I have a 1958 chevy chevy that I have restored. I have an interesting problem,need a second opinion. When the engine idles say for 20 or 30 minutes the water temperature will climb to 240 degrees or greater unless I start moving. the engine is a small block 350 out of a 80 chevy pick up truck. when the car is in motion there is no problem even on the hottest day. when you stop say in traffic the temperature climbs again until you start moving then drops pretty rapid to where it should be. I just got the car back together this week and it still does the same thing. The engine is stock not bored,has a very mild camshaft. I am running headers on the exhaust,and the transmission is an automatic. I have replaced the water pump with a high velocity pump,i have changed the fan to a steeper pitch for more air draw. I have also installed an electric fan that pushes air through the front of the radiator when the engine reaches 170 degrees along with a shroud. The thermostat is a 165 degree I have tried diffrent heat ranges but it hasn't helped. I am running a 50/50 mix of antifreeze. I have had the radiator cap off the radiator after the engine has reached temperature and there seems to be good water flow at idle. Do you tink the radiator may be partially plugged? Do you think the heat from the headers is causing the problem? I have checked the temperature gauge against a calibrated source it is accurate. The sending unit is in the rear of the block on the water jacket,where the stock one is located. This is a manual gauge. I live in the east the outside air temperature is 27 degrees f. If this does this now I can imagine what will happen this summer. There is no water in the oil or vice versa,compression is good. Any ideas? your help ould be greatly appreciated.
wayne bushnoe,massachusetts

Sounds like your fans are on backwards. What could you possibly be doing that would require you to idle 20 to 30 minutes? Quit idling.

Try taking your thermostat out and see if that helps, i had put one in my nova (160 degrees) after i checked it in hot water it open then but when i put it back in my car the temp went up to 240. anyway i took it out and it never gets over 100 degrees.

Sounds like your fans are on backwards. What could you possibly be doing that would require you to idle 20 to 30 minutes? Quit idling.

tHANKS FOR THE REPLY,WHY IDLE FOR SO LONG TRY "TRAFFIC" GOING TO WORK.

What i don't see in your post is what size radiator are you running? How many rows of core? And what idle speed? Like you mentioned, I would see if that radiator is partially plugged.
After running a while are there any hot and cold spots on the core? Run that bad boy until it gets up to temp and the shut it down and run your hand across the radiator surface (carful...might be hot) see if any areas feel cool, If you have cool spots, you might have some plugging/ blockage in the core.
If you are running a two row radiator, try going up to a 3 row core. Seems as if the 165 t-stat should be plenty low, anything up to 185 degree should be okay for a stock small block. Granted there will be some heat from the headers, but I wouldn't think enough to let her get that hot, especially with two fans running and a high velocity water pump.

Check that radiator, and 20 to 30 minutes of idle time is a bit long...even in wall to wall traffic-jam gridlock if I was going to be setting that long I would shut 'er down, even your average traffic light is only what 1 or 2 minutes? Get off of the brake and let the air flow.

Low water pressure will make you run hot. check your radiator cap & look for any pinhole leaks. I wouldn't leave the thermostat out as you can overheat from flowing water through the radiator too fast at high rpm. how's your choke?