MACS Service Reports - 2015 - JAN2

HEATING SYSTEM PROBLEMS
IF IT ISN'T A PLUGGED CORE, WHAT IS IT?

flushing the core. But first of all, you have to prove that
the core really is plugged. If you get a 15-40°F temperature drop from the inlet hose neck to the outlet along with
poor heating, that's a pretty sure sign (Figures 1, 2). You
can disconnect the outlet hose on the engine side, then
run the engine and see if there's a solid column of
coolant flowing. If not, that's a confirming event.
But before you replace a heater core, most customers will opt for a back flush. It won't be nearly
as big a ticket as installing a new core, but if you
work with an effective piece of equipment, the results are almost surely likely to be worthwhile. With
the labor time involved, chemicals, maybe a couple
of hoses, etc., it's not pocket change either. And it's
sure to boost your shop's reputation for saving the
customer money. If you measure the heater duct
outlet temperature when you start and after your
best efforts, you're almost sure to be able to show an

Paul Weissler

Everything we've seen says that a plugged heater core
is a primary cause of poor heater performance, but it's
hardly the only cause. In fact, it may not even be a majority of the causes, and doesn't always mean the system
needs a new core.
Sure, when the core is badly plugged, there's often no
choice; you have to replace it, and the job may be expensive. But often there is a choice. We're talking about back-

Paul Weissler

Figure 1: Checking the temperature drop across the heater hoses at the
core necks should show a very small drop. If you see a drop of 15 to 40°F
and heating is poor, a restricted core is a likely problem. Here we're using
an infra-red thermometer with laser sighting so we're sure we're aimed
at the right hose. Look for a temperature drop from inlet to outlet of about
3 to under 10 degrees and a high-enough coolant temperature. In this
case, the coolant is still warming up, but the temperature drop across the
hoses is running just under seven degrees. (120.8 versus 126.7). However, measuring coolant temperature on the hoses is not really accurate
and will read somewhat lower than the coolant flowing through it, so also
check with a scan tool reading the coolant temperature sensor to get the
temperature of the coolant itself.

Paul Weissler

Paul Weissler

Figure 3: After one back flush, heater outlet air temperature is up from about
94°F to 102°F, which is an improvement, but maybe the shop can do better.

Figure 2: Checking temperature drop across the heater hoses with a probe-type
thermometer as shown will produce more accurate results (assuming the thermometer itself is accurate). But it may take a few minutes for the thermometer
to reach the temperature of the heater hose.

January 2015

Figure 4: Use of a chemical flush got the outlet temperature up some more,
to 117°F.

2

MACS Service Reports



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

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