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  • Overheating Mercruiser 7.4L MPI

    Hey guys, I think I have a bad circulating pump but I wanted to get some outside opinions before I order a part.

    Engine: 1999 Mercruiser 7.4/454 MPI (not magnum) with raw water cooling (serial number 0L347xxx) with Bravo 3 outdrive. Actually it's dual engine but only having the problem on port engine.
    What I've tried: Rebuilt the raw water pump (it was needed anyway after it it destroyed an impeller last year due to a crack inside)
    Symptoms: Engine gradually reaches high temperature while idling, somewhere around 200F (and yes, it's actually overheating, the thermostat housing was HOT!). There is some cooling happening, but it's clearly not where it should be. One of my other fears is that part of the impeller that disintegrated last year is still in the loop somewhere causing a partial blockage, but I had the boat out twice after replacing the impeller (but prior to the full rebuild). The impeller I replaced today didn't have any pieces missing. Also, I took the large hose off the thermostat housing and poured water into it and hot water flowed out of the top of the manifold from the engine. I did this both to check for restrictions and to cool my engine down. Lastly, the weep hole on the circulating pump appears to have serious corrosion, probably from water, but it was dry when I inspected it. The same hole on the starboard engine shows no signs of corrosion.

    Thanks for the sanity check, I'm fairly sure I correctly identified the problem, but I'm not a mechanic so I wanted to be sure before I ordered parts.
    Dan

  • #2
    Google bravoitis

    Look for missing impeller parts
    Cheesehead boating the Gulf Coast of FLA 27.51° N, 82.53° W

    1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - https://forums.iboats.com/forum/owner...988-rogue-2420

    Past Boats
    1970 Wooster Hellion - Merc 9.8
    2002 SeaRay 190BR - 5.0 - A1G2 - "Cheeseheads in Paradise"
    1984 Avanti 170DLI -3.0 stringer- "Ship Happens"

    What’s behind you doesn’t matter.Enzo Ferrari

    Comment


    • #3
      Originally posted by Scott Danforth View Post
      Google bravoitis

      Look for missing impeller parts
      Googled, but that doesn't seem like the issue. I say that because the flow of water from the drive is VERY strong, when I went to rebuild the raw water pump the hose was gushing water until I elevated it above the water line. Also, the failure of my raw water pump previously was due to a crack in the housing, likely from the previous owner having overtightened the bolts. That's why I did the rebuild. Also, the previous impeller death was due to the crack causing the plastic "bridge" across the output hole having broken away, leaving a hard edge that the impeller was going across constantly. (kinda hard to explain). When that happened I didn't have a new housing and had to just put a new impeller in. I tried to smooth the edges of plastic to reduce damage to the new impeller, but when I went to replace the housing today the impeller was 95% okay, but there was no reason not to replace it since I was doing the rebuild anyway and I had extra impellers. I tried finding a pic to better explain this but no such luck.

      Comment


      • #4
        I went down this road last year with the port engine on my F-330. . . it was a lot of 'fun'

        Here is a link to my thread . . .

        http://forums.iboats.com/forum/engin...unning-hot-ter

        I have a couple of questions . . .

        1) First off . . . are you in salt water or un-salted?

        2) The engine slowly over heats at idle . . . does it overheat when cruising, or if you raise the RPM slightly above idle?

        I chased a cooling issue all over my engine last year, only to find that a small sea shell had grown inside the outdrive in past years and then dislodged, getting stuck at the fitting of the inlet hose in the area of the bell housing (transom assembly). The only evidence of this restriction was about a 15-20% reduced flow of sea water from the port engine versus the starboard engine.

        I had checked everything else, and the fact that I had closed cooling (rather than open - raw water cooling) compounded matters. I too checked the re-circulating pump . . . it was perfect.

        Anyway, without too much disassembly, you could do a comparison test of the raw water flow from the sea water pump of each engine. . . with a little creative plumbing. I just timed how long the sea water took to fill a 5 gallon bucket and compared that for each engine (assuming warm engine and at idle speed of about the same)

        You could also back flush the oil coolers, if not already done so.

        My transom assemblies were showing some signs of Bravoitis, but it was not the contributing factor to the over heating . . . it was the restriction at the bell housing caused by the sea shell.

