MACS Service Reports - 2016 - JAN3

funnel?), refit the pressure cap, run the engine at
2000-2500 rpm to reach operating temperature, then
let the engine idle for three minutes, shut it off, let
the engine cool, then top off.
This thermocycling procedure obviously can be
repeated as often as necessary to get the air out,
but in this case it wasn't the cause of the problem.
Must be the HVAC control module, which as shown
in Figure 5, controls both front and rear temperature. If the module has functionality in some areas,
you should want to confirm there isn't a problem
area elsewhere before you replace it. The module
did have functionality in the areas the technician
could test.
Could there be separate issues with each heater?
Perhaps there is an HVAC control module problem
with one and a heater core issue with the other?
Sure, but where to start? After going through the air
bleed procedure a couple of times, the technician
started with the front heater. Backflushing it, he
saw some garbage coming out. Heating improved
at idle, so he thought, "Why not try the same at the
rear?" Indeed why not, and it worked there too.
Why were both cores restricted with debris? We
know that the heater tubes are the smallest in the
system, and it's possible that at one point the system got a dose of sealer to correct a small leak. Or
perhaps there was residual engine block core sand
and flux residue in the system, and they got into

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thermostat? Besides, it already had been changed
for no good reason.
If only one heater were malfunctioning, you'd
naturally head directly for that one. But when both
are blowing cold air, it just has to be something else.
Well, the vehicle was a 2007 Chevy Uplander with
the 3.9-liter V6. It had to be a giant air bubble or
exhaust gas leakage into the system, right? There's
a factory bulletin for poor heat at idle, and it notes
that if you raise engine rpm above 2000, the heating
improves. Sure enough, the technician raised the
rpm to about 2000, and the two heaters started to
blow hot air.
Okay, what does the bulletin say? Well it notes
that the rear heat will not be as good as the front
in that case, because the benefit from higher rpm is
to improve coolant flow through the heater cores,
and not as much gets to the rear. There must be an
auxiliary electric pump for this, right? Sorry, but if
the system is performing correctly, the difference
between rear and front heat will fall into what we
popularly call the "normal range." Naturally that
normal range varies according to ambient temperature and the colder it is, the greater the difference.
In any case, General Motors engineering has never
validated an auxiliary pump for these minivans.
The system tends to trap some air and GM has a
static fill and air bleed procedure, which is basically this: fill the system slowly (how about a Lisle

Figure 5: Wiring diagram of Chevy Uplander HVAC module for front and rear systems. Although they are separated within module, both are controlled by
one unit.

January 2016

MACS Service Reports

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Table of Contents for the Digital Edition of MACS Service Reports - 2016