TLDR:

Two different water necks, 2 different radiator caps (one Moroso, one Stant). Neither seem to be sealing on the top flange. I can suck and blow through the overflow hoses and hear air flowing around the cap flange. The plunger is sealed, but the top flange is not.

If that flange is leaking, what actually draws coolant from the overflow back into the top hose when the coolant cools down and contracts?

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Long Version:

I have a 2 gallon cooling system. The radiator holds just under gallon, the block and heads hold just over a gallon, as best as I can tell.

Each day on Race week, I would start with emptying the overflow tank with the engine cold. Around the second checkpoint, it would be pushing coolant out the top of the 1-quart overflow tank and I'd start losing the Coolant Temperature sensor reading when it cooled down. I'd drain the overflow into water bottles, open the pressure relief valve, remove the radiator cap, and dump all 3 water bottles of coolant back into the upper hose, plus 1 or 2 more bottles of water until full.

Once it got hot again, rinse and repeat - pushes out the top of the catch can, cools down, bridge goes dry, transfer coolant from overflow to top hose.

Same thing has happened both times I've gone testing at the track and even driving it on the street.

I talked to AFCO, who built my radiator, and he said not to use the plastic push-to-connect hoses and fittings, as they can swell and distort with the heat, leading to no vacuum at cool down, ie no draw-back from the overflow.

The rep suggested switching to 3AN PTFE hose, which wont collapse and will gladly pull a vacuum at this working level.

At this point, I also did some googling and learned that the Subaru WRX uses 2 radiator caps in a stepped system - a 1.1 bar closed cap on the upper hose, which then vents to an expansion tank which has a 1.3 bar recovery cap that vents to the catch can (and also supplies the turbocharger cooling system).

So I had an expansion tank built intending to do just that - vent the steam from the upper hose to the expansion tank (closed cap), allow that steam/coolant to accumulate as a pressure vessel, then vent off to the catch can (recovery cap). But that's not the point of my question yet.

In the middle of switching to AN lines, I just discovered tonight that neither cap (upper hose or expansion tank) is sealing at the top lip.

It seems to me that if that top lip isn't sealed, it will never allow the vacuum to be created, to draw coolant back from the catch can to either the top hose or expansion tank.

Has anyone else ran into this situation?

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In regards to the WRX:

Because of the space constraints within my truck, I found the 2-cap solution to be the best way to implement an expansion tank, add fluid volume, and be reasonable serviceable.

quote:

The radiator, which is at a lower point than the turbo-tank, utilizes a higher pressure cap at 1.3bar.

Conversely, the turbo-tank is at a higher point than the radiator, utilizes a lower pressure cap at 1.1bar.

Further, the coolant reservoir is first fed by the turbo-tank relief lines and then second by the radiator.

quote:

Here is how your turbo Subaru's cooling system works:

The two caps are not only rated at two different pressures, they are constructed differently to serve two different functions.

The cap on the upper reservoir above the turbo is of a lower pressure rating AND, very importantly, has a reverse pressure valve. This cap's lower pressure rating allows it to vent pressure into the overflow hose that then goes into the overflow container. The reverse pressure valve comes into play when the system cools down, the coolant shrinks, and the negative pressure in the cooling system sucks coolant back into the coolant system from the overflow container.

The upper tank's cap is responsible for system pressurization, therefore, as well as ensuring the back and forth movement of coolant from the overflow reservoir.

NOTE: Failure of the negative pressure valve is an invisible condition... and results in the lack of a pressurized coolant system, higher and uncontrolled coolant temperature, and possible engine (head gasket) failures. Always inspect the negative pressure valve, and clean as needed. Also, periodic replacement of the cap with the correct OEM cap is advised.

The cap on the radiator itself has a higher pressure rating, it's job is to maintain system pressure, and virtually never comes into play because the upper cap's lower pressure rating means it vents to the overflow container before the lower cap's threshold is reached.

https://photos.app.goo.gl/d8pRdGotjRtzMriL7

This message has been edited. Last edited by: Magnethead,

You shouldn't need/use two radiator caps, and you should not be draining your "Catch Can"

If you plumb the expansion tank correctly you only need to have the 1 rad-cap on it. I would not run a rad-cap on the upper hose. You are just introducing air into you coolant system with what your doing.

With a properly plumbed coolant system you should never have to take the rad-cap off or drain anything at the track.

You can email me some more picture and I can help you.

jerryd@meziere.com

quote:

Originally posted by 1leg:
You shouldn't need/use two radiator caps, and you should not be draining your "Catch Can"

If you plumb the expansion tank correctly you only need to have the 1 rad-cap on it. I would not run a rad-cap on the upper hose. You are just introducing air into you coolant system with what your doing.

With a properly plumbed coolant system you should never have to take the rad-cap off or drain anything at the track.

You can email me some more picture and I can help you.

jerryd@meziere.com

Thats what I keep telling him but he wont listen, re route the top hose to get rid of the factory fill cap in the hose. The way it is now it is a huge air pocket

My SB2 headed motor would make about 3 laps and I would have drain catch can and pour back in. The 23 deg* headed motor would pull water back in with same cooling set up. Long story short I got a lifter in the SB2 at Farmington, When I took it apart I found a place on #7 head gasket that didn't look like it was sealing good . It never did lose any water.

I don't understand why everyone is concerned with a recovery system - once I fill my system I make one pass and it pukes a bit of water into the can. I empty the can and then that's it - the level drops a little when it cools and rises when its hot but I never really get any more in the overflow unless I top it off again, it just stays in the happy zone. The only time I would get continual puking of coolant is when I used to run copper head gaskets... those things suck for coolant leaks.

quote:

I don't understand why everyone is concerned with a recovery system - once I fill my system I make one pass and it pukes a bit of water into the can. I empty the can and then that's it - the level drops a little when it cools and rises when its hot but I never really get any more in the overflow unless I top it off again, it just stays in the happy zone. The only time I would get continual puking of coolant is when I used to run copper head gaskets... those things suck for coolant leaks.

this

I have 2 caps on my car, one on the radiator and one on the thermostat hsng. I have both tied together with plain hose that goes to one of those cheapo plastic tanks with the drain on the bottom. I made a tube for the top that makes it a recovery tank instead of an overflow. I put a piece of clear tubing on the drain and made a loop so I could see the level without draining. Mine functions as it should, expansion goes in the tank and when the motor cools the water gets drawn back in. Just standard caps I think they're Motorad or Stants.

I run copper head gaskets and they have NEVER leaked....not once....

Also, I plumbed my catch can as a recovery tank....so the level in the radiator stays the same no matter what. Pushes the excess into the tank and pulls it back into the radiator when it cools....I never have to empty the tank at all....I did it by putting the hose on the bottom fitting instead of the top...works like a charm...if water starts coming out of the top, I've got issues.....

Same here as BD and rusty with our cars. Fill it, make sure the air is out, make a pass, empty the overflow, done.

OP, if there an air pocket in the system it will push air out the overflow like you’re experiencing.