I don't know but I've a feeling that the thermostat may be installed backwards (been there, done that). The side with the spring around the cylinder should be immersed in the hot coolant, i.e., toward the engine and not the thermostat housing.

Chevrolet has made the t stat housing idiot proof. T stat only fits one way and in one orientation, 12 oclock/ 6 oclock, guts in the hot water flow. The above mentioned tests prove the system IS capable of cooling the car. Flow was thought of with the 3,200 rpm results. Gutting the t stat showed two things, where the problem is and the rest of the system still worked. If the gutted t stat doesn't restrict enough to get to operating temp then the t stats 1-3 restricted to much causing the over heating? Went to chevrolet and bought t stat #4 last evening. Going now to install it and give it a trial run. I'm getting way to good at this. Still working on my connections to get a full scan on it also.

Yep getting better at it. Filling it with coolant as i type.

Test and retest. FAIL. Jumped to 205*, auto fan can on, drove 2 blocks and got to 235*. turn around came home and opened the radiator drain. Thought that was not much so refilled and did a retest after doing everything to get it full, ran at 205* auto fan running, crept up to 210*, drove 7 blocks and was at 235* and climbing. returning home i tried 3,200 rpm in nutral on the way, was 205* by the time i got home. Im putting the gutted t stat back in after its cooled off. At least i can get farther than a half mile from home that way. Also did a little hammering on the gutted t stat to make the hole slightly smaller.

Does the radiator have a fan shroud?

Going back through the posts. Appears the stat is working since you feel water through the top hose. I'd suggest maybe a 180 or so Tstat. You may not have a shroud on the radiator as the blade may already be shrouded (they are on my lumina). Ok, rehash, when did this start, and what circumstances where they? The fan is comming on? I am not sure what speed those are supposed to turn. When you replaced the radiator was a direct replacement? That small of an engine I'd not think it would need more than a two row core.

Electric fan is kind of shrouded, stock. New radiator was a direct replacement, two row direct bolt up. Even chevy's t stat was a 180*. Started last fall on the road between pason and phoenix doing 70 ish down the hill. Nursed it home in short hops in what became a very long trip. This car always ran 195* up hill, down hill, rain or shine, desert or snow. Replaced the 1st t stat and got along for almost a year (no trust or trips), since then nothing has worked to fix the overheating. The fan now comes on about 205*. When it over heated it was not working unless the ac was on, (it was) which was 95% of the time anyway. Radiator fan works now with all the fluid i have been running through it. I still have the manual override i can plug into the electric fan. Its not needed but was nice to use in the tests. When i did the head job the head gasket was just about plugged solid. every 5/16 hole was a pin hole at best. Cleaned up nicely. I was told the coolant had been mixed with a different brand/type. I'm chevy to the core (except for the token ford and dodge in my youth) so it didn't come from my personal supply. I'm done working on this thing unless some ones got a sure fire fix to try. Wife wants to rent the car shes got her eye on for a weekend of testing from some agency. Think i'll say yes............

I had a chance to drive down town today, 17 miles one way. Most on the freeway. Temp ran about 170* on city streets without the fan running in 4:30 heavy traffic. On the freeway the temp came down to 145* without the fan running in 5:30 heavy slow and go traffic. This thing is sucking gas like my suburban. Filled it Saturday night, 70 miles later its down more than an 1/8 of a tank of gas. Wife says she usually goes almost 100 miles before it comes off the full peg. No leaks and no gas smell. the wife's idea is sounding better all the time.

The poor gas mileage is a direct result of the under temperature condition. The PCM probably isn't even getting into the closed loop operation. You said "the head gasket was just about plugged solid." DEX-COOL mixing with regular antifreeze can create nasty gunk that'll plug up everything. How did you clean it out (I've heard Cascade works good)? Also, silicates in the regular antifreeze will coat everything and thus it's best not to go back to DEX-COOL.

I cant stop thinking about this. Is it possible LOW flow through the bypass to the back of the t stat could be letting the t stat only open part way? I need more flow threw the t stat to stay cool. Checked the gas mileage at 23 city. None of my connections have gotten back to me yet. All blockages were at the gasket, 3/32 of an inch into the head and block, block cleaned up nicely with a shop vac and screw driver, and used a rebuilt head. Dont have any idea how many times or how long i flushed it hot and cold. Wife just checked on being "Pre-approved".......