350 Mag MPI overheating, not the impeller or t-stat?
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350 Mag MPI overheating, not the impeller or t-stat?

  1. #1
    Senior Member havasu1's Avatar
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    Default 350 Mag MPI overheating, not the impeller or t-stat?

    I have an 08 Boat with a Mag 350 MPI, Bravo 3. Boat is serviced regularly (Impeller changed annually). Last year it started doing something odd at the beginning of boating season. Randomly, it would get hot, the alarm would go off because it was getting hot. It never did it before. I had the boat serviced at the beginning of the season and the impeller was changed. I didn't think it was the impeller but I pulled it and checked the houseing. All was good. So, for good measue, I put a new tstat in and check all the hoses, no obstructions. Well, every now and then it would get hot, alarm would sound, I would get frustrated. One other thing. When the boat was serviced, they changed the exhaust manifold under warranty. Apprantly they were leaking.

    So, this year I changed the impeller (the old one looked just fine, but changed it any way). First weekend I had it out it ran great. I took it out a second weekend and here comes the alarms again??? It went off twice. I let it cool off a bit and then drained the water from the engine at the lowest blue plug. It ran good after that and was flawless the next day. I didn't have my lazer therm gun with me to check the tstat, hoses, risers and block to see if it is really getting hot or just a bad sensor. I believe it is really getting hot. Oh, this happens all all speeds. Even at idle.

    I have one friend with a 350 mag who hads similar problems. He said that a shop fixed it. According to the shop, the 350 has a problem with the bypass. Occasionally, too much water goes through the bypass and not enough to the engine to cool it. Apparently, they put a little restricter in at the bypass to force more water in the engine cooling hose.

    Anyone else have this random overheating issue? Could something have happened when they replaced the exhaust gaskets? Does it need the restricter chingus fix? Am I getting some kind of air bubble in the system?

    Thanks

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  3. #2
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    Quote Originally Posted by havasu1 View Post
    I have an 08 Boat with a Mag 350 MPI, Bravo 3. Boat is serviced regularly (Impeller changed annually). Last year it started doing something odd at the beginning of boating season. Randomly, it would get hot, the alarm would go off because it was getting hot. It never did it before. I had the boat serviced at the beginning of the season and the impeller was changed. I didn't think it was the impeller but I pulled it and checked the houseing. All was good. So, for good measue, I put a new tstat in and check all the hoses, no obstructions. Well, every now and then it would get hot, alarm would sound, I would get frustrated. One other thing. When the boat was serviced, they changed the exhaust manifold under warranty. Apprantly they were leaking.

    So, this year I changed the impeller (the old one looked just fine, but changed it any way). First weekend I had it out it ran great. I took it out a second weekend and here comes the alarms again??? It went off twice. I let it cool off a bit and then drained the water from the engine at the lowest blue plug. It ran good after that and was flawless the next day. I didn't have my lazer therm gun with me to check the tstat, hoses, risers and block to see if it is really getting hot or just a bad sensor. I believe it is really getting hot. Oh, this happens all all speeds. Even at idle.

    I have one friend with a 350 mag who hads similar problems. He said that a shop fixed it. According to the shop, the 350 has a problem with the bypass. Occasionally, too much water goes through the bypass and not enough to the engine to cool it. Apparently, they put a little restricter in at the bypass to force more water in the engine cooling hose.

    Anyone else have this random overheating issue? Could something have happened when they replaced the exhaust gaskets? Does it need the restricter chingus fix? Am I getting some kind of air bubble in the system?

    Thanks
    My 350 mag in in a vdrive, so may not be the same, but downstream of the engine, I have an oil cooler. The cooler will get small grass/debris clogging the intake side and will slow the flow, causing the motor to heat up when the imp is still a-o.k. ( and maybe bypass more??)

    Something to consider.
    Is it just me or are toilet seats getting smaller?

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    Senior Member SHADOW 21's Avatar
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    What was your engine temp when the alarm would go off, the sensor for the alarm in my boat went crazy. I removed the sensor and put it in a fry baby, heated the water in the fry baby, the ohm reading on the sensor was right on the money when the water was heating up, but when the water started cooling off, the sensor went crazy, by the ohm readings it was telling me the cooling water was around 500 degrees, then I noticed the only time the alarm went off was when the thermostat would open and allow cooler water to the sensor, replaced sensor and no more alarm. The thing that was driving me crazy was when the temp alarm would go off, the temp. gauge in the boat never indicated it was hot, it was always around 180 degrees, hope this helps.

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    Junior Member Fast Fred's's Avatar
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    Have you owned this boat since new? If not, even though you have had it serviced regularly the first guy might have had a pump impeller fail and didn't get all the old impeller blades out of the system. They will usually clog up the inlet on the oil or ps cooler ,which is the next thing downstream, and pretty much stop the water flow.
    If the people that do your service didn't see anything missing from the impeller they removed then they wouldn't think to check for a possible prior pump failure that would plug things up. Just take the hose off the inlet side of the oil cooler and stick your finger in the hole. Or look into it if you can. You should be able to see or feel any old blades lodged in it.

    Fred

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    Platinum Member BigSteve's Avatar
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    I would put a Sea strainer in the system if you don't already have one

    I once saw a block filled with sand and muck

    And I have had a perfect looking water pump have trouble at idle speeds on outboards

    thats why I spend a little more and change the housing and impeller
    If you ain't the lead dog the view is always the same !

