Quick Temperature Spikes

I have a 94 Grand AM 3.1 and I am getting temp spikes.
I will be driving along and the temp will shoot up very fast, and then stay up around the red for several seconds, then drop very quickly back to around
1/4 on the guage.
Originally it was thought that it was just low on coolant, so my garage bled the cooling system, and topped it up. This worked for a short while. Then the problem returned.
The garage also ran a pressure test, and said it didn't look like there was a leak in the system. The thermostat, and water pump were replaced, and this seemed to fix the problem for about 2 weeks, but now the problem has returned again. It is losing coolant somewhere, but it doesn't look like it is going into the oil. And it is not losing fluid when I am parked.
Also I can here a loud rushing of water or gurgling noise that sounds like it coming from right behind the dash. In addition when the temp does go up, I can usually put the car in neutral and rev tha gas, and it seems to get the circulation going and the temp drops to normal.
I had a similar problem before and it was the intake manifold, but there was coolant leaking in the oil, and the temp was rising gradually and staying up. This time the temp is spiking quickly and dropping quicky after a few seconds, and like I said, no signs of coolant mixing with the oil.
I never did have the entire system flushed out, I think they just ran it and let the air pockets come out. I am wondering if maybe there is a large pocket somewhere perhaps in the block itself that is not being flushed out.
If anyone has any suggestions of what to check for next it would be much appreciated.
Regards, Jax
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How old are the hoses. They will separate internally and collapse causing this type of problem. Replace them...

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Has anybody checked the temperature sensor......

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was
this
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up,
was
and
out.
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This sounds like a good start..
Quick spkikes must be elctrical.. Water doesn't instantly heat up the metal sensor and then cool it in a matter of seconds...
Jeff
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Sound like air entrapment in your coolant system if you can hear gurgling or boiling noise. Coolant system will not operate properly when coolant pressure is not properly controlled.

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and
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Don't know if this helps or not, but I had a 86 Ford TBird that did the same thing. Turned out to be a small voltage regulator in the instrument cluster that would intermittently "stick" and cause the gauge to climb. Next time it happens see if your fuel gauge climbs too.

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Could be a faulty pressure cap, faulty temp gauge sending unit, loose ground. Gurgling in dash is probably sounding from the heater core. Hope this helps some.
Dave S(Texas)
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had the same problem with my 92 grand am.after much testing and probing,wound up being that the electrical fan was intermittent,would work and test fine at times but while on the road or whenever it wanted would quite for intervals sending the temps up,kick on at will then temps would drop,ran the car in my drive way at idle just watching it and lo and behold it did it while i was testing it. good luck

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