I'm experiencing multiple misfire codes (P0300, P0301, P0303, P0305, P0307) on my 2004 Avalanche with a 5.4L V8 engine. A user suggested the intake manifold might be the issue—could this be possible? I'm looking for confirmation and guidance on diagnosing the root cause.
I recommend taking it to an exhaust specialist to test back pressure on Bank 1's catalytic converter. This should be done if you're unable to perform the check yourself.
How does it idle? Water in the fuel can cause P0300 (Multiple Random Misfires) and may result in poor performance, even though the engine runs fine otherwise. This happened to me recently—water contamination was the root cause. For reference: http://www.obd-codes.com/forums/viewtopic.php?t=691#p20691
I'd also lean toward a faulty exhaust converter as the primary issue.
How does the truck run under load and at idle? I suspect a clogged exhaust converter rather than intake gaskets. With multiple misfires on one engine bank, restricted backpressure from the converter is a likely culprit.