Alright, this cooling thing has turned into a major problem, the underside of the car is all back to stock and has been for awhile, but I burped the cooling system today (with the heater running) and managed to get about a quart of water in before it appeared to be full. I ran an errand and temps stuck around 175 while going downhill, on the way up hill temps were running 185. Dropped off my girlfriend and temps rose to 225 (not weird for my car this summer) went to the gas station and temps dropped to around 205. Here's the interesting part: halfway home from the gas station temps very suddenly hit 240 while stopped at a light. Normally if my car hits 230 I blip the throttle and temps drop again. But having just burped the system blipping the throttle did nothing. By the time I got home temps were over 260 with the "hot coolant" warning light on. The expansion tank had overflowed in the driveway. I let her sit while the fans brought her back to 200, when I started her again she sucked coolant down about halfway in the expansion tank, but temps rose to 225 again.

Sorry for the long explanation, but to much is better than to little in my mind. She's got a 160 t-stat installed, and ambient is 70 degrees today.

 

What condition is your upper / lower radiator hoses in? Soft spongy, collapse easily when squeezed, if yes new hoses are in order.? Is the coolant circulating properly, radiator clogged. The top half of the radiator when engine is at operating temperatures should be hot to the touch and the lower half of the radiator should be realtively cool to the touch. if not, its plugged or close to being plugged. Did you buy and install the new radiator cap yet? Do you have a single or double fan. Have you boil tested the t-stat, even new ones are know to on occassion be faulty. I would pul it just to insure it is in spec and opens. Also are you SURER the t-stst is not installed upside down? It can and has happened and will cause the cae to overheat. Spring assembly must popint up.

 

Rad hoses, tstat, and water pump (and coolant) are 3 years old, hoses feel good. If I installed tstat backwards I imagine I would have noticed sooner, but I was very careful to get it in correct. Coolant was definitely moving when I burped her this morning, wether or not it was circulating correctly is anyone's guess, will check for temp differences tomorrow 6/27. New cap has been on for about two weeks, and I've got a dual fan system. No test on the tstat, but my guess is that is the issue, and here's why:

Test drove the Monte again before writing this post. 64 degrees with heavy rain. She got up to 195 degrees during the warmup and while I slowed to make a u turn she suddenly dropped to 163 degrees and didn't go above 171 on my way back home. Makes me think that either the tstat was stuck and suddenly popped open, or there was a blockage in the rad that suddenly released.

 

Another test drive today with the exact same results, my money is on the thermostat getting stuck.