Rick's Free Auto Repair Advice

Impala P0300, P0172 hard starting

Diagnose and fix Impala P0300, P0172 hard starting condition

If you have an Impala P0300, P0172 hard starting condition, follow the testing sequence below to find the culprit

A P0300 is a random misfire so the problem is likely to be fuel related rather than spark plug related. Unless all the spark plugs are equally worn out, a failed spark plug would most likely result in a single cylinder misfire, not a random misfire.

Check fuel pressure regulator first

The fuel pressure regulator on most Impalas is a fuel pressure regulatorvacuum-operated unit that’s mounted at the end of the fuel rail. As you depress the throttle, the manifold vacuum decreases. The vacuum regulator diaphragm responds to the drop in vacuum and increases fuel pressure. The diaphragm on this unit can leak, causing it to not respond. To check, remove the vacuum line to the regulator and look for raw gas in the vacuum line. If you see gas, replace the regulator. After replacement, check the items listed below.

Check fuel trims

Using a scan tool with live data, check the short and long term fuel trims. If the engine is starving for fuel, the fuel trims will be high, as the computer tries to add more fuel. One of the symptoms of a fuel delivery problem would be a hard to start condition and lack of power.

Clean the MAF

MAF sensor

MAF sensor wire

Using a CRC MAF sensor cleaner, spray the hot wire inside the MAF. Do not use any other type of solvent and do not touch the sensor with a shop rag or Q-tip. That will break the wire.

Check the idle air control valve

The IAC controls how much air gets into the throttle body idle air control valveat idle. Using a scan tool read the IAC count. It should be 15%-25%. If it is out of range, remove the IAC and clean carbon out of the throttle body.

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Posted on by Rick Muscoplat

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