Diagnosis for running hot

Hello.  My 89' SN with 351W is running hot recently; around 200 degrees most of the time.  Use to run around 180 degrees which I thought was a little high as well.  A little history.  I replace my impeller every year and just put a new one in a month or so ago.  I have also replaced the thermostat (142 degree model).  I have checked for suction leaks and obstructions from the strainer to the thermostat and can't find any broken off impeller vanes that could be restricting water flow. Seals and hoses seam to be fine (hoses are relatively new also).   I've done the bucket test and it sucks down water quickly at idle - slightly more quickly than the hose can refill.  I have pulled the engine drain plugs out of the block, manifolds ,and j-tube on the pump (to winterize) and didn't see any evidence of obstructions there either.  What am I missing?  Bad temp sensor and or gauge?  Not sure where the sensor is located.  Any other advice on what else to check would be most appreciated!  TIA!  -Gene

TRBenj wrote: I’d confirm the actual temps before proceeding… a cheap IR thermometer comes in handy. Shoot the temp sender area- will be right near the thermostat housing area on the intake (engine water exit).

The pump is installed correctly. I’ll check temps with an IR gun and report back. Thanks so much for the advice! CCF is such a great resource!

It could if your not the original owner and not sure what the PO did. My PCM powered saltwater boats pump started leaking. When I pulled it I found out it was an automotive pump and most of the steel impeller blades were gone. Never had and overheat problem though. Some brands of automotive pumps have plastic impellers as well,I have heard of them slipping/spinning on the shafts.  

See photo below. I would check it after you have confirmed high engine temps with an IR thermometer as recommended by JQ and are certain there are no air leaks in the cooling system (including the rubber o-ring for the sea strainer bowl). 

Circling back on my thread here. First time on the lake this season (what a shame). Gauge still showing over 200 degrees and alarm sounds. Replaced the temp sending unit. Also, as suggested, checked surface temperature near temp sending unit with an IR laser meter and shows a much better 140 degrees. So I guess I have a bad temp gauge. Would a bad temp gauge cause the alarm to sound or do they work independent of each other? TIA

Since you already replaced the sending unit, what was the part number of the new one that you put in?

Your sender cross references to the PCM unit for that engine. It's PCM part number is R02003.

If the sending unit wire is shorted to ground somewhere would that make it show a Hot or Cold Temp?  Maybe he just has a frayed wire or the bad connection you pointed out.  Bfootr, Ken was referring to the electrical connection at the rear of your engine, 8 wires in one plug not your spark plug wires.

If the tan wire is grounded, it'll peg the gauge, but you're not pegged.

Okay. Found the connector. There was some corrosion around a couple of the pins (though not around the tan wire pin). Cleaned both male and female ends well with electronic cleaner and a small wire brush. Reconnected it, started the boat with the water bucket connected, and gauge showed over 200 degrees and alarm sounded within 1-2 minutes. Engine did not feel that hot and water was belching well out the back.

Another piece of the puzzle that I noticed yesterday while on the lake. In addition to the temp gauge, the oil pressure gauge started bouncing around 80 PSI and the alarm light was on next to it (along with the temp gauge light). Has always ran around 80 PSI on oil pressure. Don’t know if the two issues are related somehow. Boat seems to be running fine otherwise.

bfootr wrote: Thank you so much. I’ll check it out and respond back. I did notice that when the boat is off, the temp gauge is pegged to the far right (and not to the far left as I would think).