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  1. #1

    Merc 150 running hot a lower RPM

    Had an issue with the temp getting up to around 180 running at RPM below 3000. Idles at normal temp but with, to me, a weak stream, replaced water pump and all housings as well as popit rebuild and seemed to be ok but didn't get to run it much. Haven't been able to run it since last summer took it out yesterday and started heating up untill RPM gets around 3000 again and temp falls back to about 140. Water pressure is around 3psi at idle and comes up with RPM to around the expected 15psi.

    Any thoughts before I tear into this thing again? I'm new to these black engines and search this site often but haven't found this issue addressed, at least so far and any thoughts are appreciated.

  2. #2
    Join Date
    Apr 2001
    10 minutes from the lower Fox
    Watch the water pressure gauge when you are under 3000 rpm but running under load. You may find that you can't run it at certain speeds without it getting hot. I have a 150 Merc and can't run it under about 2600 revs without it getting hot because the poppet valve isn't open yet at that speed and it's under too much load for the thermostats alone to flow enough water. If I'm going at a slow cruise and see the water psi up around 15, I know it's going to get hot eventually. Once I'm going fast enough for the valve to open up and have the pressure drop down to around 8 or 10 psi, then it will flow enough. I've found it to be the nature of the beast. Idle is fine, and once the valve opens it's fine, but a fast mushing or barely planing speed can be a problem. Mine will go right up between 200 and 220 if I let it. If I avoid that 1500 - 2600 rpm range it will run 150-170 all day long no matter what I'm doing.

    What you see from idle to top end is a rapid rise in water pressure, then a sudden drop to about 8 psi when the valve opens, then a slow buildup in synch with the rpm up to top end. Everyone says 15 psi is adequate for top end on a 2.5L Merc fishing motor but if you're getting enough water to the intakes it should be 20+ at the rated rpm.
    Last edited by Ron V; 03-02-2013 at 05:34 PM.
    Ron V

  3. #3
    Does it have thermostats in it? Could be dumping too much water or the water tube/ adapter plate is corroded out.

  4. #4
    Take the poppitt spring out and cut a coil off. This will allow it to open sooner and should solve your problem.

    Life Begins At 100 MPH

  5. #5
    It does have thermostats as I replaced them last year when went through the rest of it. Hadn't had an outboard with a poppet before so was unsure about that section of it. The boat is light so it is not working hard at all at 2500 and the fastest speed I need to run on some of the creeks. I have another poppet kit so I will try the spring trick and see what it does. Thanks for the comments so far!

  6. #6
    Confirm your gauge accuracy first, mine reads 10 high.

    Countdown to Romp
    Ventura II 40th Anniversary Tour

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