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  1. #1
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    7.4 MPI overheating....HELP!!!!

    I have a 1999 7.4 MPI motor that is over heating when you run it over 3000 RPM's. So far I have changed the Manifolds & Risers, sea pump and thermostat. Inside manifold runs hot, outside manifold runs cool....HELP PLEASE??????

  2. #2
    Masher touches my Gold Member Outdrive1's Avatar
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    Re: 7.4 MPI overheating....HELP!!!!

    Quote Originally Posted by Donzi26ZX
    I have a 1999 7.4 MPI motor that is over heating when you run it over 3000 RPM's. So far I have changed the Manifolds & Risers, sea pump and thermostat. Inside manifold runs hot, outside manifold runs cool....HELP PLEASE??????

    What do you mean inside and outside? Are you sure you hooked everything back up correctly when you made these changes? Did you ever loose an impeller that could possibly be restricting flow somewhere in the motor or in any hoses? You could have lost it a long time ago and if you never got all the pieces out some thing could have moved around in there and slogged up a passage way.

  3. #3
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    Re: 7.4 MPI overheating....HELP!!!!

    II have a 1999 33SS Formula with twin 7.4 MPI motors, the port motor runs hot, I can touch the risers on the Starboard motor when it is running and it is cool. I can touch the outside riser & manifold on the motor that is running hot but not the inside riser or manifold on that motor. The boat runs at operating temp of 178 or so but when you go up over 3400 RPM's that is when it heats up and the alarm goes off. When I changed the impeller it had small pieces of rubber missing but noting to big?? Any suggestions?

  4. #4
    Masher touches my Gold Member Outdrive1's Avatar
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    Re: 7.4 MPI overheating....HELP!!!!

    Quote Originally Posted by Donzi26ZX
    II have a 1999 33SS Formula with twin 7.4 MPI motors, the port motor runs hot, I can touch the risers on the Starboard motor when it is running and it is cool. I can touch the outside riser & manifold on the motor that is running hot but not the inside riser or manifold on that motor. The boat runs at operating temp of 178 or so but when you go up over 3400 RPM's that is when it heats up and the alarm goes off. When I changed the impeller it had small pieces of rubber missing but noting to big?? Any suggestions?

    If your impeller was not all there then its somewhere in your cooling system. The times I've changed broken impellers I reasemble all the pieces on my workbench so that I know I've got all of them. They usually end up in your oil cooler first. You really need to blow your lines out and make sure every hose is clear. That sounds like your problem for sure. Check the lines that run into the bottom of your risers. Are you getting water out of you exhaust on the manifold that heats up? Put it on the hose and check to see if that side is barely getting any water out of it. Then you'll have to start at one end and work your way back to the sea pump until you've cleared all you lines and fittings.

  5. #5
    Registered Harper220's Avatar
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    Re: 7.4 MPI overheating....HELP!!!!

    Definitely pull the hose leading into your oil cooler and I can almost bet you'll find your blockage (pieces of impeller), which is causing the overheating on the one manifold only. It happened to me last year on one of my 496 HO's. Anything over about 3200 rpm's and it would overheat. This went on for at least 3 to 4 times of having my boat out. I checked all water lines and ended up at the oil cooler.

  6. #6
    Registered vandeano's Avatar
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    Re: 7.4 MPI overheating....HELP!!!!

    I would also check the hose in the out-drive. It is the small hose that is is your Gimbel house. Mine was almost completely closed from corrosion inside the hose. Made my impeller break apart from lack of water.

  7. #7
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    Re: 7.4 MPI overheating....HELP!!!!

    Thanks guys, I will check the oil cooler and then I will check the hose that goes to the drive. I will let you know how I make out!

  8. #8
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    Re: 7.4 MPI overheating....HELP!!!!

    I had the same probelm, the hose in the outdrive transome assembly was almost completely blocked.My mechnaic had to take out the egine to get to this part. Boat is a 2000 Baja 38 with 502's.
    do the other one too, was also 1/2 blocked already
    Alfred

  9. #9
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    Re: 7.4 MPI overheating....HELP!!!!

    It was the oil cooler, clogged! I back flushed it and it is working, temp is 178. I just had another question, should the riser be hot, like you can't touch them after the motors warm up, it has been a while since I had a boat with this kind of exhaust. Last boat had CMI headers.

  10. #10
    Registered vandeano's Avatar
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    Re: 7.4 MPI overheating....HELP!!!!

    I dont think they should be to hot to touch. They are on the warm side but not to hot to touch. If you see your temp rising over the 2500 rpm, check the hose in your Gimbal. Did you ever find out what made your rubber impeller break apart in the first place. Hot exhaust usually means lack of water.


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