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Thread: Merc 200 overheating

  1. #1
    I am looking at purchasing a boat with a 89 200hp Mercury outboard. The owner said that last year the water pressure fell to 10 psi and temps started running hot. Could this be the impeller? The motor is run hard alot. If not the impeller, would a complete hi performance rebuild solve the problem? How much could this engine be built to within reasonable expense? Thank you all you outboard fans!! I appreciate your help!!!!!!

  2. #2
    Boat Racer
    It could be the impeller, or if you live in an area with muddobbers, it could be as simple as a muddobbers nest in the intake holes or outlets for the heads...
    As for performance work, you could do some exhaust work and have the port timing raised, and let's not forget, head work to get it to scream!!!!

  3. #3
    I have quite a bit of experience with virtually the same motor (1988 2.4L 200 Black Max).
    First off, 10 PSI on water pressure is getting a little low, so yes.....this is most likely the reason the motor started to get hot.
    I have some questions which will give me a better understanding:
    1. The question is why is it only getting 10 psi? Is it on a jack plate where the guy jacked the motor up and ran it out of water? If not, it is most likely an old water impeller.
    2. Did the motor actually overheat or did it just start to get warm? How hot did the temp guage get?
    3. What kind of hull is it on? A tunnel? What kind of RPM's does the current owner spin it?
    4. Have you done a compression test on it?
    In good condition, that motor should put out about 125 PSI compression per cylinder (stock). You should also maintain about 12-15 lbs. of water psi. My motor always ran hot until I fried it. If I had that motor again, I would remove the thermostats and run stainless washers. This will increase the water flow and keep the motor cooler.
    If you remove the exhaust liner, drill out the midsection, cut the heads 50 thousandths (about 150 PSI compression), and run Boyeseen Reeds, you should see about 230hp or so. The next step after initial mods would be to do a mild port job, and cross drill the intake. I think you could get about 240hp - 250hp out of that motor fairly easily. Just make sure to run washers instead of T-stats. Also, make sure to run solid motor mounts.
    Good Luck
    PS..........I think a good rpm range is about 6,700 - 6,800 or so. I wouldn't rev it anymore than 7,000 with out putting in aftermarket pistons, rings, and rods. Make sure to disconnect the oil injection and run 32:1 with a quality synthetic lube. Mercury makes good stuff. I run Klotz KL-333. Others like Amsoil.
    [ April 25, 2003, 08:50 AM: Message edited by: HavasuDreamin' ]

  4. #4
    Boat Racer
    One thing neither of us asked, or mentioned. Are you carbed. or inj.? Of course the cross drilled intake is for carbs., but if you do ANY of the mods. we mentioned, you MUST adjust the fuel curve to handle that hungry beast!
    DREAMIN' it's been a long time since I heard anyone talk about a cross drilled intake... My father runs one on a 2.4 BP! It'll turn a little bit more than 7000rpm.!!!! Come on down to CUMBERLAND sometime, GATOR #1, ask for ROOSTER
    [ April 25, 2003, 08:19 PM: Message edited by: Boat Racer ]

  5. #5
    Just to let you know, a cross drilled intake is a performance thing of the past. More recent studies have found it to be very bad for low rpm power, and only marginal for very high rpm gains (above 8000rpm) Cross drilling the intake destroys the crankcase volume, which is very important, and also make the motor much more difficult to jet properly, and troubleshoot. Most likely, you over heat probblem is the impeller, but it is possible that the motor was overheated, and has burned the little seals on both ends of the water pipe. Throw in a new pump kit (not just an impeller) and if the problem doesn't improve, pull the powerhead and check the seal on the bottom of the exhaust adapter.

  6. #6
    Boat Racer
    Ducky: good to know that about crankcase volume, will add that to the notes...
    That is a 86 model, that was worked on, in 89 and has not been touched since(except for plugs and impellers) he runs 86 & 88 jets in it at 7700(max)rpm. with a 28 chopper on a boat that dry hulls at 900lbs....
    What the cross drilled manifold does, is allows you to throw alot more fuel to it,and because of the exhaust port work, it wants that fuel, THAT is one reason it hasn't been tore down in 14 years, and still holds 165 pounds in all 6, with dual plugged 26cc LAND AND SEA heads....
    Some people like them (the manifold)and some don't, but if you find the combo that works, you stick with it... It does hurt on the hole shot, but again, this is a fishin' boat that may go 10-15 miles, at one time, WOT, just to catch a dang fish!!!!
    Again, thanks for the info about the crankcase volume!!!!
    [ April 26, 2003, 06:37 PM: Message edited by: Boat Racer ]

  7. #7
    thanks alot guys for all the help!! I am relatively new to the outboard thing. Used in i/o's. this engine is on a checkmate starflite w/ a 14 inch manual jack plate (which I wonder if it is a little overboard) It does have lowwater pickups and a nose cone. also the heads have been milled and boyeson reeds put in. I guess my question much would a "built" replacement powerhead run? Also a replacement engine? Worst case scenario with this engine, what are the odds it is just the impeller, versus a major block problem? I would assume that the impeller would be the first thing a reputable mechanic would check, right? I just dont want to buy the boat, and then have to rebuild/replace entire motor. Wifey may not approve! Thanks again for all your help!!

  8. #8
    Boat Racer
    Do a compression check, remember to have the butterflies open, if there's not one real low then I'd bet on the impeller...
    Tell the owner to pull the lower unit. To completely replace the ENTIRE unit, and I mean impeller housing and all the gaskets and then get lower unit back on should take an hour or less.... But just looking at it, if she's missing fins or really stiff and curled up, it would be my bet!!!!
    It could be the jackplate causing the problem. It's hard to say sitting here and not seeing it in person, GOOD LUCK!!!!!
    [ April 27, 2003, 06:35 PM: Message edited by: Boat Racer ]

  9. #9
    Tom Brown
    ... or it could be leaking between the LWP and impellor. Also, it could be the down tube between the dividor plate and the water pump. Mine was a little too short when I bought my motor. It's just a piece of 1/2 inch copper pipe so it's cheaply/easily made.
    My understanding is that 10 psi at WOT isn't enough and that if it has been run very much at that pressure the cylinders will egg. I would do a leak down test before buying it.
    [ April 27, 2003, 08:32 PM: Message edited by: Tom Brown ]

  10. #10
    Boat Racer:
    I am going to be down at Dale Hollow for 4 days July 26th - 30th. You head down there much?

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