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  1. #1

    1993 Mercury 175 EFI burnt #2 Piston ??? Modified to Promax

    Hello, need some advice, I have a 93 Mercury 175 EFI 2.5 , it had the 225 promax ECU added, boysen reeds, light weight flywheel, high compression performance heads, bobs oval tuner, sportsmaster low water unit with 30 pitch mazeco prop.

    It has performed flawless for 5/6 years and number 2 piston ended up burning the temp sensor came on as number 2 is beside the sensor and the boat died in water, i restarted boat, ran for 5 minutes and did it again, the fuel pressure is perfect with only 2 psi of leak down, the injectors spray well with no signs of dirt, trying to understand what took place, is it possible it was over propped as it would only turn 5400 RPM when wide open and could that have caused detonation? All other cylinders are ok except 1 other piston that shows light signs of detonation starting.
    The pistons were factory cast.

    Any Thoughts? I appreciate any help

  2. #2
    Overpropped,and something else pushed it over the edge...octane,load temp,etc.You were living on the edge at that rpm,and fell over.When you fix it,prop it to 6200 at least,Chris

  3. #3
    Thank you Chris, I have a 25 pitch when I get it fixed, the heads are a performance mercury head, how can I tell if they should have higher octane ? I have run 93 octane at all times and several times mixed cam 2 to make sure. Is there a rule of thumb for mercury part # heads to go by on what octane to run?

  4. #4
    Join Date
    Sep 2001
    Pekin, IL
    Cranking compression much over 145 you need to look into additive or race fuel. And Chris is right about the rpms.

    Team Junk

    No sparkling wiggles in here, only dump truck grinches.

    "Screamin Heathen"

  5. #5

    One last question

    Quote Originally Posted by Chris Carson's Marine View Post
    Overpropped,and something else pushed it over the edge...octane,load temp,etc.You were living on the edge at that rpm,and fell over.When you fix it,prop it to 6200 at least,Chris
    Only one cylinder needs replaced, I keep hearing different opinions, some say to resleeve and replace the one piston, some are saying that since it didn't mess up the bore, clean out the aluminum (acid) and hone replace new piston, others are saying bore it all regardless, replace the all rod bearings, bore all cylinders new forged pistons ? The engine was a nice original low hour engine, my gut says to rebuild it all, but the majority is saying fix the one piston and go back to work after propping it correctly


  6. #6
    If you're not wanting to do more performance work you can clean the aluminum out and hone with a truing hone,NOT a ball hone,and then see what you've got.If you're round again and not more than .003 oversize you could replace the piston and rings and go again.If you choose this route you should stuff the exhaust port with foam rubber,and the rod slot with foam rubber and seal it completely with melted wax,so NO grit can get into the crankcase...if it does you'll never get it out and it will make a mess of things.This is assuming you want to leave the motor together and just pull out the hurt piston.If it is in servicable specs,put it through an extensive washout,remove the wax and foam and rewash,crank up,cylinder down,oil and put it together.
    At the first start leave the spark plug out on the replaced cylinder,and flush through the bleed line with oily solvent or diesel fuel,to give it a final rinse... If you got it clean enough it will be fine.
    if it dose'nt clean up,suit yourself about repair or rebuild,carefully check the ring placement pins in the pistons if you want to reuse them,older ones come loose...I don't reuse any pistons.I also am not in favor of forged pistons in sport motors,ie,under 7000 rpm's,Chris

  7. #7
    I guess I should have asked,are you going to do the work yourself,and how familiar with this kind of thing are you?,Chris

  8. #8
    Glad to asked Chris, I have rebuilt a 40 hp mercury and a 84 150 many years ago, I'm trying to decide to take on myself or send it off, I really enjoy doing the work and don't mind, its the starting on the right foot that gets me and that's why I am here, I can send you photos of the engineand current state? I know a good mechanic locally that will do the work for 3900 , ( bore 20 and rebuild) I have a wife in school and no funds to do that for two years so trying to get this baby back in action so I could have some sort of summer to look forward to.

  9. #9
    By the way Chris, both 40 and 150 Mercurys I rebuilt only had bore, pistons , rings, pins, etc. no crank work as the crank and bearings were solid and tight, so this would be my first complete rebuild unless I did the same.

  10. #10
    Join Date
    Jul 2002
    bore .020 means wiesco piston YIKES i know Chris he is a very good company to do business honest and great service not raining on any ones parade, i rebuild 25-30 a year personally i have had Zero luck with the acid wash cylinder deal personally i would bore it .015 buy a good cast piston from Pro marine and if the other bores are within .003 and round rering and new bearings and go from there have some one hone it who has a sunnen hone it should not be close to 3900 for that kind of money you could buy a rebuilt 200 power head just my .002

  11. #11
    Join Date
    Aug 2002
    Palm Harbor, Fl../ lake tarpon
    what type of boat is this on
    how low is the motor
    i havea 95 175 i run all the time on my mirage (few times a week) and i dont see a 30 over pitching it at all
    i was thinkin of trying a 225 pro max box on mine but figured it would be way to rich as the porting isnt like the 200 or the 225

    and tex im with you i just did the same to one and it only cost a couple hundred with the piston .boring .and all new gaskets that are needed to put it back togethr
    Last edited by toyomirage02; 02-03-2013 at 10:05 PM.

    02 mirage ski racer w/ 2.5
    lake tarpon (LTB racing)

    "Mirage Boats... often imitated but never duplicated"

  12. #12
    Quote Originally Posted by Boatbeforebeer View Post
    I know a good mechanic locally that will do the work for 3900 , ( bore 20 and rebuild).
    $3,900 That's all? lol. Hell for $3,900 I'll come get it and drop it back off

  13. #13
    93 Procraft SP180 is the boat, I'm knew to this site, I guess I can attach pictures, let me send some with the motor , boat etc to see, max rpm it was 5400' if I lifted the bow it would come up to 5600 and rise but hold on tight it would begin to flop left and right bad , it was enough to make you say a prayer within seconds of trimming up , I tasted my man hood quite a few times without meaning too. The boat up to that point ran really well.

  14. #14
    Chris, can I call you by phone ?

  15. #15
    Quote Originally Posted by Boatbeforebeer View Post
    Chris, can I call you by phone ?

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