Sticking thermostat?

Picked up my ride last night after several weeks in a shop. Had it tuned up as the timing was way out of whack and severely advanced, and now it has gone from running 195 to 260 according to the temp gauge, which means the electric fan is constantly on, but the engine (O/T stock 300 hp 1990's Vortec Chevy 350 Edelbrock performer/Edelbrock 650 cfm electric choke, in a '36) does not feel as if it is running that hot (16lb cap). I'm thinking thermostat not opening, but before I drain the radiator down and pull it apart to test the T-stat in a pan of boiling water, am I on the right track? The temperature gauge sender wire is in place and everything else seems normal.

Is there any way of testing the sensor to eliminate that as a cause? Any help is much appreciated as I am due to head out on a 1000+ mile trip beginning of next week. Thanks guys.

 

Are you sure they got the timing right? Incorrrect timing can get a motor quite warm.

 

is the radiator getting warm? if the thermostat was stuck closed it wouldn't

i'd get one of those infrared thermometer guns and check out various parts of your motor/cooling system. you could also try another temp gauge

you said they set the timing...do you know for a fact the timing marks are correct? when you mix and match parts on a sbc sometime they can be way off

 

I'd guess thermo. They're cheap and easy to replace. I'd say do it anyway, because they do go out sometimes. Timing should be checked, too. You can never be too certian!

 

guage?? they do go bad.

 

Tune up and timing was done yesterday by a hot rodder buddy who was the 'go to' senior tech at a Chevy dealership for many years - I trust him to do it right. Last time the car ran was about six weeks ago at 195˚ and it was pinging badly under load - now it runs as smooth as ****er, but the temp gauge is pegged at 260˚ with 80˚ ambient temperature on the way home last night. I stopped and had dinner for about an hour and it still showed 260˚ on restart. Don't think it is the gauge as the fan stays on all the time. I think I'll do the T-stat and drill a 1/8" hole in it as I normally do and go from there. Thanks for all the help so far.

 

After sitting all night is it still pegged? Gotta believe you have a sensor problem. Maybe a grounded/ sender wire.

 

You know that warm fuzzy feeling you get when you are right? Well you are just going to get it - bingo. Just went out to garage and switched ignition on - did not crank it. Gauge pegged at 195 and fan is running. I'll work on it as soon as I get enough daylight. Thanks fellas for all your contributions.