Strange cooling problem... Sticking thermostat?

I'm having a problem with the cooling system in my CJ that has me a little stumped.

What happens is at low speed or at idle the temperature shoots way up, 230-240 stays there for a few seconds then drops quickly to the thermostat temperature or even lower. On the highway or under load it stays right at the thermostat temperature, currently 180.

I'm running a flowkooler water pump on my 401. Also it has a 4 core radiator.

I've changed thermostats and depending on the brand sometimes the problem gets worse or it goes away for a while. I tried the Robert Shaw style thermostats for a while, but the engine takes forever to warm up.

My theory is that the flowkooler pump generates too much pressure and the thermostats can't open. Then finally when it gets hot enough the thermostat has enough force to finally open. At light loads it then closes and the process repeats. On the highway the engine is generating enough heat to keep the thermostat open all the time.

I'm thinking on changing water pumps or drilling the rivets and removing the impeller disc.

Anyone else ever experience this issue?

I had similiar symptoms and it was air in the coolant. Ran it with the cap off until it burped and all was ok. I also have the Flowkooler water pump.

That's a good idea. I'll give it another try. I thought I had got all the air out, but maybe there's still a pocket in there.

where is your temp sender? If it only holds the temp for a few seconds and isnt showing the normal signs of overheating I wouldnt worry about it. If the system is full and everything is working (pump pumps and radiator isnt plugged) then I wouldnt worry about it too much.

The temp sender is in the top of the manifold just to the left of the thermostat. It would hold the higher temps for 20-30 seconds then drop down. No other signs of overheating, ran normally. The radiator was full to the top.

are you installing the thermostat the right way? Check to make sure its not upside down, it should be marked which way it should be.

You most likely have air trapped in the system.

I have an evaporator for my propane system mounted higher than the radiator cap. Burping the system was always a pain even if I had my jeep in the ditch making the radiator cap the highest point in the system.

The best tip I found to berp the system is drill a small 1/8" hole in the thermostat flange. This allows air bubbles to pass the thermostat before it opens.

Well after fighting this problem with no luck I decided to replace the water pump. I installed a new Napa cast iron pump. And now no more overheating!! Stupid flowkooler pump. I was asking myself. If doubling the number of impellers on the pump was such a great idea why didn't the factory do that? I guess now I know.