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Yamaha 150 hp Overheating

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  • Yamaha 150 hp Overheating

    My son and I have been struggling to correct an overheating problem on his 2000 Yamaha 150 OB. He bought the boat and motor used and has had the overheating problem to some extent since he's owned the boat. We replaced the water pump last summer hoping to fix the problem w/o any change.We have the Clymer manual and after reading I felt that either the thermostats or pressure relief valve were causing overheating. Overheating primarily occurs at low speeds w/ loss of indicator stream usually, but not always. Water pressure at idle is low, almost zero on instrument panel gauge. We removed the pressure relief valve and although there was a little sand around the rubber seat, it really didn't look bad. The rubber seat was recessed ~ 1/8" inside the cavity. We did not remove the seat but cleaned w/ toothbrush, removed sand, and re-assembled. We removed the thermostats. Again some sand but not too bad. We cleaned them in vinegar overnight and verified they began to open in hot tap water, probably 120-130F. We verified overheating was real by measuring the closure temperature on the temp switches. Both were w/i spec of 183-194 F. After reassembly, the problem remained w/ same symptoms. Next we removed the thermostats entirely on advice of some locals, who advised my son that removing thermostats was first thing they did on Yamaha's. It initially seemed fixed and my son took off on a fishing trip. He got stranded and had to pull the boat quite some distance thru Laguna Madre shallow water to Intercoastal near Corpus Christi. He got a tow from a passing boat.From searching and reading previous posts on this subject, the consensus seems to be the pressure relief valve. Our current plan is to replace the pressure relief valve seat, valve, and spring w/ new parts. I read of some backflushing through the thermostat openings down to the lower unit pump outlet w/ lower unit removed. Is this worthwhile? With all the unsuccessful repairs, I'm looking for some guidance that we're on the right trail to a fix. It has been a frustrating experience, especially for my son. Thanks in advance.

  • #2
    Re: Yamaha 150 hp Overheating

    by now I would also have a leak down test done. but removing the grommet and carefully cleaning the pocket it sits in with a pick to remove all corrosion and installing a new grommet is mandatory. the valve and spring is optional. do it if it makes you feel better. the t-stats are also part of the pressure control system. reinstall them. you may end up replacing the head gaskets. they dont like getting hot repeatedly. stay away from locals, while well meaning most know less than you
    no tech questions by PM, they wont get answered.
    you have to be trusted by the people that you lie to .

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    • #3
      Re: Yamaha 150 hp Overheating

      We cleaned the relief valve pocket carefully and replaced w/ new grommet. We dropped the lower unit and flushed both coolant passages through the thermostat opening. Nothing but a little sand returned. Reassembled and my son tried the motor out today w/ same result. It idles OK w/ a indicator stream, stream stops, and overheat alarm comes on shortly. Does the relief valve allow water to bypass the coolant passages when the thermostats are closed, or does valve regulate pressure at low engine speeds to insure adequate water pressure at idle? The panel water pressure indicates no pressure at idle when the problem arises. Could this indicate the relief valve is opening too easily, perhaps due to weak spring? Spring does not seem deformed and must be compressed a bit to install the relief valve cover. The only thing we noticed on the removed grommet was that it was slightly snugger to the floating seat than the new grommet. I'm pretty sure spring pressure would cause the seat to close to the grommet. Since indicator stream is good initially, it's hard to believe the water pump (new last summer and little used due to this problem) is a factor. How is a "leak down test" performed? What does it check?thanks

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      • #4
        Re: Yamaha 150 hp Overheating

        what we are looking for with the leakdown is to insure that the combustion gasses are not filling the block. if the pressure control valve leaks all the water will dump overboard and never fill the block. if your pumping exhast gasses in the cooling system it wont work either. by chance has the power head been replaced ?there is a tiny set of plastic restrictors that if missing will cause the same problem. there is a tech bulliten on them but they do not show up on the parts list.
        no tech questions by PM, they wont get answered.
        you have to be trusted by the people that you lie to .

