V8 conversion cooling basics 101, Expnasion tank to overflow tank to...? Options

[BRAVE_HELIOS]

Cooling experts. help!

I will premise my story by saying that I will be converting my mechanical water pump to an electric unit, but I wonder if my issue goes beyond swapping water pumps.

So I fill my overflow tank to just above the add line with coolant (expansion tank is not opened at this time) when the engine is cold. I drive the car and the temp creeps up to around 190-200 but not much more. Coolant level starts to rise in the overflow to where it flows out of the overflow tube and on to the ground. Eventually it stops pissing. The overflow tank is fairly full at this point. I leave the car alone until it completely cools off. I go back to the overflow tank and notice it is bone dry. I add more coolant to just above the add line in the overflow tank and drive and the whole thing repeats.

Is this some sort of tell-tale sign of something else going on?

Although it just does not seem right, what if I leave the overflow tank empty?

Perhaps a bad pressure cap on the expansion tank?

[76-914]

If it were me, I'd put a pressure gage to it and watch that as you drive. Lower pressure promotes over heating. (IMG:style_emoticons/default/beerchug.gif)

[BRAVE_HELIOS]

Interesting.

The expansion tank and cap are from a early 2000's Passat. The markings on the inside of the cap read: 8E0-121 321. Also; the number 243 and: >PA-GF< and RE 3 Austria.

121 kPa is around 17.5 PSI so I think pressure wise I am okay... although, I did purchase the parts from the local scrap yard. May be I have a bad cap?

If it were me, I'd put a pressure gage to it and watch that as you drive. Lower pressure promotes over heating. (IMG:style_emoticons/default/beerchug.gif)

[914GT]

My guess is that the Passat recovery tank does not have enough capacity for your V8 conversion. The V8 with bigger radiator and longer coolant lines will have more coolant capacity then a V6 with an adjacent radiator. More coolant volume requires a bigger expansion tank.

[Cracker]

I don't believe the expansion tank size is your big issue. Circled below is the expansion tank I use and I have had no problem with cooling.

You can also add another "catch can" - like an oil can - and have your overfill line drain into it - fill it roughly 1/3 full and the hose below the water line (cold). This should help.

I've attached a picture of my race car with the expansion tank and overflow circled. The overflow tank is a Mobile-1 oil container.

Tony

[914GT]

Expansion tank is on the pressurized side of the system. Thought we were talking about the recovery tank that captures coolant going past the radiator cap.

[BRAVE_HELIOS]

Expansion tank is on the pressurized side of the system. Thought we were talking about the recovery tank that captures coolant going past the radiator cap.

I'm my case, the expansion tank has the pressure cap screwed on the top of it. Any coolant that gets past the cap (overcomes cap pressure of 17 PSI), flows to the overflow tank. The overflow tank fills until the coolant has no where else to go except through the hose that dead heads to the ground.

[914GT]

Yes, and it was your recovery or overflow tank that appeared to not have enough capacity to keep the system full between hot and cold conditions. But as you mentiioned in your first post, if your pressure cap is releasing at too low of a pressure then that may be part or all of the problem too. It would be worthwhile to test the cap or replace it to be sure.

Also on your first post you asked about leaving the overflow tank empty. If that happens then air will get drawn back into your system when it cools off. It's better to always have some coolant in the overflow tank when cold.

[BRAVE_HELIOS]

Yes, and it was your recovery or overflow tank that appeared to not have enough capacity to keep the system full between hot and cold conditions. But as you mentiioned in your first post, if your pressure cap is releasing at too low of a pressure then that may be part or all of the problem too. It would be worthwhile to test the cap or replace it to be sure.

Also on your first post you asked about leaving the overflow tank empty. If that happens then air will get drawn back into your system when it cools off. It's better to always have some coolant in the overflow tank when cold.

I understand now. Perhaps what Cracker suggested might work or just get a larger capacity overflow tank. The one I have I purchased from the local FLAPS... standard issue. Where can I get a larger capacity overflow tank?

[BRAVE_HELIOS]

Yes, and it was your recovery or overflow tank that appeared to not have enough capacity to keep the system full between hot and cold conditions. But as you mentiioned in your first post, if your pressure cap is releasing at too low of a pressure then that may be part or all of the problem too. It would be worthwhile to test the cap or replace it to be sure.

Also on your first post you asked about leaving the overflow tank empty. If that happens then air will get drawn back into your system when it cools off. It's better to always have some coolant in the overflow tank when cold.

I understand now. Perhaps what Cracker suggested might work or just get a larger capacity overflow tank. The one I have I purchased from the local FLAPS... standard issue. Where can I get a larger capacity overflow tank?

I will also replace the cap. Is there a preferred pressure for this application?

[914GT]

I run a 16 lb cap with a 50/50 mix of coolant, and the overflow tank varies between half full cold to the full mark when hot (around 190 or so measured at the head).

