Cooling Issue Question

[Bossloper]

I have a 2000 GMC Jimmy with a V6. I started doing something new. The temperature gauge needle starts to climb to the right and pegs the 260 mark and then it drops down to where it normally is. This only happens when I am going 45mph or faster. On a back road, the temperature rides hotter than normally and it doesn't go down until I hit the main road and get going faster. The thermostat is obviously working, so could it be the cooling clutch fan, and if so how do I check that? Any help is appreciated.

[us920669]

Thermostats stick sometimes, and that's exactly what yours sounds like, stays shut too long and then opens up.  I can usually hear the fan turn on and off while sitting in traffic, or just idling in the driveway.  Just my experiences, you understand.

[shvlhead.45]

Been driving Chevy/GMCs for a long time and that is typical from my observation.  Temp sensor is normally in the right head and thermostat is in the intake; on the 4.3L V6 and 1st Gen small blocks the water is pumped through the block to the heads and then intake, the head gets hotter faster than at the thermostat and shows nearly or reaching the over heat point when the coolant gets hot enough to open the thermostat.  When I put Jet Performance 185 degree thermostats in my 91 Sonoma w/4.3L V6 and my 99 C1500 Suburban they quit doing it.  It use to bother me but since they always drop right down to normal operating temp and I never had any problem with any of my vehicles actually over heating I quit worrying.  At one time I was drilling a 1/8 (.125) hole in the thermostat flange and it helped some but not a complete cure (read about that in a Chevrolet performance mag back around 1990).

[mechanic]

Since it seems to run too warm at slow speeds, and cools off at higher, you have a circulation problem.  It could very well be the thermostat.  Pick up a new stat and gasket, and while you are installing it, backflush the cooling system.  You can do this by temporarily hooking a water hose from a faucet to the heater hose.  The parts stores carry a kit just for this purpose.  Once flushed well, (do not use an acid based flush, the rad. core is aluminum), add antifreeze mix and you should be ok.  If you have a lot of mineral buildup in the bottom of the radiator, flushing will not always remove it.  You may have to replace the radiator, but hopefully a flush and fill and stat will take care of it.
 
Ben

[Lloyd Smale]

my take on it is if it only is overheating when going slow its probably your fan. As you increase speed more air goes accross your radiator and this allows your fan to shut off. My guess is its not comming on at low speeds. Just wait till it reads high and get out and see if your fan is running. thats considering it has an electric fan which most do now. If it hasnt been done it sure wont hurt to throw a new thermostat in it too. there cheap and do go bad. Good time to replace your colant too as it should be changed out every 50000 miles or so.