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?