Fluctuating temperature

[Lothar]

So: '92 Cavalier, 2.2L, Auto. 208k
After replacing the thermostat along with a radiator to correct overheating last fall, the car ran downright cold. About Christmas time, I got tired of covering the radiator to make the heater work and bought a new thermostat in the proper range. Since then, the car has run in a pretty normal temperature range, but the temp. needle fluctuates on about a 30 second cycle, climbing slowly up to the halfway point on the guage, then dropping quickly back to the 1/4 mark. Any ideas? The coolant doesn't seem to be leaking in any appreciable quantity and the oil has been normal the whole time. (I've replaced the headgasket before, with lots of help from the folks here). There's a coolant smell when I turn on the heater, so I suspect a small leak in the heater core. Would that allow pressure changes that would contribute to this sort of symptom?

This seems like the old "air bubble in the coolant" problem, but so far as I know, there's no provision for bleeding the coolant system in this car.

This problem has come up on other topics, but they always seem to have flamed out before the discussion came to fruition.

[Lothar]

Thanks for the response.

The heater core isn't belching clouds of antifreeze into the car yet, so it will have to wait a week or two until I have time to actually deal with it. I love standing on my head to change out a heater core.

The reason I asked about the heater was really to pose my over-complicated hypothesis, which is: could just the right sort of small leak allow periodic, limited pressure loss, and therefore a moment of localized boiling in the hottest part of the engine, which would create a bubble of vapor that would pass the temp. sensor, causing it to read low for a moment. This momentary boil would drop the coolant temperature just a little and then end as temp. and pressure came back into balance. Then would pressure begin to build again, and the cycle could start over.

Just an idea. Sound crazy?

In the meantime, I'll try your procedure for burping the cooling system.