Purge Tube

[grgrose]

I have a 94 DeVille with the 4.9 engine. I suspect I have air in the coolant system. I have seen previous posts that talk about a purge tube. Where would I look for that?

[Ranger]

There is no purge line on the 4.9. Air is purged from the system via the two way drop down valve in the radiator cap. Make sure it is not clogged. Also be sure that you have a hose on the cap that is on the overflow reservoir and that it extends below the coolant level.

[grgrose]

I will check the cap. The hose from the resevoir goes to the radiator neck.

[Ranger]

Right, but if you lift the cap on the reservoir, there is a hose on the underside of it and it should extend down into the coolant. Other wise the radiator will draw air instead of coolant back into the radiator as it cools after shutdown.

[grgrose]

That would be my problem then. No hose on that cap.

[grgrose]

Hmmm, No place to put a hose on the resevoir cap.

[Ranger]

OK, maybe I am still thinking of much older models. Where does the hose from the radiator neck (overflow) enter the reservoir tank?

[grgrose]

At the bottom.
I have had problems where the coolant gets stuck in the resevoir and will not drain back until I release the pressure at the radiator cap by removing it. Thats why I thought maybe I had air in the system.

[Ranger]

I would guess that either the overflow/return hose is clogged, the cap is gunked up or the two way valve is not working correctly.

[grgrose]

2 way valve? Where is that located?

[submariner409]

The proper radiator cap itself has 2 seal rings, a spring loaded one for the inner (radiator fill) opening and the other for the top circumference of the neck. Those 2 seals form a small chamber which allows air from the system to bleed off through the overflow tube into the coolant reservoir. The reservoir should be 1/3 to 1/2 full of coolant, cold. As the car heats up, coolant expands and an air/coolant mix bleeds off into the reservoir. The coolant in the reservoir will rise and fall, depending on engine temperature. When you shut down, the engine cools and the coolant in the system contracts. Coolant (minus air) is then drawn back into the radiator, keeping the system gas-free. You do not have to open anything to have the coolant "drain back into the engine". It's automatic. The level of coolant in the reservoir, cold, should never change if the system is sealed properly. The inner seal ring incorporates a small coolant return check valve, is loaded to the design system operating pressure, and allows air to bleed easily, but will not allow coolant to bleed until the system is up to pressure....16# ???

[grgrose]

I understand that is how it's supposed to work. The problem is, the engine was getting hot and all of the collant was in the resevoir.

[submariner409]

If the engine is not cooling, it's water pump, belt, thermostat, or plugged radiator passages. If the system is not holding pressure (squeeze the top hose, hot) it's the cap. If all the coolant were in the reservoir, it would have to be 3-4 gallons in size.

[Ranger]

Originally Posted by grgrose

2 way valve? Where is that located?

It's on the underside of the radiator cap.

[grgrose]

Never mind. It is a normal size resevoir that was pretty much full.