        Anyway, cooling system diagnosis and resolution can be evasive. With raw water cooling it is a bit simpler . . .

        Key factors are

        - Sea water pump
        - Inlet hose restrictions
        - restrictions at the oil coolers
        - restrictions at the exhaust manifolds and/or elbows

        Not saying that it is not the re-circulating pump . . . just don't be disappointed if it looks fine.

        Last edited by tpenfield; April 29th, 2017, 06:38 AM.
        Best regards, Ted . . . . Cape Cod, MA

        Current Boats: Formula 330 Sun Sport, O'Day Mariner Sail #3224, Sunfish
        Past Boats: Catalina 22 Sail #10531, Formula 242 Sun Sport
        Twin Mercruiser 7.4 LX MPI (0F802036, 039), Bravo 3's (0F806198, 199), Mercury 7.5 HP (1969), Johnson 4.5 HP (1980)

        My Boating Web Pages: http://www.tpenfield.com

        Comment


        • #5
          Originally posted by DracoDan View Post

          Googled, but that doesn't seem like the issue. . . . Also, the previous impeller death was due to the crack causing the plastic "bridge" across the output hole having broken away, leaving a hard edge that the impeller was going across constantly. (kinda hard to explain). When that happened I didn't have a new housing and had to just put a new impeller in. I tried to smooth the edges of plastic to reduce damage to the new impeller, but when I went to replace the housing today the impeller was 95% okay, but there was no reason not to replace it since I was doing the rebuild anyway and I had extra impellers. I tried finding a pic to better explain this but no such luck.
          Been there, done that. Usually when the 'bridge' breaks off in the plastic housing . . . the impeller is immediately shredded by the sharp edge. I'm not sure how effective the pump is without the bridge intact. Anyway, if you have completely replaced the housing, and impeller, all should be good in that regard.
          Best regards, Ted . . . . Cape Cod, MA

          Current Boats: Formula 330 Sun Sport, O'Day Mariner Sail #3224, Sunfish
          Past Boats: Catalina 22 Sail #10531, Formula 242 Sun Sport
          Twin Mercruiser 7.4 LX MPI (0F802036, 039), Bravo 3's (0F806198, 199), Mercury 7.5 HP (1969), Johnson 4.5 HP (1980)

          My Boating Web Pages: http://www.tpenfield.com

          Comment


          • #6
            Originally posted by tpenfield View Post
            - Sea water pump
            Sea water pump has been ruled out.
            Originally posted by tpenfield View Post
            - Inlet hose restrictions
            I don't THINK there are inlet hose restrictions. Like I said when I went to replace the pump (below water line) the inlet connection had a powerful flow of water until I elevated it.
            Originally posted by tpenfield View Post
            - restrictions at the oil coolers
            Sorry if this is a dumb question, but where are the oil coolers on a raw water cooled setup? What do they look like?
            Originally posted by tpenfield View Post
            - restrictions at the exhaust manifolds and/or elbows

            Comment


            • #7
              Pull the drive and look to see if you have bravoitus. Cant assume you dont by your methods. You may not think you have a restriction, however unless you eplore, you CANT rule it out

              Follow the raw water path. Suction side of pump should be the power steering cooler, pressure side should be oil cooler.

              Pull the exhaust manifolds and risers to look for restrictions
              Cheesehead boating the Gulf Coast of FLA 27.51° N, 82.53° W

              1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - https://forums.iboats.com/forum/owner...988-rogue-2420

              Past Boats
              1970 Wooster Hellion - Merc 9.8
              2002 SeaRay 190BR - 5.0 - A1G2 - "Cheeseheads in Paradise"
              1984 Avanti 170DLI -3.0 stringer- "Ship Happens"

              What’s behind you doesn’t matter.Enzo Ferrari

              Comment


              • #8
                Originally posted by Scott Danforth View Post
                Pull the drive and look to see if you have bravoitus. Cant assume you dont by your methods. You may not think you have a restriction, however unless you eplore, you CANT rule it out

                Follow the raw water path. Suction side of pump should be the power steering cooler, pressure side should be oil cooler.

