To the point, got a ’95 12v Cummins last August 2wd standard with 180,000 miles. Had the normal constant thermostat open and closing procedure, then twice in September it temp. gauge redlined for under 60 seconds then opened up and ran normal. So I went to Dodge and bought new thermostat for it. Swapped out thermostat and ran that evening and did fine. Next morning on way to work it overheated and didn’t cool down. It was low on coolant so I topped it off and it would still overheat and blow about a gallon out. So I bought another thermostat, radiator cap and radiator hoses installed them and ran fine for about a week then same problem. I let the truck sit for several months then took it to a guy to figure it out. He pressure checked the cooling system checked fine so he ran the check again but for 3 days straight and still no leak down. He then pulled the radiator had a flow test done and it was 80% and he said that was good enough. He changed the water pump, temp. sending unit and temp. gauge. Picked truck up last night and it did fine. Driving to work this morning it opened just after 190 and dropped down, then raised back up to redline, when I would let off the throttle it would back down a little but as soon as I throttled it would go back up to redline. I stopped the truck and let it idle and it went down it between 190 and redline. After I shut down I could here it boiling out. What could this be?

 

take one of the old thermostats and drill a half a dozen 1/4 holes in the outside edge of the valve and put it in the truck and give it a test. i feel you are just having bad thermostat problem, i know you have changed them out. but it still keeps pointing that way.