Toyota Camry Cooling System Questions

I have a 2003 Camry that has recently begun having issues. While driving or idle, the RPM's start going up and the temp guage immediately goes down to past the Cool. If I put it in Neutral the RPM's go higher and my engine is revving. If I were to take my foot off the brake, the car would go by itself. Eventually, the temp guage goes back to normal and my RPM's subside. However, if the car is in Park and this happens and I try to put it in drive, then I'm unable to start my car for about 15-30 minutes. I've taken it to the mechanic twice, and both times, they claim to "fix it." Can someone PLEASE help me? thanks.

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Is there water/antifreeze in the radiator and overflow tank to the right levels? Are you having to add water to keep the levels correct?
They've swapped out your waterpump, so you should have good coolant flow (assuming it isn't air locked, and the thermostat is opening at temperature).
If the car overheated and stalled out I would be very concerned about a blown headgasket....but I'm still slightly concerned about that ultimate possibility depending upon what else you find in trouble shooting this. You could see loss of coolant without seeing leaks, oilish film in the radiator, and or a lot of air that continually enters the coolant system.
If you had a simple air blockage (due to replacing the waterpump and refilling), those air bubbles should clear themselves after a number of heatup and cooldown cycles. As the engine heated up, the thermostat would open, air bubbles in the system would get flushed up the highest point in the radiator, and as the radiator cooled down it would draw in replacement coolant from the reservoir. After a couple of cycles, during heat up the air gets flushed out to the reservoir, and replacement coolant is sucked back in during cooling. Be sure to turn the heater on at least once so that the coolant flows thru the heater core clearing that line or air as well.
If the thermostat was stuck open, then the vehicle would come up to temperature slower. If the thermostat was stuck closed, then the vehicle would overheat. If it was sporadically sticking, then you'd see flukey symptoms.
The temperature sensor would feed the gauge, but also is an input to the engine computer which can adjust the fuel mixture, timings, rpm, etc. So a bad sensor can affect the smooth running of the engine.
So I'd personally be watching closely the air and coolant levels in the radiator and reservoir for a couple of heating/cooling cycles. Assuming you don't see air entering the coolant system from the cylinders, an oily and/or foamy substance, or loss of coolant ......then I'd replace the thermostat (cheap to do), and focus on the temperature sensor after that.
When you say it was 'full', what was full? The radiator overflow tank? or you opened the cap to look in the radiator and the radiator was full?
Assuming original coolant with nothing done to it, and it is still full......depending on your circumstances and how critical the availability of the car is, you can just watch it for a while to see if you have a problem again, or you could just swap out the thermostat since they are cheap.
I'd at a minimum make sure that the radiator is indeed full, and in a pinch if you overheat again you can run the inside heater full hot blast to help cool whatever coolant is in the engine.
I bought concentrated long-life coolant from the Toyota dealership just to top off the coolant and now not sure which water I should use to dilute coolant: distilled or tap water will do. If anybody has such experience, I would appreciate to hear your opinion.
I doubt evaporation would occur, as the system is closed. I'd recommend checking the level when the engine is cold, such as first thing in the morning.
I check the level weekly. The '07 has dropped about to just above midway on the overflow tank and the car has 7,800 miles, purchased in April 2006, so this car is less used and the coolant loss from the tank is less than the '05. I'm beginning to believe this is just normal for these vehicles as the coolant loss is not dramatic and happens over an extended period of time.
I've checked the cap of the overflow tank for resin insert and it looks good to me (no crack, no deformations).
It's hard to believe, but I can't blame dealership for blowing repair volume, because warranty covers all the costs. Apparently, that plastic plug was so reliable leakwise, that there was never a need to supply it as a part, therefore the only way to get it is together with the engine block.
I'm worried about such significant change. I asked adviser if I have to go through break-in period, because of new engine block and he said, that engine doesn't need break-in even if new, transmission does. Sounds weird to me :confuse: :confuse:
Anybody went through engine block replacement?
By the way, I couldn't find any break-in instructions in my Camry 2004 manual. So, I was just taking it easy for 2,000 kilometers.