Ok. I bought this truck and drove it 2 hours with no issues. The heat wasn’t great and the temp gauge never came off 100. Got it home let it sit till the weekend. Finally got to it. First thing I did was check the gauge by grounding it out and the gauge works. So I decided I’d buy a new temperature switch and replace it. Did the same thing. So I then went and checked the thermostat and sure enough it was stuck open. Replaced it. Started the truck and let her run for 15 minutes. Gauge moved up 1 mark off 100 and that’s it. Got great heat now. Put a thermometer in the heater vent and it read 130. So I then checked the upper and lower radiator hose. Getting 130 degrees on the upper and 80 degrees on the bottom. Not sure if that’s good readings. I wouldn’t think so because the thermostat is a 195. Upper radiator hose doesn’t seem to get overly hot or really hard. Radiator cap isn’t hot either. I’m totally baffled. I decided to clamp the upper radiator hose and see if the gauge would move. It did. Hose hit up to 160 degrees before I released it. My infrared thermostat reader said it was 170 degrees at the temperature switch for the gauge. But after releasing the clamp off the top radiator hose the gauge went down quickly. Any help would be great.

 

Are you sure you installed the thermostat correctly. It matters which end goes toward the engine.

 

Yes installed correctly.

 

You may have air in the system. Let the engine idle with the radiator cap off and the heat on until coolant starts coming out. Use a catch pan and let it drip for a while, then put the cap on and let it cool overnight, then top it off when it's cold. If both hoses aren't getting warm, you have no flow through the radiator. It could be plugged, which means it needs to be flushed.

 

Top radiator hose is at about 130 degrees. Bottom close to 90 degrees. Truck never overheats.

 

Sounds like your t-stat is staying open or your radiator is partially plugged. My 90 GMC runs at around 190. Was it closed when you put it in? You can test it in a pan of water on the stove to see when it starts to open. Should be around the rating. One thing you could do is block the radiator with cardboard and see if the top and bottom hoses get to about the same temperature. If not, you've got flow problems. My money is on a partially plugged radiator and you just haven't driven it hard enough for it to overheat.

 

It was closed when I put it in. It was brand new. I know new things aren’t always a guarantee.

 

Well, if it's not overheating and giving you heat, forget it. If it ain't broke, fix it 'till it is.......