MACS Service Reports - 2015 - JAN5

pressure cap is not sealing properly (results in low coolant level, admits air to system).

so any small amount of air in the lines to the rear will not
be a source of complaint and will eventually bleed out.

COMBUSTION GASSES
Of course, what appears to be an air pocket may actually be combustion gas getting into the cooling system.
In fact, when an "air pocket" keeps reappearing despite
your best air bleeding efforts, it's likely you're fighting
a combustion gas leak, usually from a leak at the head
gasket joint, but possibly from a mini-crack or other
engine issue.
There are several possible ways to detect a combustion gas leak. With the engine warmed up, rev the
engine to about 4000 rpm and if there's a leak, you
could see bubbling in the radiator fill neck or reservoir. But if it's nothing that obvious, the alternatives
are pretty well established, such as using a gas ana-

Paul Weissler

GETTING RID OF THE AIR POCKET
Every cooling system traps some air, and purging it
is usually a pretty straightforward procedure. Raise the
front end and thermocycle the system (warm-up, let air
bubble out of the fill neck, then top up, let the system
cool down, and repeat as often as necessary (typically 3
or 4 times). This can be time consuming, but it works for
all but the most difficult systems. Using a top up funnel,
such as the Lisle type with the adapters for different size
fill necks, is as good a tool as any (Figure 9).
If a system is likely to trap air, there will be bleed valves
that should be opened. With problematic heaters, the one
in the hose against the firewall on Nissan and other cars,
not only have to be opened, but make sure a valve isn't
plugged with debris, as they often are. Clean out that
valve before you begin thermocycling.
If a lot of coolant has escaped and you can't seem to

Figure 10: Combustion gas analyzer probe can identify leakage from engine, particularly head gasket, into coolant jackets. Lower coolant level
from fill neck, wrap plastic sheet over fill neck and push gas analyzer probe
through plastic sheet and see if it detects combustion gas emanating from
coolant.

Figure 9: Lisle funnel comes with adapters for tight fit into engine coolant fill.
Pour 50-50 mix of coolant into funnel, then jack up front of car and remove
the stopper. Run engine and air will bleed from system and system will take
coolant from funnel to maintain full system. Several efforts at thermocycling
(engine warm-up and cool down) may be necessary to get virtually all the air
out of the system.

Paul Weissler

get rid of the air pocket while filling the system, the most
practical approach may be to drain and refill. It actually
can give you a better shot at the problem.

REAR HEATERS
When the system has been drained and it's time to refill, the rear heater can pose a special issue. If you jack up
the front of the car, you've lowered the rear heater, so any
air in the rear will stay there. Hopefully you know the
routine: fill that rear heater with coolant before you go for
the front system, and turn the heater control to full heat,

January 2015

Figure 11: Combustion leak detector that indicates combustion gas in coolant
is another method. Fill tube (in this case each of two tubes) to specified level
with blue test fluid.

MACS Service Reports

Table of Contents for the Digital Edition of MACS Service Reports - 2015