Flushing Coolant System-again

[John L.]

I am getting ready to coolant flush my '66 big block and have spent much time reading the archives and stories on the web. I have flushed the coolant in the past utilizing the Prestone Reverse Flush kit, but this time around leaning to flush the system with a garden hose. I plan to do the following: 1) drain and flush radiator, 2) flush heater core thru inlet/outlet hoses, 3) flush block thru thermostat opening with lower radiator hose removed, 4) fill with antifreeze and distilled water from the thermostat opening. I assume this process will get most of the anti-freeze out of the system, have I missed anything, is there a need to remove the block drain plugs? BTW, the engine was rebuilt about 15 years ago and I have a 10 year old DeWitts radiator installed. Thanks

[Harry S.]

Re: Flushing Coolant System-again

John, I just did my 63 327. I assumed I had it all out. Being curious I removed the block train plugs and got more out than I expected. Saying that I'd remove them. Just to be clear I'm not talking about the freeze-out plugs. If you have A/C you will need to remove the hot water shutoff valve to flush the heater core.

[Duke W.]

Re: Flushing Coolant System-again

You should absolutely remove the block drain plugs. This will ensure that virtually all the old coolant is drained out. There is really no need to flush the system if there is no evidence of deposit buildup and you have changed the coolant at reasonable intervals.

If you want, fill the system with potable water and run it for a few minutes beyond where the point where the thermostat opens, then drain the block and radiator, again.

I believe you can fill/bleed the system without opening up the thermostat housing by raising the front of the car, which is necessary to remove the block drain plugs. This will keep the radiator fill port at a high level to aid bleeding.

Once the the block plugs are back in and the radiator drain cock is closed add about 2.5 gallons of Zerex G-05 then top off with distilled water. Fill the system slowly and squeeze the radiator hoses as you fill. When it won't accept any more distilled water, start the engine with the radiator cap off. When the thermostat opens, you will see turbulence in the radiator and the level should drop. Add distilled water as required until the radiator level stabilizes. Cycle engine revs a few times to about 1500 and check that the level is stable, then install the cap.

Let it cool down and check the level, add as required, then bring it back up to temperature with a short low speed drive. After it cools, check the level again, and check after every subsequent heat cycle until you are sure that the level has stabilized, which means the system has fully self bled.

I believe the coolant capaicty of C2 big blocks is about 19 quarts (check your owner's manual, shop manual, etc.) so 2.5 gallons of G-05 should yield a blend of a little over 50 percent antifreeze, which is the minimum I recommend with a maximum of 60 percent antifreeze.

Duke

[Steve G.]

Re: Flushing Coolant System-again

Tilting the car front to back doesn't really help with bleeding the air out because it's not air trapped at the back that causes the issue, it's that the air can't escape past the thermostat. The system fills from the bottom up. I've read pieces where people have wedged an aspirin in the stat to hold it open and let it flush itself out when it dissolves. All kinds of wild and crazy methods.

The best and easiest way to fill the cooling system without worry about air lock is to fill it with the heater hose that extends from the manifold disconnected, ideally at the heater end, but also works disconnected at the intake. Put the stat in and connect everything except that hose. Fill the system until coolant comes out the nipple. Doing it this way ensure that there is virtually no air in the block. With no air in the block the coolant will circulate properly and fill the heater core without issue from there. I like it disconnected at the core end because I can hold the end of the hose above the filler neck and it will fill the rad and heater core. Once it starts coming out the core I plug the core pipe until the rad is full. Pull the plug and stick the hose on.

You can safely run it now without fear of an overheat and boil over which is often what happens when you fill and try to get the air by running the engine.

I always flush with fresh water at a bare minimum. Even with a properly maintained system I see milky looking antifreeze come out after running it a few minutes.

Steve

[Frank D.]

Re: Flushing Coolant System-again

Yes. If you didn't remove the block drain plugs you only did a half-azz flush. Many replace those plugs with petcocks like your radiator probably has. A prior owner did that on my '61....makes life easy!

I've seen some cars so bad I had to poke around in those block holes with a screwdriver to break the rust free so the coolant would drain out !

[John L.]

Re: Flushing Coolant System-again

Steve, interesting, so if I understand this correctly, you suggest filling the system thru the inlet heater hose disconnected from the heater core with thermostat and everything else in place and radiator cap off and once antifreeze starts coming out of the heater core opening, block the opening, and keep filling the system until reach proper fluid level in the radiator, which in my case with the big block and no expansion tank, is about three inches below the opening.

[Steve G.]

Re: Flushing Coolant System-again

Steve, interesting, so if I understand this correctly, you suggest filling the system thru the inlet heater hose disconnected from the heater core with thermostat and everything else in place and radiator cap off and once antifreeze starts coming out of the heater core opening, block the opening, and keep filling the system until reach proper fluid level in the radiator, which in my case with the big block and no expansion tank, is about three inches below the opening.

