I have a 1995 Silverado Z71 5.7tbi. If I do not run a thermostat, the engine runs about 105 and gets me from a to b. When I put a thermostat in, the temp rises to about 250 before opening and cooling to about 140. This repeats itself indefinitely. The thermostat is a ACDelco 195. I’ve replaced it 4 times thinking I was getting a bad thermostat. I put a thermometer on the blocks because I thought the sensor was bad and the driver side is running much warmer than the passenger side. The intake, where the thermostat is held, is staying within the right temp for the thermostat. I can not figure out why it will not keep my temp regulated. Please help. My next step is changing the water pump, I’m thinking it just isn’t moving water like it should. I did check flow on the water pump and it’s flowing the right way.

 

Welcome from Florida. I would flush out your cooling system. If that doesn't help, I would then go to a colder thermostat.

 

Where are you measuring the temp at? If it’s not at the same location on both heads, the difference in readings may be normal. Try to check it at the center of the head, just above the intake. If that area is too dirty/covered, pick another location that can be checked on both heads. Post a picture if you need help finding a spot.

If one head is running hotter than the other, it’s likely a restriction or air pocket that’s preventing adequate circulation. Try running the cabin heater on full power while the concern is present - if the heat fades in and out, or fades completely without revving the engine, it’s an air pocket, and you’re most likely looking at a head gasket - pull the plugs for inspection - if any of them look like they’ve been steam cleaned, perform compression testing.