NJey1985

Hi All,

My 69 427 big block runs at about 195-200 degrees (I verified this by putting a temperature gun on the thermostat housing), however after shut down the temp rises to about 225 degrees (I would expect the temperature to rise due to the engine temperature and coolant temperature differential, no cooling air passing over the radiator and water pump turned off).

After about 10 mins I see a few drops of coolant under the water pump (which has a slight leak and is about to get replaced) and a couple of drops from the overflow tank hose on the ground. 30 or so minutes later I can hear some bubble noises (not constantly).

Engine oil has been replaced and no signs of coolant in there so a head gasket hasn't blown.

Overflow tank has about 1/3/-1/2 full of coolant when cold. The radiator coolant level is about 2 inches from the top of the cap.

Is it normal for the residual heat from the engine to boil the coolant? Are the bubbles due to air from the coolant boiling? Or are these normal noises when the coolant cools down after being heated?

Is there anything else I should be doing to the cooling system apart from replacing the pump and draining the coolant?

ronarndt

You may be hearing normal noises as the engine gets really hot in some localized areas after you shut it off and the coolant stops circulating. Later on when the coolant begins to cool and takes up less space, it sucks air from any place it can- like your leaky water pump or from the overflow tank. I can hear the heat from my shut-off big block engine bubble and boil the gas out of the fuel bowls on the carburetor. As my engine cools off there are also noises that sound like coolant moving around in the engine or past the thermostat. You are already being smart to replace the water pump before it gets worse. If your radiator hoses and heater hoses are in good shape and all of the hose clamps are tight you probably have done the preventive maintenance needed.

resdoggie

After you replace your pump/coolant it would be a good idea to pressure test your cooling system.

3X2

I would also pressure test the cap. you mention a cap on the radiator. I wasn't aware that 69 427s came with a cap on the radiator. Do you have the correct radiator?

NJey1985

Doesn't appear to be an aftermarket radiator. There is a cap on the radiator and expansion tank.

C6_Racer_X

Replace the cap(s) now. If there's a cap on the radiator and a cap on the overflow (technically an expansion tank if it's presurized), you can use the same pressure on both. It looks like most aftermarket makers spec a 16psi cap, but the AC-Delco is 13psi.

If the expansion tank is pressurized, You can also run an unpressurized (0 psi) cap on the radiator (Stant 10203 or equivalent). Sometimes that's called a "dust cover cap" or an "open system cap." You can also just cut the spring, or use your old, weak cap on the radiator, and put a new one on the expansion tank.

Always use a pressurized cap on the overflow/expansion tank if it's designed for that.

He's already pressure testing it in the system. 225°F = 4psi. Even if his temperature gauge is a little off, he's way short of the 16psi shown on the cap in the pic he posted.

A 16psi cap should hold coolant (none passing to the overflow hose) up to about 250°F +/-.

stacka

Along with pressure testing the cooling system check to make sure the cap is compatible with a recovery type cooling system.

NJey1985

The expansion tank pressure cap is 15-18 lbs

CanadaGrant

I also have a 69 427 but it is an L36. Not sure what you have but as mentioned before there is no radiator cap on mine. It is on the expansion tank only so you might want to see if you have a rad that is correct for your needs.

NJey1985

I have an L36 390 HP as well, do you have a picture of your setup? So at some point the radiator must have been replaced... as C6_Racer_X I just need to run an unpressurised cap on the radiator

3X2

I agree. I'm not aware of any 69 427 with a cap on the radiator be it a L36, L68, L71, L89, L88. or ZL1. I know my L71 does not have a cap on the radiator.

CanadaGrant

There was no rad cap on a 69 L36, only the expansion tank cap so I was wondering if you or somebody else had replaced the radiator with something that doesn't have the cooling capability.

NJey1985

I haven't replaced it, must have been done prior to my purchase.

Do radiators have a part number stamped on them somewhere?

Edit: only markings I could find was Harrison

NJey1985

Just to confirm my options, assuming 16 PSI

1) unpressurised cap on the radiator and 16 PSI non-recovery cap on the expansion tank

OR

2) 16 PSI recovery cap on the radiator and 16 PSI non-rcovery cap on the expansion tank.

C6_Racer_X

I'd go with option 1. If there's a second hose at the bottom of the expansion tank leading back into the system, option 2 could cause some problems down the road.

If the only hose from the bottom (below the "full" and "low" marker lines) is the one to the radiator, a pressurized cap on the radiator might work (your option 2), but it can make bleeding/burping the system after refilling more difficult. I won't go into refilling procedures unless you really need a refresher, except to say you need to open a "bleed" point on the engine to let the air out when filling, so coolant can get into the block. Bleed/vent it while filling at some port on the front crossover passage of the intake, probably the heater hose on yours, unless there's a coolant temp switch or sensor there (doubtful on a 1969 model).

As for the "non recovery" caps, unless you're trying to be absolutely show quality original and it originally came with that, I wouldn't worry and I sure wouldn't spend extra for it. If you are trying to be "all original," you don't need a cap for the radiator at this point, you need a correct radiator.

BTW, your existing caps are already low pressure if they're venting at 225°F. You could just leave the one on the radiator alone and replace only the one on the expansion tank. That will be fine unless the one on the radiator is leaking externally.

NJey1985

Thanks everyone for the input