Temperature Spike

Please forgive me if I am stating the obvious, but did you replace the thermostat? I saw an ad late last night that said a new type is available that fails open, causing the engine to run cooler than desired in the event of failure but doesn't lead to overheating.

It's possible you blew a head gasket. Check for oil in the water, or air bubbles in the radiator when the engine is running

I forgot to mention that the guage will indicate no temp at all untill it will all of the sudden snap up as high as it can go. Its not a graduall increase in temp. Dont know if this makes a difference. Tom

Probably a bad sending unit. Since you indicate you did get the motor to meltdown temps, you should have your radiator cleaned out by a radiator shop. With Small Block Chevy engines there is always casting sand left in the coolant galleys. When you get one over 260, you get casting sand in the radiator.

You should also check the tempiture your fans come on at. Charles

Well I'm not so sure the engine is really running at super high temps, because it doenst smoke or anything when the guage indcates that high of a temp, and I can put my hand on the intake and other parts of the engine w/out being burned. The fan does come on the second the guage snaps up. The fan is running on a hypertech cold fan switch on at 177* off at 166*. Could it be possible that I have an air lock in the system? Tom

Just as someone else asked, did you change the thermostat? Did you verify that the sending unit for the gauge is working right? Do you know how to air blead a small block chevy? Charles

The big plug on your water pump should not be disturbed. Some cars have a heater hose (or bypass) that goes there. A lot of the after market pumps come with that hole in them, even though the applications 80% are sold for do not use it.

To bleed air from a GM V-engine, fill the radiator slowly. wait until it stops purging air. Start the engine with the cap off, let the thermostat open keep the coolant level down only an inch from the radiator cap.

I don't have a book at home that covers the testing of your coolant temp sending unit for the gage. Finding out the test procedure on line should not be too hard. Charles

Or you can do what I do, drill a 1/16" hole through the thermostat. this will let air past it and prevent air lock. I read on some boards where guys drill a hole as big as 3/16, this will just let way too much water though and slow down engine warm up. Thank You, Randy

Remove 333 from email address to reply.

coolant temperature sensor

Oooooops.... I mean sending unit.

To confirm the sending unit is defective, check the resistance of the unit when the engine is cold [100* or lower]. Resistance should be high at around 1365 ohms. Next, run the engine until it is fully warmed up and check resistance again as it should now be low, around [I think] 150 ohms.