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Old 05-20-2008, 08:45 AM
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400 chevy sbc pressure in coolant

I know i's not the best of the SBC, and that 400ci chevy SBC are prone for some trouble but I am facing a sudden increase of the pressure in the coolant circuit.
As soon the engine starts, water begins to rise in the expansion tank of 2 inches, then it stops rising and some pressure appear in the hoses.
I own the car (caprice 75 convertible) for very long now (1991) and had done a major head job 7 years ago,(seats, gasket surfaces, pressure test, guides) but didn't put a lot of miles on since.
I checked for outside leaks and found some traces on each sides of the edelbrock performer intake. So I retorqued a little bit the front 4 bolts, and put a new RC33 radiator cap wchich didn't change the phenomen.
All cooling components have been changed along the years.
So my question is: is it worth and ok to try a retorque of the head bolts?
For the time I haven't finished with the checks, and I will put a release pressure cap on the radiator to help.
I'd really like not to be involved in a second head job, because, for the time I am also in the same head job on my 78 Caprice Wagon!

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Old 05-20-2008, 08:52 AM
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Do you notice any overheating issues with the vehicle? Did you recently replace the thermostat?

My first thought would be some sort of blockage in the cooling system.
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Old 05-20-2008, 09:27 AM
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No, I didn't notice any overheating, the sending unit was changed, as well as the thermostat. The curious thing is that I have the impression when the engine cools down after an hour that the overflow device under the new radiator cap stays closed due to some pressure maybe, and it only opens the passage when it is completely cooled.
Warm water seems to circulate nicely through the radiator from on side to the other too.
I am thinking of little compression following a one bolt thread in the block.
I have also the phenomenen (stronger) on a 76 Caprice also with a 400ci that seems to do that too, but there, the engine has never been openened, so it's not surprising!
Tomorrow I will try it with the release cap on and watch if pressure comes back after realeasing it at warm.
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Old 05-25-2008, 02:08 AM
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OK, here is the solution I found.
As I bought the Rc33 ac radiator cap, and the related Gates pressure release cap, pressure was present during engine use with just a 1 inch rise in the coolant reservoir, and more surprisingly the next morning after overnight complete cooled down. So I tried the Gates release pressure cap for my 305 chevy, and pressure disapeared, I recovered the little "clic" sound when after complete cool down I press the radiator hose, the circuit works great like before.
I noticed on the cap a script saying "vented", which I don't see on the 400 same release radiator cap. In the shop chart they were made both for open or closed circuit!
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