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2003 GMC Sierra 4.8 running rough with multiple fault codes
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Ramntm
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Mar 25, 2020, 6:51 PM
Post #26 of 28
(506 views)
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Re: 2003 GMC Sierra 4.8 running rough with multiple fault codes
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Do you feel a misfire when idling or accelerating? If you don't, the P0300 could be due to needing to do a crankshaft variation relearn. I have had success doing it when no misfires were detected. If you had a scanner which would show misfire data, things would be easier. The misfire history would be really helpful.
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Ramntm
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Mar 25, 2020, 6:58 PM
Post #27 of 28
(506 views)
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Re: 2003 GMC Sierra 4.8 running rough with multiple fault codes
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If you had a scanner that showed misfire data, it would be easier to solve the P0300 code. The misfire history would show which cylinder(s) misfired, and how many misfires were recorded over a number of testing cycles. Do you ever feel it misfire when either idling or accelerating? If not, you might cure the p0300 code by running a "Crankshaft Variation Relearn.' I have had success doing this when misfire data showed no misfires. The downside is, you need a scanner that is capable of running the "Crankshaft Variation Relearn" procedure.
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Ramntm
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Mar 25, 2020, 8:03 PM
Post #28 of 28
(496 views)
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Re: 2003 GMC Sierra 4.8 running rough with multiple fault codes
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For the P0449 code, the same applies as my previous reply. It would be much easier with a scanner that shows emissions data. One thing you can do is, unplug the 2 wire connector from the canister vent valve. With the key in the "Run" position, check the Pink wire for battery voltage. Tug firmly on each wire to see if either wire is broken inside the insulation. If the insulation stretches, that wire is broken and replacing the connector will likely fix the problem. If neither wire is broken and the Pink wire has battery voltage, plug the connector back in and, with the key in the "Run" position, ground the other wire (The one without battery voltage) and listen for the vent valve to make a "click" sound. You should hear the valve click when it closes and opens several times when grounding and ungrounding the wire. If all that checks to be good, you need a scanner that can show the voltage of the Fuel Tank Pressure sensor. The voltage should read 1.5 volts or very close to 1.5 with the key on, engine off and the fuel cap removed If it reads much below or much above 1.5 volts, the Fuel Tank Pressure sensor is faulty or there is a problem with the signal return wire or the ECM. I have not found a problem with the signal return wire or an ECM causing this problem, only bad Fuel Tank Pressure Sensors, when the Voltage was not 1.5 volts. Hope this helps.
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