        Comment


        • #5
          Re: Yamaha 150 hp Overheating

          I will pull the spark plugs and check for any water getting to the plugs. If the head gasket leaks gases, it should also pull some water into the engine. Engine runs fine, so this seems less likely. We decided the indicator stream which starts iniitially is weak, so we plan to pull the lower unit and inspec the water pump for damage. Where are the plastic restrictor plugs you mentioned installed? Thanks for your help

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          • #6
            Re: Yamaha 150 hp Overheating

            its possible to leak combustion gasses into the cooling system and not allow water into the cyl. sounds funny but its true. the restrictors are located on the bottom side of the powerhead. it requires removal to check them. also when ya drop the pump again remove the rubber dam and the retainer and carefully clean any corrosion from the pocket/dam. sometimes they will hold the edge of the wear plate up and create an air leak. carfully inspect the nylon housing for any signs of melting. any melt however slight is grounds to scrap the housing. but I have been bit once on a repower that someone did not install the restrictors. took me 3 days to find it and more aggrevation than I care to remember but I did get it in the end. wasnt me that did the repower but I was the one that had to straighten it out.
            no tech questions by PM, they wont get answered.
            you have to be trusted by the people that you lie to .

            Comment


            • #7
              Re: Yamaha 150 hp Overheating

              My saga w/ this overheating problem continues. We pulled the water pump and found evidence of melting of the plastic housing. We replaced the housing and installed new pump kit. Later my son tried to launch and engine would not turn over at all. Dead as a doornail. A pause of several weeks till I had time to revisit him and trace electrical problem.I checked his electrical system this past weekend and found one of the main 80A fuses was open. Replaced w/ spare and engine starts OK. An initial test was run at a boat launch, and the indictor stream started after perhaps 30 secs of idle, but remained strong for 10 minutes w/ no overheating. We declared success and went home to clean up and change clothes.Later we went for a trial run, and No indicator stream would start, no water pressure, and overheating before we could get away from dockside. The performance suggests the pump is simply not picking up water sufficently. I've seen posts about the rubber gasket and pocket beneath the wear plate. I did have some qualms about the oring that seals the plastic housing to the wear plate. It is difficult to get this oring to stay in place, and I used RTV to glue it into place. There is a gap in the oring groove of the housing that allows the oring to escape w/o some adhesive to hold in place. I have not run a leakdown test, but figured this was outside capability of DIY, and engine was running so well I discounted the head gasket problem.Thanks for help.

              Comment


              • #8
                Re: Yamaha 150 hp Overheating

                did you clean the rubber dam beneath thye pump wear plate? remove the pump and clean off all the sillycone use grease to retain the oring in the housing. I do it weekly so I know it works. have you removed the water intake screens and inspected that passage for sand? the only thing that melts the impeller housing is it ran dry. I would check it again, takes 30 seconds of dry run and the impeller and housing are shot. once you re-verify the pump is not damaged remove the pressure valve assy, start the engine on the muffs with full city water pressure and a 3/4 inch hose. anything less and it will get hot. watch for water at the valve pocket. if it starts flowing water everywhere hold the valve in place manually with a pair of needle nose pliers and see if the pump is capeable of filling the block. I am glad your experience dictates the headgasket sealing makes no difference. mine indicates otherwise. but first you must verify that the pump is acually putting the corect volume and pressure to the block. first thing that sees water is the pressure valve area. you can manually hold the valve in place to check it. this is going to sound crazy but when its in the water is the cavitation plate submerged? I did a melted pump the day befor thanksgiving that roasted due to improper use of the jack plate and trim. they pulled the intakes out of the water. all the valve does is bleed off excess water pressure but if it leaks it bleeds it all off. has this powerhead ever been replaced? have you ever seen the water pressure at 5 PSI or so at idle? if the pump is known good, and you have no water pressure, its gonna be a leak in the tube betwen the pump and the adapter,the adapter or the block. or the passages are clogged with sand/debris. if after carefully checking the pump,t-stats and pressure control valve next step is cyl head removal and the powerhead removal to inspect the 2 restrictors. if you have no water pressure at 1500RPM or so I would continue looking at the pump, the rubber dam under the wear plate and for debris in the water intake passages.
                no tech questions by PM, they wont get answered.
                you have to be trusted by the people that you lie to .