You are filling as you usually would, through the rad filler neck (or wherever your pressure cap is if you have an expansion tank) with the highest mounted heater hose ( coming off the intake manifold) disconnected at the heater core end and propped up so that the end of the hose is above the height of the filler neck. When coolant starts to come out the open heater core nipple, stick your finger over it and continue to fill the rad until the coolant is at the right level in the rad. Stick the heater hose on it's nipple.
Steve

[Patrick H.]

Re: Flushing Coolant System-again

Yes. If you didn't remove the block drain plugs you only did a half-azz flush. Many replace those plugs with petcocks like your radiator probably has. A prior owner did that on my '61....makes life easy!

I've seen some cars so bad I had to poke around in those block holes with a screwdriver to break the rust free so the coolant would drain out !

The holes in the petcocks are quite small, and I've had poor luck in general with the quality of the brass petcocks available in the auto parts stores. I changed my drain plugs to brass so that they don't corrode in place, and that works exceptionally well. I also suspect that a petcock would interfere with the "boomerang" spark plug wire shielding on a Corvette.

[Steve G.]

Re: Flushing Coolant System-again

You are filling as you usually would, through the rad filler neck (or wherever your pressure cap is if you have an expansion tank) with the highest mounted heater hose ( coming off the intake manifold) disconnected at the heater core end and propped up so that the end of the hose is above the height of the filler neck. When coolant starts to come out the open heater core nipple, stick your finger over it and continue to fill the rad until the coolant is at the right level in the rad. Stick the heater hose on it's nipple.
Steve

John,
Here's what happens when you fill the cooling system. The coolant flows down the rad then up the lower rad hose and enters the block at the height of the water pump holes, dropping down and starting to fill the block. The air it displaces can't return to the open neck through the upper hose because the thermostat blocks it. So the air travels back the way the antifreeze came in, through the bottom hose. That's the glugging and gurgling you hear.

Once the level in the block is higher than the water pump holes it is first difficult, then impossible, for the air to exit via the route the coolant is entering. Some t-stats have a small air bleed hole or a wiggle pin that will allow air to vent. That will gradually allow the air in the upper portion of the block to vent back to the open filler neck. Many do not. The heater hoses don't do anything to vent the air, they are just a loop with the lower end back at the water pump level. So the rad now continues to fill, but this air is trapped in the top of the block. If you fill the closed system until the rad is full then loosen the top heater hose at the manifold you will hear the air hissing out.

What happens next if you don't vent the air out as I'm suggesting, opting instead to run the engine to get the air out, you are operating the engine in order for the t-stat to reach the temp it will open, the air will vent and coolant rushes in the block. The problem with that is that the t-stat is not immersed in coolant and doesn't work properly. It is relying on the air to heat it enough to open. What can happens is that the coolant in the block will boil b4 the stat opens. The steam will open it and you will have a geyser of coolant blast out the filler. Squeezing hoses and jacking the car up are simply giving the non smokers something to do while they wait for the stat to open or the air to vent itself past the stat.

The other big benefit to venting with the heater hose is that initially you are not making the air travel back the way the coolant is entering. You can fill the system as fast as it will pour out of your jug. There is no waiting for the level to drop to add more coolant. Fills in a fraction of the time.

Lots of late model vehicles now have a little vent plug behind, but at the level of, the thermostat.

I know, more than you wanted to know, but I know that I always like to understand why I'm doing something and thought someone else might also.

Steve

[Leif A.]

Re: Flushing Coolant System-again

Succinct explanation, Steve...thank you.

[Jim T.]

Re: Flushing Coolant System-again

When a thermostat is installed, I drill a small hole in the thermostat before installation of a old or new thermostat.

[Frank D.]

Re: Flushing Coolant System-again

All that makes sense, but I have filled up a LOT of cars with coolant in my 50 years driving. I never once had an air pocket and certainly never saw a geyser of steam coming out of the filler. I fill up the system via the radiator cap...run the car to operating temp with cap off....fill it up some more when the T-stat opens. Replace cap - let the car cool down and top if off one more time.

[Steve G.]

Re: Flushing Coolant System-again

When a thermostat is installed, I drill a small hole in the thermostat before installation of a old or new thermostat.

Yes Jim, the vent hole is as good idea and many stats come with them. The hole has to be small enough to not allow too much coolant through so it still slows down filling.

For the hobbyest, time is not usually an issue. For me, into someone's wallet at just about $2/min, it makes sense to crack the heater hose loose and dump the coolant in.

I don't usually do it as I've described to John, disconnecting at the heater core end. I generally just remove it at the manifold end. The heater core will fill if the block is full, but in John's case where he has flushed the core and the hoses are off, it's just as easy to do it that way and fill the core at the same time.

But the vent hole accomplishes the job and prevents a boil over. That's what's important. You may never experience it, but many have. It's a mess.

Steve