, replaced both water pumps and fuel filter, but still have an issue.

    At cold, it starts first turn and runs great. I get it up to about 3,500 rpm on the lake, can run for 10-15 minutes, no problems. Oil pressure and temperature bang on normal for the whole period. Then, oil pressure starts to drop, revs drop and it almost goes into a limp mode, but if I drop it to idle, it cuts out and won't restart until it's cool, then it's absolutely fine again, until the same thing happens. Thought it might be temp, fuel or ignition related, hence all the new parts, but no luck. I've had it on the bellows for over 1/2 hr out of the water and can't get it to re-create the problem.

    Has anyone else experienced? I replaced the oil pressure sensor as well yesterday, so it's not that. Could it be the oil pump?

    Any good ideas would be very welcome. Starting to pull my hair out in frustration.

    Thanks.

  2. #2

    Default Re: 1990 OMC 4.3 Oil Pressure Drop and Stall

    Hi All,

    Thanks to those who took the time to read this, hope someone can help.

    Updated information: The oil pressure issue is a red herring. I just replaced the main ground having found it very badly corroded. Also fitted a new fuel pump (a couple of days ago). The new ground has fixed what was an ignition issue.

    The boat will still only run 10 minutes under load at 3,500rpm, then dies without losing oil pressure (the bad ground was dropping voltage on the oil pressure sensor and ignition). So now, it will immediately restart idle normally and happily continue. I did only run it at 1,500rpm after, but it kept going very happily.

    So, I think I've got it down to two possible issues. Is it being starved of fuel (cleaned the carbs again this morning, new pump, new filter)? I doubt it as straight after it dies, if you prime it, the carb jets spray fuel into the intake. But that may be because pressure has come back up. It does have a full tank of clean fuel, but no idea what gunk may be still lurking in the tank on a 30yr old boat?? It's a major job to pull the tank and drain it, but may have to do it.

    The other possibility is that there's white smoke coming up the valve cover breathers to the air filter. Does this indicate dying head gaskets? Maybe. I'm planning to replace them over the winter. But why would that cause it to die so quickly, but then restart? Surely any leaks from that would lose power and eventually die, but you wouldn't expect it to restart??

    Or could timing cause it?

    I'm making progress, but still not quite there.

    Again, any suggestions would be very welcome.

    Thanks.

  3. #3
    Join Date
    Mar 2009
    Location
    Portland, Oregon,
    Posts
    11,009

    Default Re: 1990 OMC 4.3 Oil Pressure Drop and Stall

    Quote Originally Posted by Sleathy View Post

    Or could timing cause it?
    Yes indeed! Correct ignition advance is extremely critical for the Marine gasser.

    Some gasoline Marine engine are more prone to detonation. GMs 4.3L with the GM full dished pistons is one them!

    Detonation can be caused by several factors, one of which is too much ignition lead.
    We call this "ignition induced detonation."
    Detonation will raise cylinder temperatures, and if gone Un-checked, will cause severe engine damage.
    Just prior to the damage, the engine will loose power.
    And again, if gone un-checked, it will burn the tops of the pistons.

    To eliminate this as a possibility, you will want to look and see what the progressive advance is doing up to approx 3,500 RPM.

    Compare what you see to your OEM ignition advance curve.
    Rick ... aka Ricardo..... AQ series Volvo Penta repair
    Portland, Oregon
    28' SDN F/B w/ twin Volvo Penta 5.7L DP drives

    "Intellectual growth should commence at birth and cease only at death." Albert Einstein

    Please... no PMs! Post your questions on the forum.

  4. #4
    Join Date
    Apr 2013
    Location
    Lower Columbia River
    Posts
    137

    Default Re: 1990 OMC 4.3 Oil Pressure Drop and Stall

    I suspect you do indeed have crud in your fuel. It's probably clogging the fuel pickup screen (if there is one), or the fuel anti-siphon valve which should be located on top of the tank between the top of the pickup tube and the fuel line. You could try running your engine off of a portable fuel tank to see if the problem is due to fuel.

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