I'm really not understanding what's going on here. The car had a 180 degree tstat in it. During first warmup it goes up to around 230 degrees then jumps down to 180 degrees& stays for a while, then starts climbing again & drops. Replaced the tstat witha 185 degree & still doing the same thing. water pump was just replaced.
It does this while idling or driving, will climb up to around 220-230 then drop to 180 pretty quickly
At first I thought low coolant because the car didn't seem to take that much before it was all the way full so I would let the engine run, pour some in, let it run a bit, pour some in, let it run , then pour some in. Always does the same thing.

Any idea what could be causing this?
I do know it does need a flush pretty bad, every time I drain the coolant it comes out rusty looking, like brownish tint to it.

 

Possibly have an air pocket. Are you getting good circulation through the radiator? You should be able to see flow w/ the cap off and the car running

 

After work is done on the cooling system that is a normal thing to happen due to air in the system. If this continues anfter a day or two of driving and refilling the system to proper level then you may have a problem. It sounds like the thermostat is doing its job. The temp will get around 200 with a 185 stat before it drops. If you feel like the temp is getting too high before it drops AND you already tried another T-stat with the same results, then maybe your gauge is inacurate. This could be caused by a bad sending unit, gauge, or voltage regulator in cluster. Start with the eaisiest (sender) and go from there. From the way I took your post it seems like when the stat opens the car does go back to 185ish. If, while driving for a while the car continues to raise in temp then it is an engine fault which could be a list of things.

 

i'm 100% positive the aftermarket gauge is correct.
I'm pretty sure it is air pockets. Everytime I look the coolant is low & needs more.There are no leaks for coolant to get out of either.

I'll give it a try this weekend. It's been in the shed, just got the gears swapped & already started takingthe engine out & apart. (low oil psi replacing the crank,rod, main, cam bearings) should be done by friday

 

park the car facing up a hill with the rad cap off... any bubbles in the system will work their way to the highest point in the system