chrispbrown27
Well-known member
How exactly do you know when the thermostat is stuck on an outboard without a temp gauge?
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When to replace a thermostat?
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Pappy would be the best to tell you, but why not just pull the t-stat out? simplest check, just clean mating surfaces, apply gasket sealer, and torque back in place.
To me, such an OB would always run cold and that may include cold fouling of the plugs at lower RPMs. Thing is, one usually needs to put revs on the motor, say 2,000 RPMs or so for the t-stat to open up. If there was a way you could safely run it (I'd use 2 people, one on the throttle, other checking ... ) or tie to a dock, you might be able to monitor head and pee-port discharge temps. Usually if you can keep your hand on the head for a bit, then she's not at an over-heating temp. With a stuck closed t-stat, you should be running 'cool', 170 or so all over.
On a good running motor I can feel the pee port water discharge temps rising and then hit the point where the t-stat opens and it begins to cool down. But I've never used this to diagnose a faulty or stuck closed one ... as I'd just pull it and replace it.
To me, such an OB would always run cold and that may include cold fouling of the plugs at lower RPMs. Thing is, one usually needs to put revs on the motor, say 2,000 RPMs or so for the t-stat to open up. If there was a way you could safely run it (I'd use 2 people, one on the throttle, other checking ... ) or tie to a dock, you might be able to monitor head and pee-port discharge temps. Usually if you can keep your hand on the head for a bit, then she's not at an over-heating temp. With a stuck closed t-stat, you should be running 'cool', 170 or so all over.
On a good running motor I can feel the pee port water discharge temps rising and then hit the point where the t-stat opens and it begins to cool down. But I've never used this to diagnose a faulty or stuck closed one ... as I'd just pull it and replace it.
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Thermostats in two strokes work primarily at an idle. It is extremely important to keep these clean and working. They do not need regular maint. and nothing more than a simple check by hand or a digital thermometer is necessarily. Once in a while sand or other debris will get between the seal surfaces and keep a thermostat from closing. A quick clean will be all that is needed.
On OMC product the thermo should heat the engine to around 135f at idle and off-idle. This promotes a noticeably better run quality and keeps moisture build-up inside your engine to a minimum or eliminates it altogether.
To check by hand you can rest the palm of your hand on the top of the cylinder head and you should be able to hold it there for around 5-6 seconds before it becomes too uncomfortable to keep it there. Pretty simple.
On later model engines equipped with "Quickstart" a thermostat is imperative as the engine will never exit quickstart unless it reaches 100f.
OMC thermostats were designed that, should they fail, they fail in the open position....pretty smart on their part.
On OMC product the thermo should heat the engine to around 135f at idle and off-idle. This promotes a noticeably better run quality and keeps moisture build-up inside your engine to a minimum or eliminates it altogether.
To check by hand you can rest the palm of your hand on the top of the cylinder head and you should be able to hold it there for around 5-6 seconds before it becomes too uncomfortable to keep it there. Pretty simple.
On later model engines equipped with "Quickstart" a thermostat is imperative as the engine will never exit quickstart unless it reaches 100f.
OMC thermostats were designed that, should they fail, they fail in the open position....pretty smart on their part.
They're more important than people give them credit for, and the bigger the motor, the more important they become. Some of the V4's, V6's and V8's have two thermostats. If one fails, that bank runs cold. Pistons get hot. Pistons expand, block doesn't. Sometimes seizure and/or galling happens. EFI motors read engine temperature to adjust the ignition timing and a cold engine will need more timing advance. Stuck thermostat will "fool" the ECU into thinking that the engine is cold (and it is), advance the timing to compensate. Combine that with an already lean mixture on EFI engines-especially 2 strokes-and it can be bad news.
On 4 strokes they're quite important as well.
They're cheap enough that they're considered a maintenance item. Every so often, replace it/them as normal service. I forget what they told us the interval was, but it wasn't all the time like once a year. I want to say every 2 years but I could be wrong.
On 4 strokes they're quite important as well.
They're cheap enough that they're considered a maintenance item. Every so often, replace it/them as normal service. I forget what they told us the interval was, but it wasn't all the time like once a year. I want to say every 2 years but I could be wrong.
chrispbrown27
Well-known member
Mine is an 01 Yamaha F25 4 stroke. My experience is with old 2 strokes without thermostats and thermostats in cars. In a car you typically notice a loss of heat or the temp gauge stays low. I was just curious if there was something that would clue you in with an outboard thermo. I am going to change out the thermostat. I just know that in a car the thermostat can fail fairly often and I was wondering if I should keep an eye out for anything specific.
chrispbrown27 said:
Yes. Watch for the engine oil level to change quickly, usually it will rise-and fast. And if it's used in cold water conditions, it will also affect the idle quality since the engine won't warm up if it sticks open. I've never seen a yamaha with a stuck-shut 'stat but seen plenty stuck open.
