what would cause my engine to run erratically while idling?

25

Asked by Feb 06, 2014 at 07:40 PM about the 2003 GMC Sierra 1500 SLT 4WD Extended Cab SB

Question type: Maintenance & Repair

My engine light came on and I hooked up a scanner to it and it read 5 codes, 2 of
which were duplicates.most of the time it idles normal but once in a while it will seem
to struggle to stay running. I cleaned the MAS, installed new plugs and wires, a new air
filter and PCV valve. Cleared the codes and engine light came back on. Also noticed
that the voltage meter fluctuates but when you apply throttle it rises back to 14 or 15
volts. Is my alternator on the way out and would that cause the rough idling?

5 Answers

56,845

What codes do you have? -Jim

25

original codes were: P0300,P0171 & PO174. Just went to Auto Zone and ran the codes again and the new codes are : P0300, PO174,P1153,& P0151,

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56,845

P0300 is random/ multi cylinder misfire, P0174 is system too lean, P1153 is insufficient switching of the oxygen sensor, P0151 is low voltage at the oxygen sensor. This is the upstream oxygen sensor, ahead of the catalytic converter. Sounds like either you have a vacuum leak, low fuel pressure, or a bad oxygen sensor. It's misfiring because of the lean air/fuel condition. I'm leaning toward a vacuum leak or bad fuel pressure regulator. Fix the lean condition and I bet the other codes will go away and your truck will be running right again. HTH. - Jim

1 out of 1 people think this is helpful.
25

Thanks for the help Jim! I will try the fuel pressure regulator and check for vacuum leaks.

1 out of 1 people think this is helpful.
56,845

You're welcome. Glad to help. -Jim

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