                Pull the exhaust manifolds and risers to look for restrictions
                This is by far the toughest way to troubleshoot for me. The boat is in the water at my marina which doesn't have a lift, and I don't have a trailer for it. Last time I did a short haul it was like $400. Once I've exhausted all other options I will consider this.

                Comment


                • #9
                  Have you tested the raw water pump output with the boat in the water? If you can't pull the boat, that is a good place to start, if your raw water flow there is good, (look up Merc's specs) then the clear hose test (if the pump is pulling in air) and really checking your manifolds and risers for clogging (hard to do with the boat in the water because its risky, could flood the boat with water) are the next steps. Keep in mind that these raw water systems require a large volume of cool water IN and OUT, they are not pressurized like a closed system so if the raw water flow is lower than normal, it cannot carry away the heat.

                  If your raw water flow from the impeller is low, then you have to pull the boat. No other way to fix it right. Then go through the whole water intake system, from the outdrive water intakes, to the transom fitting and hoses, oil coolers, etc. Like Ted I had a restriction, that is common on boats moored in salt water, I had barnacles growing in the water intake area of my Cobra drive. To get them all out, I had to split the upper and lower drive housings, remove the plastic water screen (I did not replace it because I always had some barnacles growing on this) clean out the passage, and replace the upper and lower water tube gaskets. And that, is what solved my running hot problem I chased for a season or 2. Not really over heating, but an increase from 160-165 to 185 or so.
                  1988 Four Winns 200 Horizon
                  4.3 OMC Cobra

                  98 Jeep Grand Cherokee 4.0 Selectrac
                  07 Jeep Grand Cherokee Limited 5.7 Quadradrive II

                  "While air doesn't freeze....rust never sleeps"

                  Comment


                  • #10
                    Originally posted by DracoDan View Post

                    This is by far the toughest way to troubleshoot for me. The boat is in the water at my marina which doesn't have a lift, and I don't have a trailer for it. Last time I did a short haul it was like $400. Once I've exhausted all other options I will consider this.
                    I had a similar predicament . . . $330 to pull the boat and another $330 to go back in. So, I did what I could while the boat was in the water, and it was not until the end of the season after the boat was hauled for the season that I found 'the shell' in the outdrive inlet tube.

                    My approach is to do the easy stuff first, and you have done some of that already.

                    Is this salt water or fresh water ???

                    To answer some of your questions. . .

                    Here are the various coolers that are in line between the sea water pump and the thermostat housing where the raw water first enters the engine.

                    Click image for larger version

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                    I am assuming that since your engine is a 1999, that you have the 'cool' fuel system and not the VST system. So, there is an additional cooler as part of the fuel system. So, you can inspect/backflush etc. those 'coolers' to see if they have any trapped impeller fragments or marine life debris clogging them.

                    As for the exhaust . . .

                    If you get to that point, then the water passages for the exhaust manifold that lead up to and through the exhaust elbow can be inspected by draining down the manifolds and taking the elbows off. You will need a new set of gaskets to re-assemble them. The old gaskets will not be reusable.

                    This really comes into play if you boat in salt water rather than fresh water.
                    Click image for larger version

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                    I'd do the coolers first and see where that leads.

                    FWIW - I tried to inspect for bravoitis while the boat was in the water using an inspection camera/borescope, but I could not really see past the fitting on the inside of the transom.
                    Attached Files
                    Last edited by tpenfield; April 29th, 2017, 11:35 AM.
                    Best regards, Ted . . . . Cape Cod, MA

                    Current Boats: Formula 330 Sun Sport, O'Day Mariner Sail #3224, Sunfish
                    Past Boats: Catalina 22 Sail #10531, Formula 242 Sun Sport
                    Twin Mercruiser 7.4 LX MPI (0F802036, 039), Bravo 3's (0F806198, 199), Mercury 7.5 HP (1969), Johnson 4.5 HP (1980)

                    My Boating Web Pages: http://www.tpenfield.com

                    Comment


                    • #11
                      Sorry, Fresh water.

