Air trapped in open system?

I recently installed a new thermostat in my 95 xj with a 4.0. The temp climbed steadily to 195 at idle, and then sharply dropped to about 150, then steadily climbed up to 195...then dropped again, then climbs up again, then drops again. It keeps doing this regardless of whether I'm at idle or driving. My first hunch after searching here was air in the system. Is this possible in an open system? I thought any air in the coolant system would bleed off itself. I did install the thermo correctly. (the little bleed hole at 12 o'clock.)
Could my thermo be cycling like this? opening..closing...opening ...closing.? I have a three core radiator, a high flow pump from Flowcooler, and I used a 50/50 mix of antifreeze. I wouldn't think i could be cooling thsi rapidly.
Could this cause me to go from open loop to closed loop on and off with the temp fluctuation like this? The gauge didn't do this before I changed the thermo.
It may not be doing any harm...but its driving me nuts watching it go up down up down all the time...
Any suggestions??

holy s**t. Looks like I stumped everybody.

What brand and temp rating thermostat did you put in there?try a new 195* thermostat from the dealer.That or try bleeding air out of the system.

I have an '88 model with the closed cooling system doing the same thing. I filled everything up and raised the rear to bleed out the air. It runs great and seems to run cool but the temp guage goes back and forth between 160 and 200 degrees.

I didn't do anything other than bleed the system. It is all stock other than that.

outlander said:

What brand and temp rating thermostat did you put in there?try a new 195* thermostat from the dealer.That or try bleeding air out of the system.

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X2 what temp thermo did you replace it with? You still have to bleed an open system cooling system. If you didn't do that yet that is my guess. Bleed it and see what happens.

i would say it the thermostat staying open too long or opening too much.

outlander said:

What brand and temp rating thermostat did you put in there?try a new 195* thermostat from the dealer.That or try bleeding air out of the system.

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I used an autozone 195 deg thermo that I had laying around.
How do you bleed an open cooling system?

An old trick I learned for bleeding air bubble is to take a small drill (1/8") and drill a hole in the lip of the thermostat. Orientate the the hole to the twelve o'clock position, fill with water, warm the engine up with the radiator cap off, and fill until it stops taking on water. And you're done.

Not sure if anyone else does this but it is the most simple way to bleed the air out of the system while filling it. I have a 87 XJ with the closed system and since I started filling my cooling system like this I have never had problems with air in the system.

I learned this from a guy who owns a radiator shop. You remove the upper radiator hose from the thermostat housing and fill it from that hose while it is lifted up above the radiator. You can hear the air rushing out of the thermostat housing while you are filling it and you know you are done when you get coolant leaking out of the thermostat housing. That is it.

jeepboy381 said:

Not sure if anyone else does this but it is the most simple way to bleed the air out of the system while filling it. I have a 87 XJ with the closed system and since I started filling my cooling system like this I have never had problems with air in the system.

I learned this from a guy who owns a radiator shop. You remove the upper radiator hose from the thermostat housing and fill it from that hose while it is lifted up above the radiator. You can hear the air rushing out of the thermostat housing while you are filling it and you know you are done when you get coolant leaking out of the thermostat housing. That is it.

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this is how i do it.