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P0507, P0300, P0306 Fault Codes on 1997 GMC Suburban 5.7L V8 - Diagnosis and Solutions

Model: 97 Suburban Fault Code: P0300 Posted: 2007-06-10 18:01

I recently purchased a 1997 GMC 1/2-ton 2-wheel-drive Suburban with 165,000 miles. After driving it 300 miles to my home without any immediate issues, I noticed the SES (Service Engine Soon) light was illuminated. Prior to purchase, diagnostic scans revealed fault codes P0507, P0300, and P0306. I replaced the spark plugs, rotor, and cap with no improvement. There is no coolant loss, but there is a noticeable oil consumption. The engine idle fluctuates between 1000 and 1700 RPMs, though it runs smoothly at highway speeds. What should be my next steps? Any technical insights or troubleshooting tips would be greatly appreciated. Thanks, ccnova

Related fault codes
P0300P0306P0507
Comments (3)
Anonymous 2007-06-11 08:14

P0507 indicates that the idle control valve is not functioning within its required range to maintain a stable idle speed. P0300 signifies a random misfire, while P0306 points specifically to a misfire in cylinder #6. Together, these codes strongly suggest the presence of a vacuum leak somewhere on the engine. Common sources include intake manifold gaskets or damaged vacuum hoses. It's also possible for the throttle body (butterfly) valve shaft to develop a leak. To test this, spray a cold engine throttle body cleaner around the exterior of the throttle body and near the intake gasket area while the engine is running. If the engine suddenly stumbles and fuel trim readings drop into negative double-digit ranges, it confirms a vacuum leak at that location.

Anonymous 2007-06-11 08:35

I forgot to mention in my original post that a friend advised me to plug the holes on the valve covers and check for vacuum in the dipstick tube. I performed this test and found vacuum present. He suggested this indicates an intake manifold leak. Is this method reliable? Thanks again, ccnova

Anonymous 2007-06-11 08:39

If you have sealed all valve cover ports and, if applicable, the oil fill cap (which is typically vented), and there is still vacuum detected in the dipstick tube, then yes — your intake gasket is likely leaking into the crankcase. This condition cannot be visually confirmed but aligns with the advice of your friend. Such a leak would also explain the observed oil consumption.