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2016 6.2L freshwater cooled running cold at idle

paulpaul Posts: 43 Baller
edited September 2019 in Boat Talk
I’m stumped on trying to fix an issue on our 2016 Prostar with the 6.2L engine (freshwater cooled). It gets up to temp very slowly (need to bring it on plane). Once up to temp it will stay there as long as it is kept on plane. Once the boat slows to idle, it drops down to 135F and stays there until speeding up again. So far I have done the following:

- replaces the TStat (no change)

- Checked the impeller bypass check valve (there was some rubber in it causing it to stay slightly open) got rid of this and it is running even cooler at idle

- Took apart the splitter manifold (directly after the raw water (impeller) pump with the restrictor plate on it. I found one piece of rubber in one of the holes but other than that it looked fine to me. The only thing that was concerning was where the spring loaded plate rests against the plate in the engine as it was definitely worn and may have not been seating properly. If this was open somewhat could it cause the engine to run cold at idle?

Any other ideas? Pics of what I found in my splitter manifold.


  • DWDW Posts: 2,115 Mega Baller
    Does the impeller bypass circuit allow the or more cool water to flow by the temp sensor? If yes, I would correct so the design level of bypass occurs. You might take an IR temp gun and shoot a few spots around the cooling passages to see if there is a discrepancy or assist in pinpointing where the offending cool water is coming from. That would also support the slow warm up. Engine may be warming up at the same rate, just excess cold water flowing by the temp sensor from the extra bypass.
  • paulpaul Posts: 43 Baller
    Hmm good point. I’ll have to search for the temp sensor. The impeller bypass feeds into the manifold that houses the tstat. From there it goes out to the exhaust manifolds I think.
  • Jody_SealJody_Seal Posts: 2,901 Mega Baller
    Go buy a cheap infra red or laser thermometer and check the housing temp where the sending unit is.
    Hobby Boats can be expensive when the hobbyist is limited on their own skill and expertise.

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