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7.4 liter cooling problems

Kay86K5

Kay86K5

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Hey all. I have a 1996 GMC 3/4 ton with the 7.4 liter 454. My wife works nights, so when she does work, I meet her at her work and swap my truck for her jeep with my kids in the back. This morning she drove the truck home so that I could head to work. As soon as I got in my truck to leave, I immediatly noticed that it wasn't running right. The idle was serging and I looked at the temp gauge and it was upwards of 250* (i'm greatful the freaking motor didn't blow on my wife on the way home. I have no clue how long the temp was like that when she was driving home this morning). So I shut it down and popped the hood and noticed that the coolant resivor was completly full to the brim with coolant. When I would barly act like I would pop the radiator cap, the fluid in the resivor would start pooring out the top. I have never had to do anything cooling wise to this truck, so I'm not sure when the last time the thermostat was changed or anything like that. And I didn't have any time to diognose before having to leave for work. So it's just sitting in the driveway waiting for me to figure it out tonight. I just thought I would post this hear to see if anyone has had a similar problem so my diognosis will go quicker tonight. Thanks for any help.
 
brian wafer said:
is the belt still there,maybe broke off! or water pomp problem ,stuck thermostat.
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yeah, belt is still there. There were zero leaks at all when I walked out to start it up to drive to work. I'm wondering if stuck thermostat too. I was thinking about when I get home, make sure coolant level is good in rad and resivoir (when everything is cooled down) and then go drive it around for a while and see if temp still sky rockets or if it starts cooling like normal again. If it cools good, I would assume a new thermostat would be a smart move and be in the trucks future

I have heard sometimes the temp sensors will go and cause it to malfunction (aka not kick on the fans like it should when getting over 210*, stuff like that). Has anyone delt with that on these things?
 
That 7.4 might have came with an optional fan, but the mechanical fan should keep it cool. You have a secondary problem somewhere in it. And really, that secondary fan will only kick on with AC, or temps around 230 or so. I would bet a failed thermostat or a bad waterpump. Thermostat takes a few minutes to change, and I always buy the failsafe ones. If and when they do go bad, they stick open instead of closed.
 
454Sub said:
That 7.4 might have came with an optional fan, but the mechanical fan should keep it cool. You have a secondary problem somewhere in it. And really, that secondary fan will only kick on with AC, or temps around 230 or so. I would bet a failed thermostat or a bad waterpump. Thermostat takes a few minutes to change, and I always buy the failsafe ones. If and when they do go bad, they stick open instead of closed.
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Yeah, I did more thinking on it and I am pretty sure it has to be thermostat (pretty sure it is stuck closed). On my lunch I went down to the parts store and picked one up for $10 (a failsafe one). I figured this is the cheapest and easiest thing to start with and will be the easiest to rule out if it doesn't work. We shall see tonight. I guess if that doesn't work I will go with a water pump after that. Thanks for the comments and input :waytogo:
 
Another possibility is the fan clutch went bad and isnt turning the fan correctly. There is a way to test them, as they are like 80$ and that's alot of money to guess compared to a 10$ stat, but I cant remember how to do it anymore. I just went with windstars when my clutch went bad. :haha:
 
Sounds to me like your radiator cap is bad. Once it overflowed the reservoir, was the radiator low? Overfull reservoir, with low radiator, means the cap isn't operating properly. It should be allowing coolant to be returned to the radiator as the motor cools, and if it's not, you'll end up with an overflowing reservoir.
 
dyeager535 said:
Sounds to me like your radiator cap is bad. Once it overflowed the reservoir, was the radiator low? Overfull reservoir, with low radiator, means the cap isn't operating properly. It should be allowing coolant to be returned to the radiator as the motor cools, and if it's not, you'll end up with an overflowing reservoir.
Click to expand...

Incorrect. It was just the thermostat. Running and cooling fine now. The resi was so full because the motor wasn't cooling at all cause the thermostat was stuck in the close position
 
So it WAS pulling it back in when it cooled, you were only talking full reservoir hot? I was assuming you meant cold or hot. What you found makes sense then. Hotter the coolant gets, more will be pushed into the overflow.
 
Yep. Sorry, don't know if I made that clear or not. Once the motor cooled it pulled back into the rad
 
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