What to do with this 350?

[Zenwren]

My 69 C10 has what appears to be it's original 350, the odometer shows (1)06k which I believe is original. I've only driven the truck a couple hundred miles but the engine seems fairly sound... EXCEPT about 30% of the time the cooling system doesn't hold pressure. It doesn't seem to get hot or lose massive amounts of coolant, but the last time it did it I noticed it was bubbling through the upper hose and into the coolant overflow bottle. The oil looks clean. I have tried a new radiator cap, and replacing the intake gasket. About the only thing I can think of now is head gaskets. Someone slapped some L98 aluminum heads on it about 20 years ago which doesn't fill me with confidence. So my options are as follows...

1. Have the L98's resurfaced and replace the gaskets and hope the rest of the engine is good.

2. Replace the heads with some halfway decent junkyard iron heads

3. Crate motor and be done with it

What I'm looking for is a simple reliable cruiser in this hot Arizona desert. Is there anything else I should check before pulling this pig out? Any other options I should consider?

[truckster]

Go to your local auto parts store and rent a combustion gas detector. It's a clear tube that you put a fluid in. You put in on the open radiator neck and run the engine. If you see bubbles coming up through the detector, watch for a color change in the fluid. That will tell you if combustion gases are making it into the coolant.

[mongocanfly]

Truckster... I never knew such a thing existed..thanks for showing it...

[vince1]

I would do a torque check on the head bolts. If they are loose you could probably get away with just a gasket change. I like to do a second re-torque after the engine has been run a few days. You could try some dye penetrant on the heads to check for cracks if you have them off.

If you have an old radiator to sacrifice you could try some block weld.

[Warrens69GMC]

I'm in Safford AZ, where in S.E. Arizona ?

[Zenwren]

Originally Posted by Warrens69GMC

I'm in Safford AZ, where in S.E. Arizona ?

Hello neighbor! I'm in Sunsites/Sunizona area.

[geezer#99]

When the coolant overflows into the bottle do you fill the rad back up. Or do you have a return bottle set up.
Might be nothing wrong with your motor.
Do a compression and leak down test to determine your motor condition.

[Zenwren]

I have a return bottle.

[tdangle]

After having gone thru the crate motor deal (New GM SP350/357 Deluxe), I would pull the motor, go thru it and pick up a set of newer iron vortec heads, a new intake manifold go with EFI (probably the Holley) and be done with it. Would end up spending 1/2 of what I did.

[Dead Parrot]

By 'lose' do you mean:
1 moving from the radiator into the overflow bottle
2 it is vanishing
3 winding up on the ground
4 other?

It is normal for coolant to move from the radiator to the overflow when hot and move back when things cool off. While I think the intent is for there to be no air in the system, some will wind up with a small air space in the radiator. My 75 often has an inch or so of air even when the overflow is properly filled. Been doing that for the 43 years it has been in the family with no ill effects so I just accept it as a quirk.

[Zenwren]

Originally Posted by Dead Parrot

By 'lose' do you mean:
1 moving from the radiator into the overflow bottle
2 it is vanishing
3 winding up on the ground
4 other?

It is normal for coolant to move from the radiator to the overflow when hot and move back when things cool off. While I think the intent is for there to be no air in the system, some will wind up with a small air space in the radiator. My 75 often has an inch or so of air even when the overflow is properly filled. Been doing that for the 43 years it has been in the family with no ill effects so I just accept it as a quirk.

It was vanishing, but I just discovered and repaired a pinhole I had on the little nipple at the neck of the radiator (non-pressurized side). Now the overflow is working properly I have driven it a couple times and noticed the level went down about half a quart in the overflow. I plan to drive it around some more and see what happens tomorrow.

[Dead Parrot]

Very possible that was the problem. Sounds like you might of had some air in the heater core that is working out of the system. With the motor cold, take the radiator cap off, and start and rev the motor a bit. When the thermostat is closed, all the flow is though the heater core and any remaining air should be forced out and into the radiator. Keep 'burping' it until the coolant level quits dropping, then top off, button it up and drive.

Hope the pinhole was the problem. A lot simpler fix then head repair/replacement.

Thanks to truckster for mentioning that combustion gas detector. Nice thing to know exists.

[Zenwren]

*UPDATE*
I took the truck for two drives today, one for about 15 miles and let it fully cool for a few hours and then a slower cruise for about 30min. Both times I checked the upper hose before shutting the engine off it didn't have a lot of pressure but did have some. I noticed a distinct pulsing feeling when squeezing the hose, even a little while after the engine was shut off. The upper hose also collapsed a time or two when revving in park. The overflow bottle seems to be working as it should and the level hasn't gone down. The temperature fluctuated a little between 175 and 195 but mostly stayed right at 180 (180 degree thermostat).

Hopefully I'm just being paranoid here, but it seems to me like the hose should be rock hard (like it is some of the time) after a long drive in 90 degree heat.
Next step for me is to pull the plugs and see if i get a squeaky clean one and maybe pick up an coolant exhaust gas detector kit next time I make the voyage to civilization.

[68Gold/white]

Looking for a clean plug is a good idea. Tiny coolant leaks like yours can leak, un-noticed. many times a pin hole in a radiator tube (that is not seen formt eh front or back of the radiator) can be difficult to detect. A pressure test usually finds these.

I have one of those combustion gas detection kits, they DO work. Mine sets on the shelf between uses, and the fluid goes bad between uses, I have to buy a new bottle, every time I use it....

[vince1]

I've had 3 astro vans with 4.3's. The first two, an 89 and a 90 leaked coolant into the left rear cylinder from cracked heads I believe. The 90 I owned since nearly new and took it to almost 300k or 400k kms using bars leak. At one time the fan came off and went through the rad. I was surprised to find the rad not clogged at all. For a tiny leak I wouldn't hesitate to use that stuff.