Add me to the list of people with this problem. Mine seems to be a bit different though. It just started happening yesterday.

Heres what I have done to my car in the past couple weeks:
installed shorty headers (broke coolant temp sensor in the progress)
installed new plugs and wires
oil change with royal purple 5w-30
added RP purple ice to coolant
replaced coolant temp sensor
accidently left coolant cap loose and boiled some coolant off (shut w/in 5 min, as soon as I could pull over, after this is when I noticed the problem)

Symptoms:
temp quickly rises to 240ish shortly after starting
temp seems to stabilize around 239F while driving ~50mph
looked under hood and only passenger side fan running at 251F (drivers side dead)
all fuses are good and all relays pushed in all the way
temp seems to track normally while increasing (not making huge jumps in temp)
temp sometimes increases to ~250F at stoplights
little bit of junk in front of radiator, but not a lot (buildup at top inch or so)

First, I want to figure out why my fan isn't turning on. If that doen't fix my problem, I'm thinking its probably a bad t-stat or maybe even water pump. One other thing I thought of was that since I left the coolant cap unscrewed and the coolant was at atmospheric pressure, maybe some boiled inside the head by the temp sensor and there is a vapor pocket stuck in there. I would appreciate any help/suggestions.

I took it out today and ran it rather hard to see how it behaved. Collant temp would steadily climb to 250-260F both while driving and when sitting at a light. I did notice that whenever I would rev the engine past about 4k, temp would drop rapidly by 20-30 degrees and then gradually return to what it was before. I have a new t-stat to install to see if that fixes the problem, but if it doesn't I don't know what to do. I don't want to start replacing stuff ***** nilly, because that gets expensive and could possibly lead no where.

 

wow! so it cools off after running 4000 rpm or higher for 10 min or more?

bad pulley on the water pump? one guy reported that in a thread in here

seems the pulley separates inside and the pump isn't spinning as much. i would do water pump and T stat first.

if it is a vapor lock, the pump is prob too weak to clear it out

just guessing ...hard to figure not seeing the Vette

G/L

 

Ok, I have officially proved that I am a moron. I went out and checked the coolant level and I couldn't even see it. I guess I didn't clue in on it because I never got a low coolant message. I guess I boiled off more than I thought. After adding about a gallon of distilled water, all is well again.

 

 

oh myyy gaawwdd!!!!!

you owe the world ten good deeds! get to work!