hello! I have 2001 zr2 with a 4.3eng & I have P0300 random & P0302, P0304, P0305 I did a smoke test &
1 test you do to see if it's a vacuum leak connect a code scanner that shows fuel trims & rev engine up to 2000 rpm
& short term fuel trims should drop to zero or lower that will show vacuum leak take a can of brake clean
on the intake manifold & watch the short term fuel trim should drop & there's your vacuum leak!
couldn't find anything & short term fuel trim was all over the place replaced the intake gasket (stock intake gasket)
2X the same problem removed the intake manifold & bought a straight edge took a feeler gauge &
found. ,002" gap by #2 cylinder & #5 cylinder, so the intake manifold was warped but I couldn't find a manifold I researched &
found that Felpro makes a thicker gasket & it's not cheap! but it fixed the problem no more misfires!!
short term & long term fuel trim was great!
AND BOB'S YOUR UNCLE & FANNY'S YOUR AUNT!!