Mercruiser 3.0 : Engine Temp not coming up and bogs

Mercruiser 3.0 on a 2003 Mariah SX18 with 150 hours.
Craft and engine meticulously maintained. Runs (ran?) perfectly.

Yesterday boat ran perfectly for 2 hours. Engine temp steady at 170. During idle cooldown, temp reduced as expected.

Today, a totally different story. Engine temp did not raise at all and engine bogged/died when i tried to advance rpms.
I ran about 20-25 minutes between idle and 1200 and the temp never rose. The eng temp gauge does go "live" when
turn the key/run switch on.

Here's the chronology of what happened in case anyone can sleuth this.

5 minute warm up at dock. Idled smoothly, oil pressure up.
5 minute troll at 1200rpm to warm up engine and engine noted engine temp was not rising.
Tried taking it slowly to 2500 and engine bogged and died.
Restarted and trolled at 1500rpm for another 5 minutes, still no rise in engine temp.
Tried again to take the rpms up to 2500 and it bogged and tried to die before I pulled back to idle.
Idle it back to the dock and trailered it.
20-25 minutes at low-rpm and the engine temp never rose.

As soon as I got it on the trailer I felt the hoses.
The hose between the water pump and thermostat was WARM, not too hot to hold.
The hoses coming from the thermostat to the manifold and oil cooler were both COOL.
I keyed the instruments on and off and the engine temp cam alive (slightly more than peg left).

So, Ii'm no boat engine expert, but it seems this is a thermostat issue. BUT, I don't
understand why it is bogging when the throttle is advanced. It's carburated not fuel injected.
I'm not aware of any "computer" controlled carburation taking an engine temp calculation,
but as I say I'm no boat expert.

Is there some "guardian mode" on these boat engines that keep from going above a certain rpm.
before engine's temps register above a certain temperature?

Thanks. If you have any questions please let me know.

Dave

Re: Mercruiser 3.0 : Engine Temp not coming up and bogs

Sounds like thermostat for your heat issue, since you say after shutdown when the motor was able to heat up things without cooling water circulating, the gauge moved a bit. Thermostats are cheap, do one.
But the bog is not likely connected to the cool motor condition unless you have the choke that uses engine temp to pull it off. What's the upkeep and maintenance history on this motor?

Re: Mercruiser 3.0 : Engine Temp not coming up and bogs

remove fuel filter and empty contents and see if it is dirty or any water droplets.replace with a new filter not the one u removed.report back

Re: Mercruiser 3.0 : Engine Temp not coming up and bogs

Probabally just the thermostat. When mine failed open in my 3.0L a few weeks ago the engine bogged and ran like crap because it couldn't warm up. $20 worth of thermostat and gaskets and you should be fine.

Re: Mercruiser 3.0 : Engine Temp not coming up and bogs

The engine has been winter/summerized/oil+filter every year, garage kept.
Valve cover 3 years ago, other than than routine maintenance and minor nits.
Probably replace the plugs/wires in spring.

I'll check the water/fuel separator and replace the thermostat.

Hope it's not a water pump or water pickup on the outdrive.

I'll report back.

Thanks
Dave

Re: Mercruiser 3.0 : Engine Temp not coming up and bogs

gotta be the stat

Re: Mercruiser 3.0 : Engine Temp not coming up and bogs

Was there any popping during throttle up? If so I would highly recommend a compression test I had this issue when head gasket let go! But I would go with the things that are least expsensive 1st filters, stat or electrical? Look up in the adults only stickie at the top of this forum for troubleshooting tips on WOT. Good luck and post back!

Re: Mercruiser 3.0 : Engine Temp not coming up and bogs

OK, boat back from the shop.

The thermostat was sticking, that's it. Replaced it and temp comes up nicely.
Had the carb adjusted and it's smooth through the range.

As a precaution had a "once over" done. Compression great, plugs looked good.
Cap and rotor pitting, so replaced them. New impeller (it was the original), filters
and battery.

Vessel ran perfectly and put 6 hours on it over the weekend.

Thanks for all the tips