Pressurized coolant reservoir

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My ecoboost Fiesta has a plastic coolant tank that is pressurized to 21 psi(!) Is this common? Why does it need to be pressurized? I assume there is still a radiator cap for over pressure protection? I realize it raises the boiling point but many cars get by with an open system.

The problem is it has three lines in and out. The middle one has a small, continual leak because there is no way it’s going to hold pressure over the long term given the engine vibration and cheap design of the fitting. I used JB Weld today to hopefully seal it once and for all.

Seems like a prescription for failure given the coolant is critical for cooling the turbo bearings. Ironically the hose to the engine has a conventional clamp. The pipe to the turbo has a clamp on a fitting that on the other side has silly rubber o-rings that Leak.
 
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I first noticed this design on my 2012 Nissan Frontier. It doesn't make a lot of sense to me either. I had quite a discussion with a parts guy when I went to get replacement caps. He thought I was mistaken when I told him the pressure cap was on the expansion tank.
 
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Most vehicles have a "de-gassing" tank that maintains the system pressure with a related cap that is remotely located away from the radiator. Long gone are the days of a radiator cap and overflow bottles.

My old and rusty but very trusty is like this as well. A pressure cap on the remote, dual walled tank with two lines going to it.

One is the simple reservoir that gravity feeds into the closed loop. The other line is of a much smaller diameter, I believe it's called a steam tube. It let's over- heated vapor condense back into liquid and returns it into circulation.

Of course all connections and lines have to be in good repair as you found out. Nothing wrong with the JB Weld as long as it holds. But if any plastic bits and rubber hoses leak on my rig?

Replace. OEM. Done right. Bodgey repairs can bite your fingers and wallet before you know it. :(

Cooling, especially on a boosted engine is max critical.
 
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Pressurized cooling systems are quite common. 21psi is not that much. On race cars you will see 50 psi. A pressurized system prevents air pockets and hot spts. The higher the pressure the higher the boiling point. Typical coolant has a boiling point of about 223F. At 15 psi the boiling point increases to about 268F. That's a considerable benefit.
 

Burt

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Pressurized cooling systems are quite common. 21psi is not that much. On race cars you will see 50 psi. A pressurized system prevents air pockets and hot spts. The higher the pressure the higher the boiling point. Typical coolant has a boiling point of about 223F. At 15 psi the boiling point increases to about 268F. That's a considerable benefit.
I appreciate all the feedback. Obviously I’m behind the times, but I think Ford has a design flaw here on a low volume model. Too much vibration on a weak connection.

I understand the benefit of pressure, but cars have gotten along well for decades with unpressurized reservoirs and pressurized radiators. I guess the pressurizing the overflow tank extends the life of the radiator at the risk of having to replace the cheap tank and fittings? This started leaking at about 35,000 miles.
 
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On Euro cars (Fiesta is an Euro vehicle) pressurised expansion tanks are standard for 40+ years. Last vehicles with older system were Japanese cars.
 
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On Euro cars (Fiesta is an Euro vehicle) pressurised expansion tanks are standard for 40+ years. Last vehicles with older system were Japanese cars.
Ford still used it until at least 2011. The Rangers with the 4.0 had a non pressurized overflow tank with a traditional radiator cap. The 2.3L used a pressurized tank.
 
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Not sure about American vehicles, but Euro Fords used pressurised expansion tanks just as every other Euro make since mid 70s. Had a 93 Escort that have had failed one when 20y old. My 2002 Nissan 2.7 TD have overflow tank and some 90s Japanese makes used them to that point.
Ford still used it until at least 2011. The Rangers with the 4.0 had a non pressurized overflow tank with a traditional radiator cap. The 2.3L used a pressurized tank.
 
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My 88 Cherokee used a pressurized tank ( and it was garbage and converted to a traditional system used after 91 on the Cherokees...). So even 30+ years ago there was a north American application...

As I am remembering all my F150s going back to 1997 were pressurized tanks. Zero issues.
 
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That pressurized tank is not the reservoir, it's the surge tank. You should have an unpressurized reservoir attached to the surge tank.
My previous 1995 Corvette had a surge tank and a reservoir so these kinds of systems have been around for a long time.
 
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That pressurized tank is not the reservoir, it's the surge tank. You should have an unpressurized reservoir attached to the surge tank.
My previous 1995 Corvette had a surge tank and a reservoir so these kinds of systems have been around for a long time.
Not all are that way, my 1994 BMW had the pressurized tank but there was no separate reservoir. It was a pressurized reservoir.
 
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If you'd like to eliminate the weak link, check the internet for an aftermarket aluminum tank. Several companies make one for the Mustang and a lot of Mustang guys have replaced their ugly plastic tanks with a very nice looking aluminum units. As far as servicing the unit you've got, I would replace it before that fitting fails. And be careful with the cap. The cap on my 2018 Mustang with only 12,000 miles broke as I unscrewed it, due to excessive friction between the O-ring and the inside of the filler neck. I put a dab of grease on the O-ring of the new cap before putting it back on.
 
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That pressurized tank is not the reservoir, it's the surge tank. You should have an unpressurized reservoir attached to the surge tank.
My previous 1995 Corvette had a surge tank and a reservoir so these kinds of systems have been around for a long time.
expansion tank + overflow bottle is a rare setup
 

Al

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Most vehicles have a "de-gassing" tank that maintains the system pressure with a related cap that is remotely located away from the radiator. Long gone are the days of a radiator cap and overflow bottles.
Wish someone would tell subaru about that. Trying to save a buck.
 
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