5.0 MPI in my sons boat. I just replaced the intake due to corrosion and used the T-stat called for, 160. Gauge reads 180 at idle and running, maybe a bit more like 183 or so. This indicates that cooling flow is sufficient since idle and running does not change much. Initially, I though stat (or gague) may have been incorrect but the more reading I do I see that the T-stat BEGINS to open at 160 and is not fully open until 15 to 20 degree higher. Assuming gauge is correct and no other cooling system anomalies should the engine be running at 160 or 180? When Mec spec'd the 160 did they intend the actual water temp to be 180 or 160? My 502 with a high flow 140 runs at 140 but that is a HP500 carb. I question if I should lower the stat to a 140 and hopefully see 160 on the gauge and actual water temp in manifold. For now, we are assuming the gauge is correct. I plan on inserting a mechanical thermometer into one of the brass plugs to verify actual temp at the inake water housing. With that said should I be seeing 180 or 160 at cruise RPM with the 160 stat? If gauge is acurate do I want a lower temp stat to get the actual water temp in intake to be 160 or 180? This is not a performance setup, just a nice cat deck boat but the hull likes 3600 to 3800 with the SBC to get it to balance well. So with that said would the lower temp be better for oil temps and engine life to be at an actual 160 or 180?

I am not certain of oil temps but that was one of my concerns, this package does not have the oil cooler so I thought that an actual temp of 160 may be better as he is running fairly hard for a SBC. Once we get a setup to check oil temps we may put an oil cooler on it. Not building a sport boat, just a good family deck boat for now. I really wanted to put a 383 in but that was out of the budget and what he needs the boat for at this point. I know, in general, Merc spec'd 140 for carb and 160 for MPI. Also 140 for saltwater due to deposits etc. We are fresh water so not concerned with that.

My 502 MPIs ran between 170 and 180 on 160 degree thermostats. Raw water cooled. Never an issue.

I’m now running carbureted and closed cooled to save my aluminum heads. Same temperatures and no issues.

If the oil is staying clean and the engine maintains consistent oil pressure after a hard run, I wouldn’t worry about it.

I am really wondering if Merc wants them to run (as measured actual) 10 to 20 hotter than the thermostat rating. It seems they would have to run that much hotter as I have observed that the opening area at rated temp is very small and does not fully or appreciablyhave much flow area until 15 or so more than rated. Many have holes drilled in stat. The holes may allow just enough extra water thru the stat that when the cracking temp is hit that there is enough flow to keep it very close to the rated temp of the stat.

My 502 runs just at or slighlty below 140 but I have two 3/16 holes in the high flow body stat. I believe that this allows enough additional water to keep it at 140. The holes alone would likely not cool enough but the added small opening at 140 ish allows enough to cool to 140. I do see below 140 (with the two holes) so that makes me think there is a fair bit of cooling water at idle. As most to this stuff is made in China it could be just an "off" rating.

“ 5.0 MPI in my sons boat. I just replaced the intake due to corrosion and used the T-stat called for, 160. Gauge reads 180 at idle and running, maybe a bit more like 183 ”

totally fine. Unless this is a new big jump in temp.
Your gauge, wiring resistance (ohms) , and sender may be off a bit.

Same temps that I see on the gauges on my 502mpis