2004 grand am gt air in cooling system

[nathan709]

i thought it was a head gasket for sure so i did a compression test... surprisingly it passes.... about a difference in pressure of about 4-5 psi.. i am wondering wut are the common ways the cooling systems gets air... thanks alot

[GTP Dad]

Welcome to AF!!

There are several ways to get air in a cooling system. The most common is repairing the system where you drain the system and refilling it. Less common are bad head gaskets, bad intake gaskets, cracked heads or air leaks in the coolant recovery system lines.

If you need to bleed the system fill it up entirely and place the nose of the car up hill. Put the heater on high and start the vehicle. Locate the thermostat housing and on top there should be a plug that uses an allen wrench. Once the car is hot open the plug until coolant comes out. Repeat until all you get is coolant. Top off the system. Repeat as necessary.

[Tech II]

Only one way it can get air, it has a leak somewhere......

3.4L? Usual places are lower intake gaskets(look in engine depressions on top of block under the throttle body, for coolant collecting....w/p weep hole.....heater core.....radiator.....hoses(radiator/heater core)......bad radiator cap or reservoir cap......head/head gasket......

If I were checking for a head gasket leak, I would be doing a leak down test, rather than a compression test.....

[nathan709]

hmmm there is not any coolant leaking from anywhere besides the overflow tube... where could the cooling system be getting air and not leaking coolant? lower intake gasket? the system clearly holds pressure.. the radiator tube both get hard.. i was wondering.. has anyone herd of a car passing a compression test and failing a leak down test? could the head gasket still be bad after passing the compression test? thanks!

[Tech II]

You didn't mention that the coolant was coming out of the overflow tube......

A compression test shows a cylinder's ability to compress and build up pressure.....a leakdown test shows it's ability to hold pressure.....A leaking head gasket would show bubbles in the cooling system, during a leak down test....