Thermostat Partially Stuck Open?

[hoytkimbershimano]

I have a 2009 Pro XS 250. 2009 250 ProXS (Serial 1B750829)

I watch the temperature religiously on my Merc Monitor. My motor was warming up to about 105 today. Getting up on plane the temp went up to about 125 and then came back down to the 110-120 rang3 while running. After coming off plane it cooled down to 105-110 again. Normally it stabilizes at 120. Normally, It might get up to 125-135 before cooling back down and staying ar 120.

I was idling after my last spot and it seemed to get to about 118 so I got up on plane and it held 109-113 after blipping up to 126 and coming down which is fairly normal for the water temp here in Michigan. Then stayed in the 118-120 range idling in the canal.

1. How hard are these to change? I have some wrench ability, but not a ton.
2. What’s the risk of damaging them during inspection? Should I order replacements first?
3. Any special tools needed?

How urgent is this to fix?

Thank you.

[lmoore]

Sounds like probably do have something stuck. Very easy to inspect/test. Loosen the lock nuts on the fuel rails so you can slide them back (don’t need to actually remove them). Thermostat housing is above the fuel rails with a large hose running into it. Two 8mm bolts hold the housing in place, once those are removed you can slide the housing out of the way and pull the thermostat. Test in a lot of hot water (should open at 130 degrees) and then clean any debris out. Not a bad idea to have a thermostat housing gasket on hand just in case, although I’ve only needed to replace once and I’ve done this job many times. Before you slide the fuel rails back, I usually take a small brush or long q tip and apply a thin film of 2 stroke oil to the injector O-rings. Probably not a requirement but I’ve torn one before when skipping that. All in all it’s maybe a 1 hour job for both sides.

[hoytkimbershimano]

Sounds like probably do have something stuck. Very easy to inspect/test. Loosen the lock nuts on the fuel rails so you can slide them back (don’t need to actually remove them). Thermostat housing is above the fuel rails with a large hose running into it. Two 8mm bolts hold the housing in place, once those are removed you can slide the housing out of the way and pull the thermostat. Test in a lot of hot water (should open at 130 degrees) and then clean any debris out. Not a bad idea to have a thermostat housing gasket on hand just in case, although I’ve only needed to replace once and I’ve done this job many times. Before you slide the fuel rails back, I usually take a small brush or long q tip and apply a thin film of 2 stroke oil to the injector O-rings. Probably not a requirement but I’ve torn one before when skipping that. All in all it’s maybe a 1 hour job for both sides.

Thank you. I found Don’s post with directions that line up with what you said. The fuel rail part is the one that makes me nervous. Is the place you apply the oil obvious? I’m going to hold on doing anything until a I get a replacement thermostat and gasket.

Aappreciate the feedback!

[lmoore]

Thank you. I found Don’s post with directions that line up with what you said. The fuel rail part is the one that makes me nervous. Is the place you apply the oil obvious? I’m going to hold on doing anything until a I get a replacement thermostat and gasket.

Aappreciate the feedback!

When you slide the rails out, the injectors are on the backside of them, between the rails and rest of the motor. The injectors will either stay in the rails or stay in the motor, if they stay in the motor you will see an o ring where the injector slides into the rail. Apply the oil to that o ring. Not sure how well i explained that but it will be fairly obvious when you get into it.

[SilverFox579]

I'd replace both stats. Pretty sure these were later updated to 130deg vs 120. You could clean/test the old ones, and if they pass, keep for spares.

[Bryanmc57]

I'd replace both stats. Pretty sure these were later updated to 130deg vs 120. You could clean/test the old ones, and if they pass, keep for spares.

Not sure when the change took place but my motor (delivered in July, 06) came with 130 degree stats.

[SilverFox579]

Not sure when the change took place but my motor (delivered in July, 06) came with 130 degree stats.

You're right, his should already have 130s. Pulling and inspecting will confirm.

[JOE54]

^^^^^^^ +1 yep

[hoytkimbershimano]

Strange, I did buy the boat one year used, but there is no record of the thermostats ever being changed. The motor has always regulated to 120 on the button.

[SilverFox579]

I'd still pull 'em and check, would rather know than wonder.

[EuropeanAM]

Given the age- if they've never been out of the engine, plan to replace them.

When testing- always have a pan of COOL water that you can use to watch the thermostat CLOSE. Should be able to go from fully-open to fully closed in 70 degree water, within 10 seconds (sometimes less).

If it takes considerably more, replace.

And of course, debris holding one open is a common cause of problems.

NOTE: DO NOT accelerate above 1500 RPM's if engine doesn't come up to 120F. There is a high risk of cold-seizure.

[hoytkimbershimano]

I’ve been out a couple times since the trip above and things seem to be behaving OK. It typically gets a bit above 120 then comes down to 118-120-122. It used to be 120 consistently, but I’ve been seeing 118 and 122. When I accelerate, I see my temps climb to 125-129 and then come back down. The water is still cool here and stabilize at 114-118 while running 3800-4200 RPMs. After running temps bounce back 125-130 before opening and then back to 118-122.

I assume that blip up in temp is normal until the stats or poppet open?

I have the new Thermostats and Gaskets, but don’t want to fix something that isn’t broken.

Thanks,
Tim

[SilverFox579]

Once the poppet opens, the thermostats are "bypassed" and out of the engine cooling circuit until the poppet closes again. On plane temps will vary depending on water body surface temperature and engine demand.

Your thermostatically controlled temps are what I'd expect to see from a good pair of 120° stats...just saying.