I have seen this before...as a matter of fact, it actually happened to one of my cars...the temp would get extremely hot then all of a sudden drop to the normal range...(like a stuck stat), I did replace the stat and the problem continued...I knew however that the stat was not opening at the proper time, because when it did, I could watch my temp hand drop to normal. The problem was air in the system...took a while to get it all "burped" out, now it works perfectly. Keep an eye on the coolant levels and the resevoir fluctuation. This was the defining moment for me, the resevoir level dropped suddenly, I refilled and the problem went away. Good Luck! I hope you find this to be very helpful!
By the way, a pressure test will not necessarily reveal a cracked head...depends on the temp that the crack expands at...check your oil for coolant...(make sure you have no oil in the coolant), secondly, depending on where the crack is, (If you have one), you could be loosing coolant thru the exhaust...same is true with a head gasket. Hopefully however, it will be the air issue and you can get it worked out!
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two garages pressure tested system, one of them for 3 days. Originally the van would start to overheat but if I kept driving it would g back to a normal temp. I thought maybe the thermostat was sticking so I had it replaced. This is when they did the first pressure test. After I got the van back the temp gauge seemed a little higher than normal but everthing seemed to be okay, for the first hour. Then the van started to overheat everytime I used it. Towed to second garage, pressure test for three days, no problem found. Water pump removed and checked, no problem found. Now I am stuck. Spent$700 and still have the problem.
How do you know the head gasket is good...you could also have a cracked head, exhibits the same symptoms as a blown head gasket. Also, air pockets in the water jacket can cause overheating...tell me more and I can help solve this.
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