My temp gauge started spiking real fast when coming to a stop then getting back on the gas. I flushed the sys. but still doing the same thing So here's where I'm at. I replaced both thermostats with 180 degree. I replaced the fan clutch because it was weak and new radiator cap. The first time I started it I bled the sys. and the temp still crept up slowly while idling so I cut it off thinking head gasket. So I got a block tester to see what would happen. I started it with cap off but the water would come out until the thermostats opened. I put the cap back until around 180. It released pressure but stayed at 180. I put the block tester on and it would start bubbling by itself. I left it on for 3 or 4 minutes and the liquid stayed blue. I done this 3 times so I'm thinking maybe head gasket not blown? I put the cap back on and let it idle for another 10 minutes or so and the temp stayed around 180. I let it cool down to hook everything up to take it on a test ride. After doing all that I top it off and start it up. I crack the bleeder just to make sure no air. The temp trys to creep up so I take the cap off. It cools back to 180. I top it off and put the cap back on and let it idle for a few minutes, still 180. I take it down the road at 60 come to a stop and turn around still 180. Head back and come to a stop. After I turn on my street It starts to heat up. Pull in my drive way and try to take off the cap. After getting it off and hot water coming out the temp goes back down to 180. So now what

 

I started it kinda cold again. There was still some pressure on the top hose. It crept up to almost 210 so I to the cap off and it went back down to 180. I top it off again bled the sys. and put the cap back on. It idled for about 40 min at 180. I hit the bleeder screw a couple more times. Went down the road again. When I came to a complete stop it was @180. When I took off again it went up to 210 fast turned in my driveway and went down to about 200 and stayed there. When the gauge is at 180 the heat works. When the gauge goes up the heat blows cold. Is there still air in the sys. causing all this?

 

your issue sounds kind of awkward, if you start the truck with the radiator cap off does it bubble out like maby a blown head gasket? and are you reading the temp from the stock electronic gauge? I have heard of the electronic gauges going bad and showing erratic temps. if you had water coming out of the cap until the thermostats opened that tells me that there is a leaky head gasket or something related, like a cracked head or sleve but the temp spiking at a stop is not making sense.