305 engine gets hot?

[My 86]

Have been having problems with my 86 GMC 1500 pickup. When I first start up the truck and head towards work about after the 3rd mile the temp gauge has been pegging to 260F. If I let off the accelerator it will drop below 200 but then go back up when I accelerate. Just replaced the radiator and water pump today along with all new hoses. I also have changed the thermostat for the second time. No water in the oil and exhaust does NOT have a sweet smell. The temp gauge still pegs out to 260 after it has been driven a few miles. All hoses seem quite hot to the touch. Could it be a gauge problem or sending unit?. I tried an experiment. I unplugged the connector from the sensor on the thermostat housing and the gauge didn't peg out after driving awhile. Stayed between 200 & 210F. Is this the sending unit for the temp gauge on the dash? If not where is the sender located? And should I replace it? Need some help here I'm out of ideas. I appreciate any comments or suggestions from you all . Thanks!

[1967KaiserM715]

I would get an infrared thermometer, to verify temps. The sensor for coolant temp gauge should be in the drivers head near the front by number 1 exhaust port.


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[My 86]

Thanks 1967KaiserM715.  I'll look for one of those thermometers today. I also notice when I start the engine each time I notice a sloshing sound by the heater core. Is that normal? I have owned several of this style truck over the years and have never noticed that .

[bd]

Air is trapped in the heater core circuit.  Recheck the coolant level.

[My 86]

Ok I replaced the coolant sensor and replaced some of the wire that was bare and possibly touching a ground. Went and drove it again and it still pegs out the temp gauge. I purchased an infrared thermometer and ran some checks. 177 to 180F around thermostat housing and water pump. 197F on the temperature sensor. This is idling. The temp gauge on the dash reads between 210 & 220F .

[My 86]

This morning I did a pressure test on the cooling system. The gauge on the tester did not drop so I believe I don't have a head gasket problem. Turns out the gauge in the dash appears to not be working. So I went to NAPA and purchased a new temp gauge. It mounts under the dash. I don't like that but it will work for now until I get an OEM replacement. So I went head and drove it on the highway several miles. The gauge reads between 190 & 205F which is well within range.