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Old 04-04-2018, 09:21 PM   #1
jpmartinson
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Cooling problem

Hi everyone,
I am having the same problem with two different trucks. One is my 71 GMC 3/4 ton 4x4..

When I start it up, it slowly heats up to the thermostat opening temp, the thermostat opens and the temp drops and stays cool/cold and never heats up again. When driven, temp slowly goes up and just keeps going up. Acts like a stuck thermostat. Have replaced the thermostat a couple times, even put it in some hot water on the stove with a thermometer and verified that it works properly but the problem still persists. New fan clutch, new water pump, new radiator, new temp sender and new cap. Anyone have any ideas?

Thanks for any help.
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Old 04-04-2018, 10:01 PM   #2
mongocanfly
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Re: Cooling problem

have you changed hoses? does it have a shroud?
might try a different gauge.....there's not much left with all you've changed....
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Old 04-04-2018, 10:23 PM   #3
YoungPup1977
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Re: Cooling problem

Qq

Last edited by YoungPup1977; 04-04-2018 at 10:34 PM.
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Old 04-04-2018, 10:25 PM   #4
jpmartinson
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Re: Cooling problem

Yea, my thoughts exactly. Has a shroud and new hoses. Was just out messing with it and noticed that when I run it with the cap off, can't seem to get all the air out or something. It burps out coolant, I refill it and it does it again endlessly. I just replaced the heads. I had a problem awhile back where a saddle tank cap didn't vent and filled the motor with fuel. When I tried to start it, it hydrolocked and bent a valve on number 8 cyl. So picked up a pair of rebuilt corvette heads on put them on. Maybe buggered a head gasket during re-assembly and now getting compression bleed into the cooling system? Maybe damaged the block somehow when it hydrolocked?
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Old 04-04-2018, 10:33 PM   #5
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Re: Cooling problem

Hey YoungPup1977,
The one I am working on now is my 71 GMC with a 350 4 bbl. The other one that is having the same problem is one of those "other" trucks that is a trash truck for my son's business. Just posted some info on the GMC about the head swap so that's the scoop on that one.

Don't have much history on the other one except that I replaced the water pump, thermostat, radiator and temp sender.
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Old 04-06-2018, 08:55 PM   #6
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Re: Cooling problem

How hot is "hot"? Over 220?

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Old 04-06-2018, 10:54 PM   #7
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Re: Cooling problem

Air in the system can cause a cracked head but hopefully this bridge isn't crossed. Did you pull the Tstat and bleed air from system? I'd run a leak down at that point and see if compression is bleeding into coolant system.
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Old 04-07-2018, 05:57 PM   #8
jpmartinson
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Re: Cooling problem

Hey GASoline71,
Not sure of the exact temp as I am using the stock gauge that just registers hot or cold. But judging by where the 195 stat opens on initial cold start, I would say at least 220. When it gets almost to full hot on the gauge, I pull over and let it idle and the temp drops to the "normal" range. Then as I drive, the temp creeps back up.

Someone I talked to yesterday mentioned that the 400 heads have cooling passages that don't line up with the 350 block. As I said, I purchased some "rebuilt corvette heads" on Ebay. I don't believe they offered the 400 in the corvette, so unless the heads were not as described, that shouldn't be an issue?
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Old 04-07-2018, 06:03 PM   #9
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Re: Cooling problem

Hey Nuckingfuts,

I think I got all the air out. Yesterday, I did a cooling system flush and replaced the Tstat again. As usual, it starts up, gauge goes up to Tstat opening temp then cools slightly and stays at about that temp. Have not taken it out to see how it does on the road yet so will see if it still overheats and will let you all know what happens.

Will do a leak down test also and see how that goes.
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Old 04-07-2018, 06:05 PM   #10
jpmartinson
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Re: Cooling problem

Thanks to everyone for your input. Hopefully I will get this figured out without having to pull the heads back off.

Really puzzled as to why I am having this issue with two different trucks.

Thanks again.
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Old 04-07-2018, 06:06 PM   #11
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Re: Cooling problem

There is a dye you can out in your fuel that will show up in the coolant with a UV light if you have a leaking head gasket, cracked head, whatever the case may be. Another thing you can do is use a test kit to pressurize the system and see if it leaks down.
If you are heating up under load and not at idle that would make me think of air in the system and only drawing significant coolant while loaded with more rpm.

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Old 04-07-2018, 11:33 PM   #12
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Re: Cooling problem

Running SBC400 heads on a 350 block will not cause any cooling issues. It has always been my understanding that a cooling issue when driving is related to a coolant flow restriction. Does your lower hose have the coil spring inside? If not that hose could be collapsing at higher RPM's restricting coolant flow.
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