ACtion Magazine - June 2017 - 14

Cool runnings
By Steve Schaeber, MACS Technical Editor

he main purpose of a thermostat is to help the cooling system regulate engine operating temperatures to within the manufacturer's
specification. This is normally done by first remaining closed to
allow engine coolant to warm up, and then gradually open all the way as
temperatures near spec. But sometimes this doesn't happen, as in the case
of a fully or partially stuck open thermostat.
When a thermostat sticks open, engine coolant is allowed to circulate without much restriction throughout the cooling system. This sounds
great, and it's important for this to happen after the proper operating temperature is reached. However, if it happens too soon the engine coolant
may not be able to reach a high enough temperature. This problem occurs
because modern cooling systems are actually quite efficient, and are able
to transfer a lot of heat from a lot of liquid in a relatively short period of
time. When the coolant is not allowed to warm up fully, it basically circulates from the engine to the radiator and back to the engine, losing so
much heat with each pass that it stays relatively cool.
Problems attributed to a cold running engine include lower HVAC
heater output, engine performance issues and increased tailpipe emissions. But it also prevents the evaporation of condensation that forms on
the inside of the engine block. If this water doesn't burn off it can oxidize
the metal (producing rust) and cause an oil sludge buildup in the bottom
of the pan.

ACTION * June 2017

Engine oil is also affected by the thermostat, and it's important for it
to reach an operating temperature of approximately 190°F. Oil that is cold
is not quite as viscous as the substance when it's warm. Transmission oil
coolers are also affected because they are generally in direct contact with
engine coolant.
An easy way to determine if a thermostat may be stuck open is to
simply monitor coolant temperature. Some vehicles have a dashboard
temperature gauge, but many technicians prefer to use a scan tool with
graphing capabilities. Begin when the engine is cold and connect your
scan tool before starting the engine. Graphing the coolant temperature
sensor PID will allow you to watch the thermostat to see what happens
over time. If the thermostat is only partially stuck open, you may notice a
rise in value to a relatively warm number, perhaps around 150°F. But if it's
stuck open all the way, temperatures may not increase much past 120°F.
Many PCMs will also monitor thermostat operation, illuminate the
check engine light and produce a trouble code if there's a problem. Ford
uses P0125, Insufficient Coolant Temperature for Closed Loop Fuel Control, along with P0128 Coolant Thermostat (Temperature Below Thermostat Regulating Temperature). Issues could be related to slow warm up
times, low coolant level, or sensor output issues, as well. Sensor readings
may be double checked by using a mechanical thermometer or a thermocouple connected to a digital meter. ❆

http://www.lilandglobal.com/

Table of Contents for the Digital Edition of ACtion Magazine - June 2017