                Comment


                • #9
                  Re: Yamaha 150 hp Overheating

                  did you clean the rubber dam beneath thye pump wear plate?No, we didn't clean that part. We will remove, check, and clean. I'll remove the silcon sealant and use grease (silicone grease?) to hold the oring this time.have you removed the water intake screens and inspected that passage for sand?Yes, we removed the outer screen, inspected, and back-flushed several times. We'll check for damaged pump housing due to dry running.We've removed the pressure regulating valve, cleaned the pocket, and replaced the grommet. We also removed T'stats and checked both for correct opening temp. W/ T'stats out we removed lower unit and flushed downwards through the T'stat openings and got a nice healthy stream out the bottom. I am concerned about removing the pressure regulator valve and restarting engine. On previous occasions, I removed the ECU to access the pressure regulator. I don't think I will be able to access that area to force the valve closed w/ ECU in place. How about installing a temporary blind (small plate) over the opening of the regulator and snugging the fitting? I can verify the water pump intake was underwater during testing. I was standing directly over the engine during first test and checked engine position at boat dock on 2nd test. From appearance of engine paint, bolts, and wiring, I don't believe this engine has ever been touched before, however my son bought boat 2nd-hand and I can't be certain. We saw no water pressure on these last tests, however see below. On earlier occasions the pressure was above zero, but low at speed. I'm assuming it should be 5-10 psi at speed? In writing this reply, it just hit me that I noticed the small tube from the lower unit was disconnected when we examined the motor and flushed the engine after the overheating incident. This tube must be the pressure tap for the water pressure gauge. Would this tube being open release enough water to prevent pumping? I'm pretty sure we reconnected during lower unit installation (it was tricky to get back through the upper housing), when we last installed the pump. It did not have the plastic wire tie clamp, and may have popped loose due to inadequate clamping. I have solved some fairly tough engine problems (auto engines) in my years, but this problem has been hands down the most difficult to resolve I've ever encountered. Thanks again for your time and encouragement.

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                  • #10
                    Re: Yamaha 150 hp Overheating

                    the speedo tube has nothing to do with cooling. and yes occasionally its a pain, I manage to snap off about 3 of the plastic nipples from the case a year. I am good at removing the broken part though . the rummber dam and the holder can build up corrosion and lift the edge of the wear plate and cause low/no pressure at idle. if you have a good pair of needle nose pliers you can clamp off the hose from the pressure valve but I would rather you run it and verify water is getting to it then use the needle nose to manually close the valve. instead of removing the ECU just unplug the wires going into the bottom and move them out of the way. I know its frustrating but most of us full time techs have paid our dues with these type problems. normally on that motor I see about 5 psi at idle and 10-20 PSI at speed. I forget the exact spec but its in the manual someplace.
                    no tech questions by PM, they wont get answered.
                    you have to be trusted by the people that you lie to .

                    Comment


                    • #11
                      Re: Yamaha 150 hp Overheating

                      thanks for help. The boat is in Corpus Christi (Padre Island) and I'm in Houston, so I get down on weekends to help. I'll post back when I've had a chance to investigate again.regards

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                      • #12
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                        Re: Yamaha 150 hp Overheating

                        I have found numerous times that the use of any thing other than grease to hold the water pump o-ring in place will cause the pump to leak and cause these symptoms. The pump body must not be warped in any way, either through heat causing it to distort or the use of gasket sealers to hold the oring in place during its replacement. The overheating may have caused other problems such as a leaking head gasket.

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