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OMC 5.8L 351 Overheating..need help

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  • OMC 5.8L 351 Overheating..need help

    I had the drive replaced with a new water pump. The mechanic said he flushed the engine and parts of an old water pump came out. He said he replaced the thermostat but it seems to still be running hotter than normal. I took it out on the lake ran it for about 45 minutes and blew an exhaust hose causing water to start coming into the engine compartment, though only trickling. I was towed in and someone said it blew because it probably overheated. The gauge only read about 190 at most. The mechanic said the engine is screwed because there is something stuck inside causing it to overheat. I really need help with this as we just purchased the boat and havent been able to use it yet. If it's an easy fix such as taking different exhaust parts of to find the blockage I would like to do it on my own. Please help.

  • #2
    Re: OMC 5.8L 351 Overheating..need help

    Sounds like you have pieces of the impeller blocking your water flow, it happens and is pretty common, I have a feeling that rubber is causing blockage inside of your water pump,alot of the time the impeller will go before the thermastat. first thing I would do to trouble shoot is check out your water pump in the stern drive lower unit.

    1991 Bayliner Capri 2050CX,4.3L V6,Mercarb 2bbl,Alpha 1
    (QUOTE): When our boat is happy, THEN we are Happy


    • #3
      Re: OMC 5.8L 351 Overheating..need help

      The mechanic claims he flushed everything out and there are no more pieces of an old impeller coming out. He's scaring me by saying the engine is screwed and there is a blockage inside there. What else could he do to find the problem if he flushed it and cant find the problem along with installing a new water pump and changing the thermostat?


      • #4
        Re: OMC 5.8L 351 Overheating..need help

        ohh...just wanted to add. It appears the power steering fluid is very low/near empty as I believe when it was summerized it was never replaced (long story) Any chance the fluid being low would cause the overheat?


        • #5
          Re: OMC 5.8L 351 Overheating..need help

          No, low fluid would just fry the power stearing pump if used to long without fluid and make your stearing stiff, If you haven't fried your pump, fill it, did your mechanic replace the impeller or just flush it out??

          1991 Bayliner Capri 2050CX,4.3L V6,Mercarb 2bbl,Alpha 1
          (QUOTE): When our boat is happy, THEN we are Happy


          • #6
            Re: OMC 5.8L 351 Overheating..need help

            he replaced the impeller since i was putting a new drive in. He said he then flushed it out and was able to find small pieces of an old impeller in there. He said he continued flushing it with nothing else coming out but it still is running hot. I ended up burning up an exhaust hose and i was only running at maybe 190, Any idea where the blockage may be?


            • #7
              Re: OMC 5.8L 351 Overheating..need help

              OMC water pump is in upper unit, Other place for impeller pieces to go is in the heat exchanger. Follow the hose that comes thru the transom to a long cylinder most likely on the back of the engine. Pull the hose off that side. Look in there for impeller pieces. You may have to pull both hoses off to see thru. This cools the power steering fluid. It has tubes about 3/8 diam. inside for the water to run thru to cool the fluid. the 2 fittings on the side of this are the power steering lines. Check here for PS leaks too.

              Just because the mechanic sez its flushed dont mean there is not something in there. the small pieces wedge in the tubes and wont flush out and must be extracted with a hemostat or small needle nose pliers.

              other causes of blowing an exhaust hose is rust/scale restricting water flow in exhaust manifolds or elbows. OR starting/running without water to drive. And this probably wont show up as hig temp on gauge.

              Cant stress this enough...NEVER, not even for a second, start your engine without water to the drive intakes. If you dont have a flusher (ear muffs) get one and find out how to use it.

              it is important that water be there for 2 major reasons: 1. you will fry your water pump impeller in less than 15 seconds (rubber impeller turning engine speed in a stainless housing). 2. Engine exhaust needs the water in it to cool your exhaust couplers (big rubber hoses) or they will fry from the inside.

              If all else fails get a second opinion. I have never had a cooling problem I couldn't find, but some do take time to track down.

              One other thing is to check the impeller housing and o-ring. If the SS housing (little SS basket thing in the drive that the impeller spins in) is not perfectly even in the plastic housing it will not work. Check the housing for any signs of melted plastic, if ANY replace the housing.

              One last thing is to check the hose that goes from the outdrive to the transom (outside of boat) if it is leaking you are loosing water. Turn the drive to the starboard (right from back of boat) and find what looks like a heater hose, this is the hose that takes the water from the drive and waterpump to the inside of the boat.