  8. #6
    Senior Member havasu1's Avatar
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    Quote Originally Posted by SHADOW 21 View Post
    What was your engine temp when the alarm would go off, the sensor for the alarm in my boat went crazy. I removed the sensor and put it in a fry baby, heated the water in the fry baby, the ohm reading on the sensor was right on the money when the water was heating up, but when the water started cooling off, the sensor went crazy, by the ohm readings it was telling me the cooling water was around 500 degrees, then I noticed the only time the alarm went off was when the thermostat would open and allow cooler water to the sensor, replaced sensor and no more alarm. The thing that was driving me crazy was when the temp alarm would go off, the temp. gauge in the boat never indicated it was hot, it was always around 180 degrees, hope this helps.
    To be honest, the temp gauge on the boat sucks. It really does not give a good reading. I know this sounds dumb but it goes from 0 then to 100, 140, 190, 220 and that's it. It is really hard to read the temp but the alarm goes off around 190 best I can tell. Boat generally runs around 160 when there are no problems. Like I said, I'm going to bring my lazer term gun next time and get readings when the alarm goes off to see if it is a faulty sensor. Though I do need to get a better engine temp gauge that is easier to read and make sense of.

  9. #7
    Senior Member havasu1's Avatar
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    Quote Originally Posted by Fast Fred's View Post
    Have you owned this boat since new? If not, even though you have had it serviced regularly the first guy might have had a pump impeller fail and didn't get all the old impeller blades out of the system. They will usually clog up the inlet on the oil or ps cooler ,which is the next thing downstream, and pretty much stop the water flow.
    If the people that do your service didn't see anything missing from the impeller they removed then they wouldn't think to check for a possible prior pump failure that would plug things up. Just take the hose off the inlet side of the oil cooler and stick your finger in the hole. Or look into it if you can. You should be able to see or feel any old blades lodged in it.

    Fred
    I'll check that again. The first time this started happening, I checked all the hoses best I could. I bought the boat in August of 09 and the guy I bought it from said he had it serviced in June of 09. They guy I bought it from bought the boat new in September 08. Just to give a timeline. I used the boat from August 09 through October 09 before it got stored in my garage in Havasu for the season with no overheating alarms. For me, it did not start giving me problems until after I had it serviced in May of 2010. If I remember right, it was one of the first trips out for the season just after the May Service.

  10. #8
    Member nordic 572's Avatar
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    Default collers clogged

    all the above info is worth doing , worth the money, 1st get a strainer,2nd pull both hoses off coolers ,put a light at end that would be difficult to see thru ,look thru other end. on oil cooler you will see many tubes , or passages ,should be able to see thru all cooler tubes on the oil cooler , on the p/s cooler the cooler element is a larger single element type ,only 1 tube (mine mag mpi 454) is around a 1/2 " most likley oil cooler would clog 1st, possibly w/ debris from your lake /river ,or debris from impeller deterioraition.
    in new england we "ran low water last year , " i sucked in enough sand to prematurly kill impeller witch i just installed .the small particals from the desintergraition ,plugged cooler ,this is only my opinion hope this helps you ,ps if you find o/c plugged i would recomend you try to clean it w/o using a coat hanger try some thing like plastic the guts of coolers are fraigle ,and a puncture will add water to oil good luck

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    PASO ROBLES CA cougar63's Avatar
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    I have a friend that lunched a motor in a Carrera with a big block and bravo one .I just built a nice 468 and before i started it i hooked up a set of manual gauges as i dont trust the electric gauges. Anyway as soon as we fired the temp rose to 200 of course we shut it down. He a bad temp gauge and no thermostat. The mercruiser has to run a thermostat and the white plastic diverter that goes under it.This plastic diverter if broken or not installed correctly will allow most of your water to run out the exhaust and not thru the block. Just my 2cents good luck

  12. #10
    Senior Member havasu1's Avatar
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    Quote Originally Posted by cougar63 View Post
    I have a friend that lunched a motor in a Carrera with a big block and bravo one .I just built a nice 468 and before i started it i hooked up a set of manual gauges as i dont trust the electric gauges. Anyway as soon as we fired the temp rose to 200 of course we shut it down. He a bad temp gauge and no thermostat. The mercruiser has to run a thermostat and the white plastic diverter that goes under it.This plastic diverter if broken or not installed correctly will allow most of your water to run out the exhaust and not thru the block. Just my 2cents good luck
    This is what I am thinking. As I said above, there seems to be a restricter that Mercruser uses as a fix to stop too much water going through the bypass. This fix is a plastic washer (restricter) that is placed at the "Y"at the bypass to force more water through the motor for cooling. Though this is not done on all boats. Only on the ones that seem to have a random over heating problem. Anyone else heard of this.

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    Default Fender washer

    I was having the exact same problems that you were after replacing impeller, pump, and t stat.

    After reading this to my mechanic, we decided to insert a 1" stainless fender washer with a 1/4" hole in the hose to the left riser. It fixed the problem! Risers are staying cool and the engine temp is normal!

    I am a little worried though stress on the main circulation hose.

    Any thoughts?

    Thx.

  14. #12
    Senior Member ol guy's Avatar
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