AIr lock: just for the record

[LMP]

Just picked that on another forum:

"I spent a winter with no heat on my old Sidekick did the thermostat fan clutch thing to no avail turned out to be a leaking head gasket which was air-locking the heater core "

Since there were numerous allusions to that condition recently on this forum, this quote shows that is non uncommon. We are all justified in hoping to clear this problem by simple means and explanations, but unhappily, the hard fact is that blown head gaskets do happen.

The problem is how to obtain a sure diagnostic before committing to expensive head gasket job . Here is one trick I used once, and I suppose it is used elsewhere:
Required: compressed air supply
tire valve.
Remove radiator cap and fill up any void
Remove spark plugs ...(ok here, you have to see them in the first place, which is not obvious)
FOr confort, I use a air hose attachment allowing to have the air valve screwed to it. By the way, remove the inner spring valve from the tire valve.
Then, by applying the bottom of the valve to each spark plug hole one after another and forcing air in the cylinder, the head gasket leak will allow air to enter the cooling circuit and air bubbles will show right away popping out of the radiator.
However, given poor accessibility to spark plugs on E-W V6, , running the engine and watching air/exhaust bubbles popping out of the radiator is easier. But may be some of you also have another 4 banger to use the trick...if necessary.