Hello. I'm replacing the *cracked* 305 block on my 82 z28 and I'm transplanting in a 350. I have run into the problem where my temp gauge in my dash is reading 250*, but when I take a reading off the block, coolant temp sensor plugs, or the heater hoses, I find it to be 180-200*. I'm not sure if the car is actually overheating or if my gauge is not working. It's a factory gauge too.

 

What did it normally read before the engine swap?...

 

Normally hovered around 210, but I also had to replace both of the thermostats because one was bunk and the other didn't fit into the engine block.

 

Two thermostats??...

 

I misunderstood the wires on top of the thermostat housing. The only temp sensor is on the block.

 

The thing in the block is NOT a sen-sore. It is a gauge sending unit.
Did you replace the thermostat in the water neck on the intake manifold?...

 

I replaced the thermostat, the sending unit, and the sensor on the thermostat housing.
unless that is also a sending unit.. regardless, it has been replaced.
also, my mistake on calling the sending unit a sensor only comes from what it is sold as z.B https://www.summitracing.com/parts/ado-g1852

 

The sending unit for the gauge is located in driver's side head.

Pull the wire off the sending unit (driver's side of the head) and see if the gauge drops to nothing. Then ground that wire and see if the gauge pegs. If it does, then the gauge is good and so is the wire.
Then either it's a bad sending unit (cheap Chinese stuff), or possibly the gauge was just pegged and that reset the sweep.

 

There are as many as THREE temp things on these cars: that one, which works ONLY the gauge and is not tied into ANYTHING else whatsoever; the one on the water outlet (aka thermostat housing) with 2 wires, yellow & black, which reports coolant temp (NOT engine temp) to the ECM for spark & fueling decisions, and in some configurations, some of the electric fan operation; and the fan switch in the pass side head in the same hole on that side as the gauge SU is in on the driver's side, which either operates the electric fan exclusively, or supplements the ECM's operation of it.

As far as the gauge:

When you turn the key to Start, it's supposed to mimic the operation of the idiot light (sofakingdom light?) which lights at that time, as a "bulb check". The gauge should read near zero when you turn the key to On with the engine cold, and peg when you turn it to Start. If this happens, then the gauge is somewhat good. From there, check the SU resistance with your handydandy ohmmeter. The SU will have VERY high resistance when cold, and VERY low when hot. I don't have numbers in front of me but expect some 10s of thousands of ohms when cold, around 1000 ohms (let's say 750 to 1500) at a normal operating temp like 200ish, and less hundreds as the motor overheats.

Tidbit of wisdom from over 50 yrs in this hobby: AT LEAST 90% of all "overheating" problems are in the owner's mind. About 50% of all the others are a defective radiator cap, and/or not using antifreeze. VERY FEW "overheating" problems observed in the wild are ACTUALLY due to engine temp being too high. Since the "real" (infrared) thermometer says it's OK, it's probably OK, and it's just a gauge problem. About 99.999% of those are due to the sending unit or faulty (hacked) wiring.

Work the odds. See where we're going with this.

 

When he said "block", I ASSumed he meant cylinder head...

 

Not heads. the engine itself.
If u have an infrared temp sensing gun, aim it at the thermostat housing & check the temp there. That gives u pretty good idea as to engine temp. Then aim it at the engine block & check temp.