Overheating issue - I am stumped!

[Mattchu60]

Hey Folks, hoping for some ideas here. I have a 87' TBI 350 truck that last week i swapped a rebuilt 350 sbc into. The swap went fine and everything got hooked up ok. I am having a super-frustrating/scary issue that I cannot figure out.

Heres what happens - I start the truck up from sitting cold, it warms up to the normal operating temp and just keeps going up, up, up until she hits almost 250 degrees, hangs for a bit and comes down to 200ish for the rest of the time. Its pretty dang scary to watch it happen, once I see it start climbing past 230 I usually shut the engine off because it scares me so much.

Its not a really wild engine, I estimated the CR around 9:1, it has a pretty mild comp cam in it (12-388 i believe is the number), summit 72cc heads and flat top pistons. It shouldn't be putting out a lot of heat.

My first guess was air pockets in the system, so I sat it on a hill and ran it with the cap off to see if any air came out, no change.

Last night I decided to replace the thermostat and also the sender for the gauge to see if that made a difference. After refilling with coolant through the thermostat hole on the intake, then topping it off in the radiator (i wanted no air in the system!) i went for a drive, i was disappointed to see the same result again - climbing temps to a very high point, although now its not settling down as quickly as before.

Heres the kicker - I have the EBL flash system on my truck so I could use my TBI still. I get a full readout from all the sensors the ECM uses on my laptop - and the Coolant Temp Sensor on the intake manifold is not getting any higher than 210 degrees at the high point. This information makes me think its a sender/gauge issue for the unit in the cylinder head (TBI has one sender for the gauge in the head, one in the intake for ECM).

I am confused and frustrated. My father said to just ignore it, but its pretty darn hard to ignore watching the gauge climb so high, because I have no idea if its sending false signal or maybe its right.

Any thoughts are appreciated to help me figure this out. I may try removing the thermostat all together to see if it makes any difference.