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1995 SEDAN DEVILLE-4.9 & 1978 SEDAN DEVILLE-425
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1,673 Posts
Discussion Starter #1
Been thinking of changing coolant in my `78 Deville 425 V8.
Is it self bleeding or not?
If not--what`s best way to get air out of system?
Thanks
 

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1980 FBC
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979 Posts
No bleeding necessary but the level will drop after the thermostat opens. Run it a while. When the level drops top it off.

ALWAYS change the thermostat when doing any cooling system work that requires draining. A crapped up thermostat is one of the easiest ways to ruin an otherwise good engine.
 

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1926 Model T street rod, 2000 Jaguar XJ8, 1999 Corvette.
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6,737 Posts
On any cooling system, I keep the idle up after adding maybe a gallon to an empty system. A friend holds the idle at about 1500 RPM, heater on max heat, fan on low. As I add the remainder, the level will drop quickly. I continue to add and keep the idle up. Once it tops off I quickly install the cap and tell my buddy to release the throttle. Because the engine and radiator are about the same height underhood on your car, you have less chance of trapping air. Burping the system started when hoods were sloped down and the radiator was mounted lower than the engine or the engine faced to the right.
 

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1995 SEDAN DEVILLE-4.9 & 1978 SEDAN DEVILLE-425
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1,673 Posts
Discussion Starter #4
Thanks for the info.
Read the 1977 Cadillac service manual also and it says any excess air in cooling system will just be purged thru the overflow tank then thru the
hose that comes out the top of it to the outside.
Got under car today and found the radiator drain plug is "seized" the fins on the plug just spin and do nothing---looks to be original radiator.
Suppose I could disconnect lower radiator hose to drain---but more work to do so.
 

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1990 Brougham, 1990 Brougham d'elgance 1979 Coupe
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1,196 Posts
Take the theromostat out, and take the bottom hose of the radiator and run a garden hose into the thermostat hole and radiator.

I spent the extra 1 or 2 dollars and alaways use distilled water when refilling.

Ive never had any trouble with air getting locked into a old GM car.

If you can find a radiator shop you may be able to get a new pet cock soldered in.
 

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1995 SEDAN DEVILLE-4.9 & 1978 SEDAN DEVILLE-425
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1,673 Posts
Discussion Starter #6
Take the theromostat out, and take the bottom hose of the radiator and run a garden hose into the thermostat hole and radiator.

I spent the extra 1 or 2 dollars and alaways use distilled water when refilling.

Ive never had any trouble with air getting locked into a old GM car.

If you can find a radiator shop you may be able to get a new pet cock soldered in.
Had another look at the drain plug closely and found the "nut" at the top of it just had to be loosened.
I forgot to relieve pressure with the rad cap first--loosened the drain plug nut and it the coolant exploded out of there
enough to shoot the drain plug a good 10 feet away.
I never thought I could move that fast---I got my hands in some cold water fast and luckily no burns--but
skin is real sensitive.
I wish I would have read your post first about distilled water.
Got a good deal on some extended life coolant from Napa and good to go.
Feel better knowing there`s fresh coolant in there and saved myself some $$$ doing it myself.
 

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1990 Brougham, 1990 Brougham d'elgance 1979 Coupe
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1,196 Posts
Had another look at the drain plug closely and found the "nut" at the top of it just had to be loosened.
I forgot to relieve pressure with the rad cap first--loosened the drain plug nut and it the coolant exploded out of there
enough to shoot the drain plug a good 10 feet away.
I never thought I could move that fast---I got my hands in some cold water fast and luckily no burns--but
skin is real sensitive.
I wish I would have read your post first about distilled water.
Got a good deal on some extended life coolant from Napa and good to go.
Feel better knowing there`s fresh coolant in there and saved myself some $$$ doing it myself.
Glad your "OK" and found the plug.

I put the oil cooler gaskets on my 90 the other day and took a shower in oil. Im pretty sure that the car is now completely leak free, which is amazing for a 25 year old GM product, especially a Chevrolet engine.

I have to put the speedo housing seals on my 79 but cant seem to find the parts. Im going to the transmission parts place next time I go to the city. Any chance anyone would know the NAPA part numbers? Theres a o ring and a seal.
 
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