What I like to do on any truck is disconnect one heater core house and dump fluid in there. Then take the thermostat out and fill the block up. Finally fill the radiator let it idle for 15-20 minutes and top it off, I never had problems

 

What’s a good way to make sure the radiator is full? Autozone said it takes about 20 quarts. Not getting nearly that much in there!

 

Thats only if you drain the block. If you are just doing a thermostat I usually have to put in close to 2 gallons

 

I finally flushed the radiator.... I followed the directions from this form mostly.... I drained and filled the radiator several times with distilled water. Each time I let it run, cooled it down, drained and repeated. By the end I was getting mostly clear water coming out.

But the coolant temp sensor stopped working! I’ll see if it comes back to life. If not, I’ll replace it again. I just replaced it last year...

 

Have a seat kiddies! I'll give a little history lesson. Back in the 1930's, Plymouth cars were known for being reliable and as trouble free as any car built back then. Suddenly, dealers were getting the cars in with over heating problems. After looking into the trouble, it turned out that they had changed suppliers on thermostats. The old vendor had sloppy quality control but the new one was spot on. That's where the problem was. It sealed TOO good. An air bubble would form and until the air heated up, Eventually, the heat would transfer through the air pocket and the thermostat would open. However, air is very poor at heat transfer. The fix was to drill a very small hole in the flat metal of the thermostat. New units often have a jiggle valve to let the air through. If there isn't, sometimes when there is, I drill a 1/64 hole. Air will bubble through it but more viscous water can't.

Replace your thermostat with a regular 195 degree unit. Don't bother with a Fail Safe as every one of those I've dealt with were either bad, or went bad. Just a good part with a itty bitty hole. This will prevent the air pocket. You can often hear a slight flatulent sound coming from the fill neck of the radiator as the air under the thermostat bubbles through. This will prevent the volcano effect. This is where the hot air finally opens the thermostat and the water rushes through and hits hot metal needing cooling. It will blow out the fill neck. Since late model engines have a lot of aluminum, it can warp the heads. The old solid iron engines from the 1950's through the 70's.you could overheat and let cool over night and they would usually still be good. Today's engines, not so much.

 

You're right, I may just have to replace the thermostat. Trying to find a link on the steps for a 5.9 right now!

 

I’m getting heat through the vents, so wouldn’t that indicate the thermostat is working properly? It’s just the gauge or the sensor that isn’t reading the temperature.

 

Oh, that's old school simple. Drain the coolant to below the level of the thermostat. Remove the upper hose to give yourself room. There are two bolts that hold the thermostat housing down. Remove those and pull the housing off and the thermostat out. Clean the mating surfaces really well. You want nice clean smooth metal on both the intake and the housing.Get a new thermostat and gasket. I believe you have the paper type gasket on yours. I like to put a thin film of grey RTV water pump and thermostat gasket sealer on both sides of the gasket. Make sure the thermostat sits in the little recess and the gasket is on top. As you start to tighten the bolts, make sure it can wiggle a little. If you pinch it in the housing, it can leak or break the housing. Reinstall the hose and refill the system. A little tip, if you have things like a/c compressors and alternators in the way, get your self a distributor wrench. This will let you get under things to loosen or tighten bolts.

It's really easier than I make it sound. I just like to ramble on. Which is a problem as when Chrysler bought AMC, they didn't make the Rambler anymore.

 

Get yourself a remote infrared thermometer. You can check the temperature of the thermostat housing. These are handy little tools. You may have some heat but not all of it or you may have a bad sensor. You can also use these to find a dead cylinder. The exhaust manifold will be cooler there as the gasses aren't burnt. You may just need a sensor.

 

Heater is a constant flow system, and the thermostat has nothing to do with it.

What year is your truck?