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2002 GMC Sierra CEL On with a Slightly Rough Idle
This GMC came in with a Check Engine Light (CEL) on and a slightly rough idle. Our
first step was to hook up Escan and check for
Diagnostic Trouble Codes.
We have three. A P0171, P174, and a P0158. The first two are lean bank codes. The
third is a problem with a rear O2 sensor. Since we have a lean problem it would
be a good time to graph some sensors. We'll start with Escans Info Pids
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The info sidebar tells us a few things right off the bat. We know that we have good
Engine Vacuum, Charging Voltage, Bank to Bank fuel trim and Complete Monitors. We
can also see that we have a fuel trim problem as noted by the red fuel trim buttons
and the 2nd and 3rd rows on the graphs showing a large trim adjustment. Ideal that
would be 0 to 10 but we have 22. This is our lean bank condition. A very good candidate
for a lean idle would be a vacuum leak. We decided to switch the pids to show all
four O2 sensors as shown in the pic below. |
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We can learn right away from the pic above that O2B2S2 is flatligned at a high state
like the code P0158 stated. With both banks running lean that O2 signal should have
been in a low state like O2B1S2. We can deduce that O2B2S2 needs to be replaced
at this time. Since we think we also have a vacuum leak we decide to introduce a
small shot of propane in the area of the intake gaskets a known weak spot of this
engine. Remember always use caution because propane is extremely flamable and could
catchfire. O2B2S1 and O2B1S1 both show the propane entering a leak. Look for the
large hump where the O2's went high showing the entering of the propane. The pics
below shows the trims after the repairs and they also show the O2"s |
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We decided to follow up a also check to make sure the
Catalytic Converters
are still functional. They pass! |
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