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  1. #1
    ripcuda's Avatar
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    SLT750... running too cold?

    Hi Gang,
    I put my SLT750 in the lake today for the first time since I swapped in a different engine. Started quick and runs great. Sa-weet!

    So as I'm idling around letting it come up to temps (lake is still pretty cold)... I'm doing the fingers on the head test for temps. The MAG was hot... the CEN and PTO were fine. I continued to idle around for a while... only pushing revs up to around 3k... mostly idling. The whole time, the MAG was hot. Like can't hold my fingers on it for 1 second... whereas the CEN and PTO are the normal 3 second touch until uncomfortable.

    It occurred to me that the t-stat likely hadn't opened (and the bypass only works at high rpm water pressures) so that would explain no flow... and the MAG getting hot. So I shut it off and pulled off the T-stat/bypass housing. I removed the t-stat and took it back out. Now I have even cylinder temps. But they were colder than normal. I ran it good at this point. Full throttle, couple good runs across the lake then checked again. The cylinders were warm... like melt butter warm... but not the normal 3 second touch warm I'm used to.

    I tested the t-stat in a pot... opened/closed at 140 as expected... so it's fine.

    So... should I run with no t-stat? The lake is only going to get warmer throughout the season.
    Or put the t-stat back and run it hard to ensure I get it to open... then test for even temps?

    What would you guys do?

    Cheers!


  2. #2
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    When you take out the thermostat, then there is constant water flow, even at idle, which will make the engine run colder. With the thermostat installed, there is very little to no water flow at idle. Did you check the popoff valve for wear/proper operation? If that is sticking, it can cause what you are talking about. I don't think the thermostat by itself can flow enough water for proper cooling at speed.

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  4. #3
    LaveyT's Avatar
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    Same issue with an outboard I have. Pop off sticking and the thermostat will not flow enough water to cool the engine.

  5. #4
    ripcuda's Avatar
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    Yeah... sticking bypass is a possibility... I'll check that. I thought that the bypass (popoff) only came into play at higher rpms where the water pressures are higher... like full throttle. I was only idling around, so I don't think the bypass would even open at those rpms to have any affect on cooling.

    Has anyone ever checked engine temps on an engine that was just started and idling in the water? Do you notice a hotter MAG until the t-stat opens? I'm wondering is that is just the issue I detected.

    Cheers!

  6. #5
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by ripcuda View Post
    Yeah... sticking bypass is a possibility... I'll check that. I thought that the bypass (popoff) only came into play at higher rpms where the water pressures are higher... like full throttle. I was only idling around, so I don't think the bypass would even open at those rpms to have any effect on cooling...
    The plastic pressure bypass valve can jam in the open position too.

    Polaris has specific wear check mentions in the service manual for that plastic piece.

    I have seen them worn with a notch or step on one side, or very tapered, or just sort of rounded off.

  7. #6
    LaveyT's Avatar
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    I don't even understand why the mag runs waaayy hotter on a hose than the pto and center. The pop off on my old 75 mariner should start working just slightly above idle. I wasn't even aware it had a warning horn until last year.

  8. #7
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by LaveyT View Post
    I don't even understand why the mag runs waaayy hotter on a hose than the pto and center...
    Think about how the water actually flows when pushing water into the water manifold from the garden hose via the 'reverse flush' fitting.

    The following is generalized...

    The first place the water wants to go is directly out through the thermostat housing and exit the back of the hull. The only thing preventing this is the thermostat and bypass valve or pinching off the water exit hose.

    If both the thermostat and the pressure bypass valve are present and closed then the hose water is forced to flow through the water bar and into the cylinders.

    The PTO cylinder is nearest the water feed so it always gets water. The flow rate from the garden hose into the cylinders is limited by the restrictions in the water manifold and by the amount of water pressure and flow available from your garden hose.

    The MAG cylinder is not only slightly uphill from the water source but the water pressure arriving at the MAG is diminished by the water disappearing into the PTO and CEN water jackets. There is an air pocket inside the MAG water jacket. If the water flow is not strong enough to push that air out then the water simply falls through the air pocket. Properly cylinder cooling does not happen if there is lots of air trapped inside the water jacket.

    In normal engine operation (riding) the water comes in underneath the air in the water jackets and the trapped air naturally rises and exits through the water manifold, with the water level following the air upwards. When you are riding and the jet pump comes out of the water, it only takes a couple of seconds for the water jackets to refill and re-pressurize once the jet pump hooks back up to the lake.

    In reverse flow using a garden hose, the trapped air wants to go up but the water is flowing downwards.

  9. #8
    LaveyT's Avatar
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    That makes sense, any clue why Rip Cuda's mag would run hotter at ide in the water? Especially if it was revved up and the air left the system?
    On my old mariner the rubber swelled up and made the poppet sticky. Once I revved it up good it would run just above idle without overheating. Evidently the pop off valve comes in at about 2500 rpm on my outboard. I'm not sure if it only opens partially until the rpm is high enough from the water pump to overcome the spring. I only open up the old houseboat a few times a year just to make sure every thing is working properly.

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