Air Trapped in Block Coolant?

[jec]

Well, I've always relied upon the "Board" for hard to get parts and/or information so let's see what kinda tips you'all have for a strange problem with my Coolant System.

This is on my restored '72 Longorn with big-block and 4 core radiator. The truck originally had air conditioning, however, I decided to build with an air delete option. This was an 8 year restoration project that I just completed this spring. I have had it out to a couple shows and it's been alot of fun.

My problem is that after the truck has been running and it's at normal temperature (as verified by factory gauge in dash), upon shutting it off, it sounds like the coolant is boiling in the block. The sound lasts for about 30 seconds and it dissipates. Now, I know what it sounds like but I'm pretty sure it's not overheated.

This happens after just idling in garage for 20 minutes or driving around just the same. Here's what I know.......I can see coolant flowing thru radiator with cap removed. Both upper and lower radiator hoses are hot. The thermostat seems to be fine. I have a factory clutch fan and shroud in place and there is a strong pull of air by the fan moving from the grille toward the engine.............The temp gauge moves just about 1/3 of the way over just as it always did before I restored the truck. Same radiator, water pump, gauge, and sending unit as before I restored the truck.........I don't have the heater hoses hooked up at all, they are capped off.

Any ideas???? Seems like there is just air trapped in coolant system within block but I don't remember ever having this problem on any of my other vehicles over the last 40 years.

Anybody know what to look for????

Thanks,
Longhorn John

[hugger6933]

As long as the rad cap it the highest point in the cooling system there should not be a air bubble trapped for very long. If there is one it should have been burped out by the flow of the water and the motion caused by the pump. Do you have the big block? If so do you have the bypass hoses on the water pump? They are kinda needed so get those back on and try that. Jim

[T3AMCH3VY]

Three things can cause this.
1: Trapped air bubble
2: Water is actually boiling
3: Head gasket

I would be surprised if the air bubble hasn't worked its way out yet unless you are introducing air into the cooling system some how. Do you have an over flow jug? Water COULD be boiling if the cooling system isn't building up the correct pressure. Check for small leaks or replace the radiator cap. Head gasket Get or borrow a block tester to check it out.

[jec]

Dear "Hugger",

Yeah, I'm a big-block. H'mmm..............bypass hoses on water pump? There is the curved one going up into manifold but no others. There is a pipe plug on the passenger side water pump but I was thinking that was for the heater hose.

I'll check my manual but where does the bypass hoses go??

Do you mean to create a loop in place of heater core???

Thanks,
John

[davepl]

Bypass hose is that little bent one that goes from the water pump to the intake manifold. It's about 4 inches long.

It's purpose, among other things, is to allow air to escape. Otherwise you'd have to drill a hole in the thermostat, which is what people wind up doing as "fix" for having broken it by removing it in the first place.

I just did mine today and I hate them... even formed it's hard not to kink it. But you need it.

Not picking on anyone in this thread, just a general observation: it drives me crazy when people remove equipment or deem it unnecessary, usually because they don't understand it. Fan shrouds, vacuum advance, bypass hoses, heat stoves, thermacs, heat risers, heat riser crossover, and so on.

The factory was CHEAP. I was just on a thread talking about the door pins that were optional to turn on the dome light. So if a truck had something from the factory, it generally needed it. Other than emissions stuff like AIR or TCS there's very little you can take away from one of these that doesn't adversely affect things.

But, back your truck, other than the gurgling is there anything wrong? It doesn't sound like its overheating, it's not puking out the overflow, so is it just the sound?

As someone mentioned above, it could actually be boiling if the system doesn't hold enough pressure. I don't know the numbers, but the higher the pressure, the higher the boiling point. It's 212F at sea level but might be 250F or something at 15psi. So you would have to test the cap and rad to be sure they're holding pressure. If they're not, you could be getting boiling at what is otherwise a 'normal' operating temperature.

I don't know your elevation there in Ohio, but that could lower the unpressurized boiling point too. I just still think you'd see it puking out somewhere if its boiling.

Edit: I have the tools for pressure testing cooling system parts and don't remember them being very expensive (I'm cheap like the GM factory). But you might be able to rent them at your local auto parts store. Or just put a new rad cap on and see if changes, though that won't tell you if the rest of the system is holding pressure.

[jec]

I know what you mean about trying to improve on or eliminate factory parts. These trucks were designed with the bare essentials and I've owned mine since 1999.

The upper bypass is on my truck. It did puke out the overflow tube the first time out last Saturday but I think I was over filled. Now I'm about 3 inches from top of filler neck and getting no overflow.

It's just the sound bothering me because it's one of those "Sounds Like" things and it's not normal.

I had many other old chevs including big blocks and don't remember anything like this unless I was really hot.

Thanks for your comments,
John

[geezer#99]

Change out the rad cap. I had one do the same. I thought it was air being pulled in past the seal.

[So.Cal.Super]

Hey John, just a couple of questions. I'm racking my brain and it might help others answer.
Do you have an over flow tank? I saw you only mentioned over flow tube.
What is your coolant mixture, 50/50?

I had your same setup with the only difference of high flow water pump and 180 high flow stat and never had block boil. I hope it's not a head gasket, but you could check that with a hydro carbon test.

I would replace the cap Just because it's cheap. I had one cause me a headache on a truck once. Never thought that simple cheap part in front of my face was the problem.

[Andy4639]

I would jack the front of the truck up as high as you can safely get it and let it run with the cap off and see if it does in fact have air trapped. Then as stated buy a good cap and see how it does.

[jec]

Good news, new radiator cap solved the problem!!

At first, I still had the problem after replacing but after 2-3 cycles of hot to cold and topping off radiator, the noise is gone. Apparently air needed some help to work it's way up and out.

Thanks for all the comments.

[Daman]

Always drill a 5/16 hole in the thermostat.