                      Comment


                      • #12
                        At a bit over 1000 rpm, the raw water pump shuld fill a 5 gallon bucket in under 15 seconds. 20 seconds if you have a bit of restriction.
                        Cheesehead boating the Gulf Coast of FLA 27.51° N, 82.53° W

                        1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - https://forums.iboats.com/forum/owner...988-rogue-2420

                        Past Boats
                        1970 Wooster Hellion - Merc 9.8
                        2002 SeaRay 190BR - 5.0 - A1G2 - "Cheeseheads in Paradise"
                        1984 Avanti 170DLI -3.0 stringer- "Ship Happens"

                        What’s behind you doesn’t matter.Enzo Ferrari

                        Comment


                        • #13
                          good to know, that's the first test you should do. If you have good flow, its not pulling in air (clear hose test) and your manifold and riser ports are open (most likely so in fresh water) then you should not have an overheat, as long as you don't have other problems, like leaky head gaskets.
                          1988 Four Winns 200 Horizon
                          4.3 OMC Cobra

                          98 Jeep Grand Cherokee 4.0 Selectrac
                          07 Jeep Grand Cherokee Limited 5.7 Quadradrive II

                          "While air doesn't freeze....rust never sleeps"

                          Comment


                          • #14
                            Just looking back at my cooling system issue thread from last year . . . I did a flow test of the raw water through the cooling system at idle and got the following results:

                            - Port engine took 40 seconds to pump 5 gallons of water. 7.5 gallons/minute

                            - Starboard engine took 33 seconds to pump 5 gallons of water. 9.1 gallons per minute

                            The engine idle speed was about the same for each engine (620-640 RPM), but the flow was 20% different as measured, even though the flow looked healthy and sufficient.

                            The 20% difference in flow caused the port engine to run about 15 F degrees hotter than the starboard engine.

                            Point being that if you had an even more restricted flow (say 30% reduced), it could result in an overheating situation.
                            Best regards, Ted . . . . Cape Cod, MA

                            Current Boats: Formula 330 Sun Sport, O'Day Mariner Sail #3224, Sunfish
                            Past Boats: Catalina 22 Sail #10531, Formula 242 Sun Sport
                            Twin Mercruiser 7.4 LX MPI (0F802036, 039), Bravo 3's (0F806198, 199), Mercury 7.5 HP (1969), Johnson 4.5 HP (1980)

                            My Boating Web Pages: http://www.tpenfield.com

                            Comment


                            • #15
                              Sign up today
                              I finally had time to work on the boat today, and I was able to improve things slightly and rule some things out.

                              Changed so far:
                              • sea pump and impeller (housing was cracked)
                              • circulating pump (major rust around weep hole)
                              • oil cooler (>50% of paths blocked by rocks?!)
                              • thermostat housing (rusted more than I trust)
                              • thermostat and associated parts (too cheap not to)
                              Results:Tests performed:
                              • One of the concerns was about the inlet being blocked. I realized I have an engine coolant flush tank and hose (normally used for winterizing) that connects in just before the sea pump. I ran cool water through this tank and the temperatures didn't change at all. So I have ruled out an intake restriction.
                              • Checked temperatures around the engine (by hand, forgot my thermometer) and I found something unusual... the port side exhaust elbow is cool to the touch at all times. The starboard side is mildly warm. On my good (starboard) engine both are warmer.
                              • Checked flow out of two coolant hoses that go to exhaust manifolds, neither seem to be restricted.
                              Suspicions at this point:
                              • I did not check my fuel cooler, it's going to be a nightmare to get to, but I'm starting to think that's where the blockage could be. What do the insides of that look like? I don't think it could be similar to the oil cooler considering how much had been trapped there and how big the pieces were, wouldn't things get blocked before that?
                              • Could it be that the engine block actually has a blockage that's slowing water through it and forcing water to go out the exhaust cooling paths instead? I 'm guessing this could explain the cooler than normal exhaust elbows despite the engine overheating.
                              • Is it possible that it's actually the oil overheating for some reason? If so, what should I check on the oil cooling system?
                              • Edit: Also, I just realized there's also a power steering cooler hidden somewhere in the back, Could the blockage be here?
                              Thanks a ton for all the help everyone, I was hoping to take the family out for memorial day, but at this point I think maybe I should buy paddles... ;-)
                              Last edited by DracoDan; May 27th, 2017, 08:29 PM.

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