Temp Fluctuating??

[masher]

We have a 99 Tahoe with the 5.7. I just replaced the water pump and now the temp gauge will go up to about 210 and then drop to about 170 and then back up to 210 etc.... It takes about a minute or two to go through the cycle. I thought maybe I had an air bubble but I have pumped the top hose for a while, and ran the truck with the radiator cap off for about a half-hour with no luck. We have also driven the truck about 500 miles since it has been doing this I would think the air would have worked its way out. Anyone have any ideas or seem this before??

Thanks

[Whifflebat]

Sounds like the thermostat is just opening and closing...

[josh89chevy]

My 89 does the same thing. I believe it started doing it after I replaced the water pump also. Might have done it before, I don't remember. I replaced the thermostat recently hoping to fix the problem, but it didn't. I'm stumped.

[iceman7329]

My 89 starting doing this also after replacing my T-stat.

[Whifflebat]

I think its normal. Think about it, engine warms up to 210, so the tstat opens up. Coolant flows out of the engine, to the radiator, cools off, goes back to the engine. Its cold outside (winter), so the engine cools down. This causes the tstat to close up again, until the engine heats up again, then the process repeats.

[iceman7329]

Makes sence, only does it in the winter but I have a 195 t-stat and my truck runs at 180 all day.