I have installed a new water pump and thermostat on my 305. Everything seemed okay at first, after I installed the water pump twice because I ruined the gasket. Now my temp. guage starts out reading fairly normal, but then within maybe 15 to 20 seconds it will go from 190 to 240, the immediately cool back off. I think this is too fast and too high for a motor temp to rise in normal 35 mph driving. While the motor was running I waited for it to rise to 240, then I unplugged the sensor wire from the block, and plugged it back, and the guage was magicaly at 180 again. Here's the other thing. At speeds over 55 MPH my service engine comes on. It always has done this. When it lights up, now, after all these repairs I made, the temp guages freezes at 180-190. Do I have a faulty guage, sensor, or is the problem elsewhere? I pulled the old thermo this morning, boiled it on the stove and it did not open. I put a new one, but no changes with the guage. I figured it was the thermo, but its not. There's no way this motor is cooking off all the way to 240 from 180 in only about 2 to 3 blocks of easy, easy driving.

 

my 89 used to do something similar when the thermostat would stick it would vaper lock when the radiator got low it would read almost 250 then once i would get it to open it would fall to 160

 

I just took my NEW thermostat out, and now it's staying around 120...nice and cool. But see, I boiled the new on the stove and it opened at maybe 200? I guess where ever water boils. what worries me now is that I have a hissing leak on the front of the intake. I hope I didnt blow, or make an existing problem worse cause I drove a lot today with the temp. flucuations.

 

my motor has almost 300000 miles on it i guess i have just been lucky but with mine bouncing around like it did i never really had any problems like you , no offence but thank god although i did have my fair share the replacement thermostat for that truck should be a 195 that is stock

 

ok i see others have had the same problem with their trucks, i have had all the same overheating problems, i have changed the thermostat, fill the coolant level , and the problem went away until this morning. in less than 4 miles the truck overheated four time each time i had to pull over to the side of the road to allow the truck to cool before i burned the engine up. as i was pulling into my drive way the temp was headed back to the 240- 245 mark and then it started to cool off. i pulled the thermostat and placed it in hot water only to have it open up when the water started to boil. my guess around 212 once i pulled the thermostat out of the water it started to close, i did this a few time to make sure it was working. on the second time it overheated i felt the radiator to make sure it was overheating and the top hose felt cold but the radiator was hot.i replaced the sending unit at the same time the thermostat was replaced. as for the fan clutch it feels good ???? whats your thoughts

 

have you FLUSHED the coolant?

 

You're not getting it burped properly. There is still air in the system.

 

How do you burp a cooling system?

 

1992 Z71 350, radiator blew up, replaced it and thermostat and is still overheating with white smoke from tail pipe. Engine oil clean but no coolent flow from top hose. Could it be the water pump? Thanks