TEMP going up and down after gasket change.

If you think it is air, try getting the front end up high while it is running with the radiator cap off. Preferably you'd do this before it gets hot. You can also gently/slowly squeeze the upper radiator hose to help push out any air that may be trapped.

This assumes the obvious things like correct installation, 195ºF thermostat, etc.

2001BlackonBlack said:

After replacing my leaking thermostat housing back on my 2001 L6, I have a problem now where the temp gauge is moving up and down, the Heat works fine and it is full of coolant.

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Couple of possibilities:

You've still got air trapped in the system. Park with the nose up, remove the radiator cap (just opening the overflow bottle up won't do it), and let it idle for a few minutes. Keep an eye on the coolant level; it's possible for the level to look good at the radiator or overflow bottle, but still be low in the engine itself.

Your coolant temperature sensor (located in the thermostat housing) was collateral damage of the housing replacement, or was on its way out and coincidentally chose right now as the time to croak. It can be replaced without having to remove the housing again, but give the wiring and connector around it a check first - the plug on the connector gets brittle over time and doesn't necessarily make good contact; the wiring can do the same.

The thermostat isn't aligned properly with the opening it sets against. It's really easy for them to get moved out of place when replacing the housing; if the two ideas above don't clear the issue, you may have to pull the housing and double-check it for correct orientation.

There are a couple of other more remote possibilities, but having been through this before those are the things I'd start with if I were in your shoes.

So it turns out that over lunch it fixed itself, thanks for the help I'll keep that in mind next time I have to replace my coolant.

2001BlackonBlack said:

So it turns out that over lunch it fixed itself, thanks for the help I'll keep that in mind next time I have to replace my coolant.

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Likely then it was an air pocket. Although open systems are self-burping it often takes a number of heat-cool cycles to get all of the air out.

JPs01XJ has a good suggestion to speed up the process, but I would change the cap off to cap on. Let the air get to the cap and allow the open system to do its job--just make sure the reservoir bottle has the correct level in it.

There are a number of vehicles with open systems that simply won't self-burp. You have to raise the front end WAY up in the air to get some of them to cooperate.