[BRAVE_HELIOS]

Just looked on Amazon and most of the replacement pressure caps are rated at 22 PSI. Is this a good pressure? I think most American type caps are around 15-17 PSI.

Yes, and it was your recovery or overflow tank that appeared to not have enough capacity to keep the system full between hot and cold conditions. But as you mentiioned in your first post, if your pressure cap is releasing at too low of a pressure then that may be part or all of the problem too. It would be worthwhile to test the cap or replace it to be sure.

Also on your first post you asked about leaving the overflow tank empty. If that happens then air will get drawn back into your system when it cools off. It's better to always have some coolant in the overflow tank when cold.

I understand now. Perhaps what Cracker suggested might work or just get a larger capacity overflow tank. The one I have I purchased from the local FLAPS... standard issue. Where can I get a larger capacity overflow tank?

I will also replace the cap. Is there a preferred pressure for this application?

[BRAVE_HELIOS]

I guess I don't understand the advantage of a higher pressure cap... such as 16 psi to 22 psi.

Why do VW systems have a high pressure rating of 22 PSI?

I run a 16 lb cap with a 50/50 mix of coolant, and the overflow tank varies between half full cold to the full mark when hot (around 190 or so measured at the head).

[914GT]

Raises the boiling point of the coolant. Higher pressure cap may be needed running plain water or at higher operating temps.

I guess I don't understand the advantage of a higher pressure cap... such as 16 psi to 22 psi.

I run a 16 lb cap with a 50/50 mix of coolant, and the overflow tank varies between half full cold to the full mark when hot (around 190 or so measured at the head).

[BRAVE_HELIOS]

If I assume that the cap I have now is faulty; and the standard replacement VW cap is rated at 22 PSI, it sounds like it can only be advantageous to go with this setup because of what you stated. So far I have not seen a lower pressure cap for the VW expansion tank.

Raises the boiling point of the coolant. Higher pressure cap may be needed running plain water or at higher operating temps.

I guess I don't understand the advantage of a higher pressure cap... such as 16 psi to 22 psi.

I run a 16 lb cap with a 50/50 mix of coolant, and the overflow tank varies between half full cold to the full mark when hot (around 190 or so measured at the head).

[Chris H.]

You're on the right track. Highly doubt it's the size of the expansion tank. I'm using the much smaller VW Beetle "globe". It does not have a recovery tank either. Haven't had a problem at all. I keep it right at the full line just over 1/3 full. When it gets hot the level goes up about 3 inches. Never overflows.

Can't hurt to switch that cap out. I'm running the stock VW cap that came with mine. Sounds like there is definitely a sealing problem somewhere. Those caps are cheap.

[BRAVE_HELIOS]

Would you happen to know if yours is a 22 PSI cap?

You're on the right track. Highly doubt it's the size of the expansion tank. I'm using the much smaller VW Beetle "globe". It does not have a recovery tank either. Haven't had a problem at all. I keep it right at the full line just over 1/3 full. When it gets hot the level goes up about 3 inches. Never overflows.

Can't hurt to switch that cap out. I'm running the stock VW cap that came with mine. Sounds like there is definitely a sealing problem somewhere. Those caps are cheap.

[Chris H.]

I'd say yes, although there is nothing on the cap that says it's a 22 lb cap. I bought this one:

link to ebay ad


And when I cross reference it on the Rockauto parts site it says that the stock cap for a car you'd use it for is a 22lb cap. Different brand but looks exactly the same. Try 2004 Beetle 2.0L and you will see the cap pressure reference. The Meyle one is also compatible with the same car.

So I'm like 99.999% sure it's a 22 lb'er. Pretty sure all of the modern blue VW/Audi ones that use the pressurized system are 22lbs.

Go for it.

[Cracker]

Each additional pound of pressure increases the boiling point by 3 degrees....the critical components to consider is matching the cooling system (radiator, expansion tanks, caps, etc.). If the entire system is a hodge-podge of parts - be careful. I've never known anyone to have this much trouble with cooling - start a "Go fund me" account and fly someone out to fix it with you. All the best.

T

[BRAVE_HELIOS]

I'm telling you... I should start a Go Fund Me account!

FWIW; most of my components started life as brand new such as the Griffin radiator and the water pump and hoses and sensors. The fan was used but as you can see from the video, no issues with drawing in air from the bumper opening. Speaking of bumper opening; it's 94.5 square inches. I am wondering if it might be too small. The front bulk head opening is maxed with it opened completely between the headlights except for the area in the middle which has the hood latch attached to it.

Each additional pound of pressure increases the boiling point by 3 degrees....the critical components to consider is matching the cooling system (radiator, expansion tanks, caps, etc.). If the entire system is a hodge-podge of parts - be careful. I've never known anyone to have this much trouble with cooling - start a "Go fund me" account and fly someone out to fix it with you. All the best.

T