Totally Awesome
Street Race? |
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I
was sitting at the stop light in
my red, ‘86 Mustang GT 5.0L.
In the next lane was an orange, lowered,
ominously rumbling Chevy step-side
pickup. My Mustang revved up and
cut back down to idle over and over
again. Was this a totally awesome
street race? No, it
was the throttle position sensor
on my 5.0L whacking out. I sat helplessly
as the high revs got higher and the
low revs got lower until the Mustang
stalled out. The old pickup calmly
drove away.
It was great news when computerized
fuel injection and air intake replaced
carburetors. Emissions laws had
gradually turned carburetors into
overly complicated and finicky
beasts. However, modern induction
systems still need throttle plates
and other mechanical, carburetor-like
parts. Some “modern” induction
systems have now been on the road
for over twenty-five years. After
enough time and miles, a modern
system can start acting like a
cranky old carburetor.
When one of my newer cars starts
behaving like a ’77 Nova
with a bad choke, the first place
I look is the throttle position
sensor. The throttle position sensor
mounts over the end of the spindle
supporting the throttle plate butterfly
valve. The sensor tells the engine
computer the position of the throttle
butterfly valve. A throttle position
sensor on a ’91 Camaro looks
very similar to the sensor on a ’02
Camry. Typically take the hose
off the air intake, find the throttle
butterfly valve, and then look
for the small, often round box
covering the end of the shaft supporting
the butterfly valve.
Problems with the throttle position
sensor are most noticeable when
the engine is at idle or just coming
off idle. When the feedback loop
between the throttle position sensor
and computer is broken, the computer
does not know how open or closed
the throttle is so the car stalls
or idles strangely. Replacing a
flaky throttle position sensor
can immediately transform an engine’s
performance. If the car stumbles
at idle then it is also probably
subtly hesitating and fumbling
at higher rpm.
Throttle position sensors are
usually variable resistors mechanically
connected to the butterfly valve
spindle or Hall effect sensors
detecting the movement of a magnetic
field. The sensors might get dead
spots or fail intermittently and
never activate the right fault
codes on the engine’s diagnostic
system. Your car’s repair
manual might give the specifications
for testing the throttle position
sensor using an oscilloscope. Looking
up various throttle position sensors
in the www.RockAuto.com
catalog (under Emissions), the price for
a new sensor typically varies between
about $20 and $50.
Installing a new throttle position
sensor usually just requires opening
the hood and removing two screws.
So check out the throttle position
sensor before pouncing on oxygen
sensors, mass airflow sensors,
transmission solenoids, engine
rebuilds, or other big and possibly
unnecessary jobs!
Tom Taylor,
RockAuto.com
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