MACS Monthly Newsletter - 2020 - DEC6

What Do You Expect? It's A Diesel!
If you don't have the scan tool software for this test, using an infrared thermometer ( " ray gun " ) on each heater
hose next to each sensor might seem to be an approach
that should work. But with the thick thermal conduit,
there might be too much of an insulating effect. In that
case, you might need a pointed probe-type thermometer
and very, very carefully puncture the insulation at the locations where a hose takes a U-bend. See Figure 11. Of
course, a temperature drop at any inlet and outlet points
past the points where the hoses bend should confirm the
temperature drop issue, and once you confirm that, you
can proceed to close in on the exact location of a restriction.
When the system is warmed up, there should be normal
flow through the heater core, when the controls are set for
full or nearly full heat. With any conventional heater circuit, you'll typically see a temperature drop of 10°-20°F
from heater inlet to outlet. That might be something on
the order of 150°-160°F to 130°-140°F. When there's a kink
or twist (in either the inlet or outlet hose) that is responsible for poor heating, the drop is more likely to be in the
range of 40°-60°F, maybe even higher if the restriction is
severe. n

acdelcotds.com

A complaint of poor heater performance on a diesel often leads the technician to say it's a generic issue, that the
high efficiency of the engine, particularly at low load, is
the cause. But often the motorist will say, " It used to be
much better. There is something wrong. " And although
some diesels have oil-fired (or even some PTC electric)
heaters to improve cabin comfort, the motorist may be
right. Here's an example that occurred on new Chevy Silverado / GMC Sierra models with the 3.0-liter in-line sixcylinder, which like the 90° 6.6-liter V8 diesel, also carries
the " Duramax " brand name.
The poor heating on the Silverado / Sierra has been
caused by kinking of one or both heater hoses, restricting
coolant flow. It might seem that a kinked or twisted heater hose, unless in an area covered by an underhood panel
of some type, should be visible. However, these vehicles
have thick thermal conduit wrapping over the hoses (Figure 10), so any twisting, etc. may not be readily visible.
A good diagnostic approach could begin by comparing the temperature readings on the heater core inlet and
outlet temperature sensors with the engine fully warmed
up, the heater temperature set to full hot and the blower
motor on high.

Figure 10: A thick, thermal conduit covers the heater hoses
on the Chevy Silverado / GMC Sierra with the new 3.0-liter " Duramax " diesel. This obscures some of the bends in
the hose assembly. Arrow points to one hose area of possible
kinking.

December 2020

Figure 11: This illustrated view of the hoses (sans thermal
conduit) in the Silverado / Sierra with the 3.0-liter diesel
enables a technician to look for a sharper-than-normal bend
in a hose, that could be restricting coolant flow.

MACS Service Reports

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MACS Monthly Newsletter - 2020

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