              Good luck and I hope this helps.
              When will people learn that impellers require regular
              replacement and are not drive it till it pukes items?

              1984 Regal Medallion 195
              3.8 V-6 with the dreaded
              OMC Stringer Drive.
              2003 Avalanche w/Hypertech

              If it ain't fixable - don't break it!


              • #8
                Re: OMC 5.8L 351 Overheating..need help

                this is a good start for me but who knows what the mechanic will actually do. The boat was ran on muffs after installing the drive and flushed and I only ran it after putting it in the lake. This may actually answer the question as to why the power steering fluid is flow. May be a blockage going into the pump? As I am not a boat mechanic would I be able to troubleshoot/repair this myself? I believe I can replace the exhaust hose easily. And there are no hoses leaking outside at the drive or inside anywhere, except the one which just cracked.


                • #9
                  Re: OMC 5.8L 351 Overheating..need help

                  any reason why the gauge would not show an overheat but then I'd blow an exhaust hose? I mean it ran higher than normal around 190 but definitely no more than that.


                  • #10
                    Re: OMC 5.8L 351 Overheating..need help

                    I'm no expert but I wonder...is 190 an overheat on 5.8L? Maybe you just blew a hose and the 2 aren't connected?

                    I tried to contain myself...but I escaped!

                    Scott & Roxanne
                    1989 Carver Santego 30
                    Twin Merc 5.7L Alphas


                    • #11
                      Re: OMC 5.8L 351 Overheating..need help

                      Fix the exhaust boot that has cracked/melted and then hook up the garden hose directly to the raw water side of thermostat housing. Run the engine and see if the temperature still overheats. You will be able to eliminate the possibility of an engine or exhaust blockage this way. You must REMOVE THE IMPELLER on the outdrive before doing this so you don't burn it up.


                      • #12
                        Re: OMC 5.8L 351 Overheating..need help

                        I have not finished the thread but I saw something that lit me up...

                        POWER STEERING

                        The power steering cooler is located behind the shift cable and is a tube about the size of a 16 oz coke bottle. Cool water straight from the impeller pump flows over the return line of the power steering pump. When this animal breaks, you end up with HOT power steering fluid mixed with water going straight into your engine. It will raise the temp of the engine.

                        Tale tale signs are this:

                        Power steering fluid low after a just filling it and no ps fluid seen in the bilge.
                        Noticiable oil slick coming behind the boat at low speeds as this fluid ends up mixing with your engine water and going out the exaust.

                        Now I will get back to reading the rest of the thread...


                        • #13
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                          Re: OMC 5.8L 351 Overheating..need help

                          As for the hose, on a boat like this that has overheated from lack of water they will crack and with age, eventually become unsound. Also, when you remove your ex. manifold you have to loosen them. If you dont get them back on good and snug, and they are not in great shape, the dry rot from heat and old age will make them blow. (Trust me, I have been 7 miles out in the Gulf of Mexico and now I double wrap mine using a larger size hose I got from Ace hardware. In big water you can never be too careful.)

                          Back to the list of items it could be causing the over heat.

                          NOTE - Over heating is almost impossible to diagnose on muffs. In the water under load is your true measure. Don't say you haven't been told. This comes from years of 800 stringer experience.

                          1. Check the sending unit and the temp guage. (When in doubt, replace it for 15 bucks with one from the auto store. Even better go get an infra red temp guage for 30 bucks you can point at different objects and get temp readings.)

                          2. The swivel bearing housing HAS TO BE GREASED. (Lack of grease causes exhaust fumes to mix with water and jacks the temp up.) A cracked swivel bearing seal will do you in.

                          2A. Clear hoses from the intermediate to the thermostat will show bubbles if you dont have a good seal in the upper/lower connection.

                          3. The exhaust manifold and risers have to be clear of scale. Rust is allowable, scale will do you in...scale holds heat.

                          4. The thermostat must be good.

                          5. The timing must be set correctly. Premature ignition will make them run hot.

                          6. Get an empty gallon jug and pull the hose off of that comes from the intermediate houseing and disconnect where it goes into the thermostat. With the engine on, and the lower unit under water IN THE LAKE, crank your boat and see how long it takes to fill that gallon bucket. A good pump will push out a gallon in less than 30 seconds at idle.

                          7. Water pump on engine can be bad. It usually leaks.

                          8. Blown head gasket. Sometimes tough to diagnose but the heads can warp at real high heat.

                          9. Don't forget most big boats have a hot water heater that circulates water from the engine water pump back to the intake manifold to heat the water in the heater cheaply.

                          There is your list. Good luck.