Temperature gauge or thermostat?

[jmarty10]

I have an '08 4.3 merc TKS fresh water cooled engine. On Saturday when I started the engine the temp gauge got up to normal temp. When i opened it up I hit about 175 and it dropped a little to 165. When I started idling it dropped pretty significantly around 120-130. For rest of day it didnt get back up past 150 when running around 3000 RPM. Boat ran great. High temp alarm never went off. I'm wondering if gauge is faulty or bad thermostat? Boat is 3 yeras old with 80-90 hours on it. FYI.....Water pump was changed last fall. Thanks

 

[Doernuth]

Re: Temperature gauge or thermostat?

Sounds like you got air in the system.

Run it again and see if it stays normal. The other thing is to get an IR thermometer and check the temp near the sensor see if it is near the temp on the guage.

 

[Capn G]

Re: Temperature gauge or thermostat?

Do you have a "run safe" thermostat installed? I've had mine lock open before temp really got hot... same problem you describe, temp normal, then run it hard for a bit to get temp up and then it drops off 20-30 degrees with no alarms.

Another thing I've wondered about, is that the fresh water cooled engines don't seem to have a "filter' between the lower unit pickup, thought the impeller, hose, or finally the distribution point on the front of the engine... Seems plausible to me that lake "stuff" can get sucked up into the water system and eventually make its way to the thermostat/water pump... Could it be possible you might have taken up a bit of lake debris that got wedged in between your plate on the thermostat not allowing it to close completely???

 

[04fxdwgi]

Re: Temperature gauge or thermostat?

I have an '08 4.3 merc TKS fresh water cooled engine. On Saturday when I started the engine the temp gauge got up to normal temp. When i opened it up I hit about 175 and it dropped a little to 165. When I started idling it dropped pretty significantly around 120-130. For rest of day it didnt get back up past 150 when running around 3000 RPM. Boat ran great. High temp alarm never went off. I'm wondering if gauge is faulty or bad thermostat? Boat is 3 yeras old with 80-90 hours on it. FYI.....Water pump was changed last fall. Thanks

Almost sounds like the stat is stuck (or working very slowly), or maybe had a little piece of something stuck in it, keeping it from operating as it should. Shouldn't drop that low if stat is working properly and it indicates the stat is stuck slightly open. The stat should maintain temp from going too high OR too low, pretty closely. Seen that many times before. Try the IR thermometer trick and see if gauge reads the same as the hand held (fairly closely). If they are close and it does it again, I would replace the stat. If the engine was ever overheated, then replace the stat, since an overheat can mess them up pretty bad.

 

[jmarty10]

Re: Temperature gauge or thermostat?

If the stat is stuck or not working correctly would it prevent the over heat alarm from working?

 

[04fxdwgi]

Re: Temperature gauge or thermostat?

If the stat is stuck or not working correctly would it prevent the over heat alarm from working?

If it's stuck open far enough, engine won't come up to temp and never set off the alarm. Seen em stick open closed and everywhere in between.....

 

[jmarty10]

Re: Temperature gauge or thermostat?

Ok - I purchased a RYOBI IR gun for Home Depot yesterday. I am going out tonight to to check temp at different locations of the engine. Block, manifolds, riser, and thermostat housing correct? Also, I think I am going to want to warm boat up and cruise around at 1500 rpm for a while just to make sure therm opens up and no overheat issues off the bat since the therm seems faulty correct? Thanks

 

[jmarty10]

Re: Temperature gauge or thermostat?

Ok - here's an update: Took the boat out tonight and ran it for about an hour and a half varying from 3000 RPM to WOT. Took a long time for gauge to get up to op temp but it did after 20 minutes. Stopped boat at idle opened up hatch and used IR gun to measure risers, manifold and t-stat housing and circ pump hose. Both risers I could keep my hand on and measured about 120-130. Manifolds way to hot to the touch. Star side measured about 185 and port side measured about 204. Circ pump hose measured about 165 and was consistent with the gauge.

2nd cruise of night I ran at WOT. Before I really opened her up I was cruising at about 3000 rpm and gauge was struggling to move up. Once I went WOT she got up again to 165-170. Stopped and took reading and had same temp measurements as before.

So, I definitely dont have a low temp problem because I'm getting temp. The gauge eventually gets to temp and doesnt go past 175 and no overheat alarm. So I will just keep my eys and ears open for problems. BTW: How hot do manifolds get?