Recently purchased a 1990 Supra comp with pcm 351 ford. Recently changed oil and impeller. Old impeller was missing rubber so I took apart thermostat housing to check for pieces. I found nothing. Mechanic friend said chunks probably blew through exhaust and not to worry so we took it out.

Temp gauge alarm goes off as soon as key is turned and gauge is either bottomed our pegged out or jumping all over. If I put finger pressure on it it shuts up.

Ran it about 15 min at idle and was cool to the touch. Drove about 15 min at mid throttle and felt warm but nothing alarming. Parked and fished and drove 25 min when we stopped it was boiling water in the engine. Stopped for 2 hours. I checked the thermostat and I had put it in with the spring facing up away from the engine. Made a phone call, the orientation seemed incorrect so I flipped it over. Restarted boat after cooling and immediately upon applying throttle felt water flowing in exhaust elbows. Boat ran another 20 minutes at 3/4 throttle and felt lukewarm to the touch but if I give it throttle would be cool. Noticed that after stopping it would heat up with engine off and as soon as I’d start it it would cool and as soon as I hit the throttle it would cool a lot more. Took it in. Parked it.

Where is the temp sending unit located? I need a working heat gauge! Where could I order new gauge and sender?

How bad do you think I screwed up the motor by overheating it? It wasn’t smoking or smelling funny but was obviously boiling internally.

Was my thermostat likely the reason for overheat? I made sure my water pimp was installed the same way it came off and I pulled the outlet hose and what I could see of the impeller was in good shape. I’ll pull it all back apart again but my mechanic had originally told me it didn’t need an impeller and the alarm was only because of a loose belt so I don’t really trust the guy!