Temperature Gauge Fluctuations

[avaid]

I have a 94 pontiac grand am and recently replaced the engine. put in a brand new water pump but did not change the thermostat because the mechanic said it appeared to be fine.
Ive noticed the temperature stays around the middle mark most of the time and when i let it sit to idle from cold, it heats up and the fan turns on when it reaches halfway. however, when driving, ill notice the temperature change from the middle mark, go up to right before the red, and then go back way down to the quarter mark. Ive seen it go from 260 degrees F to 160 in a matter of 30 or 40 seconds.
my best guess is that its the thermostat needing to be replaced but any other ideas from the more experienced folks out there would be greatly appreciated.

 

[melsg5]

You have air in the cooling system

 

['03 Grand-Am GT]

Sometimes when the mechanic at the shop tells you that the thermostat doesn't need to be changed could be either that it is actually still good or that he saw the location (too hard to reach) and figured that if there's no problem, just leave it there. This is based on a pass experience with my '03 Sunfire 2.2 ecotec. Was scheduled for cooling syst. flush and I asked if they could changed the thermostat since they were working on the cooling syst. They told me that they don't change it unless there's really something wrong. Well sure enough, couple of weeks later, you guessed it, no heat, therm. stuck open! And on some cars when that happens, you can get a check eng light or, on the Sunfires, no check eng light but cooling fan runs all the time...by default I figured. So anyways, ended up changing it myself and when I saw the location, I understood why the GM dealership didn't want to do the job...lol.

 

['03 Grand-Am GT]

Melsg5 is right though, check for air in the cooling syst. And since thermostats are fairly unexpensive why not change it while you got your hands in there?

______________________________

 

[richardgriese]

^x3

 

[melsg5]

x4 on the thermostat and proper bleeding procedure