Overheating?
 

When I bought my tantrum (stock 502 MPI) the thermostat typically held at about 180. Last year it overheated (pump went out) so the pump was replaced and everything was cleaned out. This year the thermostat has been reading just below the buzzer at 240. I haven'd noticed anything wrong performance wise. Do you think this could be a thermostat problem or can my boat run normally at 230ish?

Oil temp stays below 180 (bottom of gauge)...

240 is bad if that is the real temp. Did you back flush the system to get the pieces of impeller that shredded out? Run it at 2000 rpm in neutral and see if you have good water coming out the the back, should be enough to fill a 3 gallon bucket in seconds

Boat should run right where the thermo is set or slightly higher (10 degrees) when u do wide open runs

Did you pull the pieces out at the oil cooler intake line. Thats where they will be.

All of the pieces were flushed out of the system and the temp was checked and found to be OK last year...

Do you have a water pressure gauge?
Have you verified with a laser temp gun?

I'll have the marina verify the temp this week. What is the normal water pressure reading?

The alarm should go off around 200-210*. The alarm temp sensor is separate from the gauge sensor. I doubt you are actually running as hot as the gauge shows. It could be a bad gauge or sensor.
230-240* is way hot and you would have all kinds of steam coming out the exhaust.

Great info. The gauge always shows the temp just below 240, but the buzzer never goes off, performance is not affected, and there is no steam. I'm going to remove the plugs (one at a time) and flush the system out tomorrow.

Before you do that,

1) Running on hose see how much water is coming out of your exhaust.
2) Get a real temp reading from your thermostat area using a temp gun.
3) Pull the feed hose off the transom assembly, just follow the sea water pump hose back. It should be an 11mm bolt make sure its not restricted.
4) keep in mind when your impeller went last year it may have worn the pump housing and now the pump can not make enough pressure.

Good Luck.

If the wire is loose or falls off the sender it will peg the gauge. Don't think your motor would still be running if it really ran at 240 all summer

I took each plug out individually and started the motor for 30 seconds. A small amount of sand came out with the plugs, but other than that the water was running clear and the pressure was strong. I ran with MGH's tantrum up the middle channel to Deckers averaging 50 mph at 3500-4000 rpms, no problems with the engine. The water pressure gauge was reading a bit over 10.

The water temp gauge was constant at about 200. Thats what it read in July as well when I did a 160 mile round trip to Grand Bend ON.

i threw up a video of ur boat - looks good running

If its really 200 that is too hot.

You have to get a laser temperature gun on the thermostat housing when the motor is at 200 degrees to see if the guage is right or wrong. That will really help tell whether it is a cooling issue or a wiring issue and where to start diagnosing the problem. At 240 degrees you should see a noticable drop in oil pressure from the oil thinning out. That would be at least an indication that the engine is actually overheating as opposed to needing a new sending unit, etc.

Just because it is running lower now doesn't mean it is not a problem. I do not know where you live but cooler water temps can cause such a reduction.

I'm having the marina check it. I just hope it is the therm and not the actual temp, can't afford to replace that engine right now if something's facked up.