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  1. #1
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    2.5 150 too cold at idle?

    Im in the process of rigging a 2.5 150 and putting it on my varmint project. When running the engine, Ive noticed that it never really gets to operating temp, 95F - 100F according to my infrared temp gauge, after a 15-20 minute run time. Tell tale stream is pretty much instant on start up, and does get lukewarm to the touch. Water pressure gauge is about 8 PSI and steady. Brand new impeller, and thermostats. The thermostats are aftermarket ones with the tiny bypass holes in them. Is it possible that the poppet is stuck open? Id like to see a little more heat in this motor, not really in the mood to cold seize it. Any ideas?

  2. #2
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    8 sounds high for idle, open popets should have it very low under 3.

    But 8-9 is normally open pressure......

    i would look for whats shooting pressure so high.

  3. #3
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    Quote Originally Posted by Motv18 View Post
    8 sounds high for idle, open popets should have it very low under 3.

    But 8-9 is normally open pressure......

    i would look for whats shooting pressure so high.

    I guess ill run a compression check to rule out a head gasket. Other than that, I’m at a loss for right now.

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    Just went through a similar situation with a 92 150XR6.
    Water pressure 6psi at idle and would not increase with engine rpm. Replaced T stats with Merc brand t stats. Replaced poppet valve diaphragm and spring. Now have 2psi at idle and water pressure increases with engine rpm. At idle you should have about 2 to 3 psi, somewhere north of 1500 rpm is when the poppet valve begin to come into play and you should see somwhere around 9psi. At WOT water pressure should be a min of 12. Sorry I cant be more specific but I dont have my manual handy. Hope this helps

  5. #5
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    Quote Originally Posted by Iron Horse View Post
    Just went through a similar situation with a 92 150XR6.
    Water pressure 6psi at idle and would not increase with engine rpm. Replaced T stats with Merc brand t stats. Replaced poppet valve diaphragm and spring. Now have 2psi at idle and water pressure increases with engine rpm. At idle you should have about 2 to 3 psi, somewhere north of 1500 rpm is when the poppet valve begin to come into play and you should see somwhere around 9psi. At WOT water pressure should be a min of 12. Sorry I cant be more specific but I dont have my manual handy. Hope this helps

    very good info, thanks. I’ll pull the poppet apart and see if anything stands out. I searched and search last night for a similar scenario. I found way more overheat posts than I did too cold posts.

  6. #6
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    Quote Originally Posted by Blacknsilvr View Post
    very good info, thanks. Ill pull the poppet apart and see if anything stands out. I searched and search last night for a similar scenario. I found way more overheat posts than I did too cold posts.

    Here is an additional tip about your condition. Higher than nromal water pressure at idle can cause a rough idle condition also. If your engine is carbed it most likely will have a Thermal Air Valve on the odd cylinder bank. If CHT. doesnt reach 100 deg. The valve wont open and port air to the carbs to lean the mixture out. Just an fyi for you.

  7. #7
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    Quote Originally Posted by Iron Horse View Post
    Here is an additional tip about your condition. Higher than nromal water pressure at idle can cause a rough idle condition also. If your engine is carbed it most likely will have a Thermal Air Valve on the odd cylinder bank. If CHT. doesn’t reach 100 deg. The valve wont open and port air to the carbs to lean the mixture out. Just an fyi for you.

    I know what valve valve you are talking about, and it was somehow bypassed when I got the motor. Just the valve with no hoses hooked up to it basically.
    Another problem..... yay.
    88/12 reaction 19
    77 hydrostream varmint project

  8. #8
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    I would verify t-stats opening correctly in a pan of hot water. 143 deg is correct for most v-6 mercs.

  9. #9
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    Quote Originally Posted by XstreamVking View Post
    I would verify t-stats opening correctly in a pan of hot water. 143 deg is correct for most v-6 mercs.

    The aftermarket thermostats have weep holes drilled in them. Im thinking that may be factoring in to the equation. I have a set of merc thermos on the way.

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  11. #10
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    Quote Originally Posted by Blacknsilvr View Post
    I know what valve valve you are talking about, and it was somehow bypassed when I got the motor. Just the valve with no hoses hooked up to it basically.
    Another problem..... yay.

    Only one of those ports on the TAV has a hose connected to it. The other port is open to the atmosphere. If that valve is not connected to your carbs you probably need to re-Set the idle mixture screws after replumbimg the valve to the carbs. Do you have a maintenance manual for that engine? I would highly recommend getting one if you do not.

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    Good point!! On the after market T stats.
    Last edited by Iron Horse; 09-27-2019 at 04:45 PM. Reason: Additional info

  13. #12
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    Quote Originally Posted by Iron Horse View Post
    Only one of those ports on the TAV has a hose connected to it. The other port is open to the atmosphere. If that valve is not connected to your carbs you probably need to re-Set the idle mixture screws after replumbimg the valve to the carbs. Do you have a maintenance manual for that engine? I would highly recommend getting one if you do not.
    I’ve got an older one I can reference off of. I just want to tackle the heating issue right now, the TAV valve I will mess with later.

  14. #13
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    Quote Originally Posted by Blacknsilvr View Post
    I’ve got an older one I can reference off of. I just want to tackle the heating issue right now, the TAV valve I will mess with later.

    I also keep keep the search button on fire on here anyways

  15. #14
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    I agree with poppet and double check stats. If either dont fix. its in adapter

  16. #15
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    Quote Originally Posted by Merc 2.5 View Post
    I agree with poppet and double check stats. If either dont fix. its in adapter

    Fingers crossed lol.

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