Engine Misfire or Rough idle on GM 4.8L, 5.3L, 6.0L

Engine Misfire or Rough Idle on GM 4.8L, 5.3L, 6.0L is becoming more common and getting harder to diagnose. 

With this getting worse it is also setting off MIL illuminated with a stored code of P0300.

Most of these engines are calibrated for ethanol fuel (E85). Should the rough idle, engine misfire, or DTC P0300 diagnostic procedure point to an air leak in the intake manifold gasket, following the below directions will improve diagnostic accuracy.


As a result some customers may comment that the vehicle may have;

  • A rough idle
  • Engine misfire
  • MIL illuminated
  • Astored DTC P0300.


  • The L59 engine is calibrated for ethanol fuel (E85). Due to the low volatility of ethanol, the PCM provides higher fuel flow through the injector. This may pool on the upper manifold to head gasket material. Consequently over time (usually 12 months and longer), the gasket material may degrade resulting in an unmetered air leak. Manifold warping may be the cause as well. Follow instructions on manifold information to determine if replacement needs to happen.
Green Gasket
Green Gasket


  • To correct this problem, Replace the upper intake manifold gaskets with the teal green gasket material, P/N 89017589.
  • NOTE: This gasket is also made by VICTOR # MS16340
Orange Gasket
Orange Gasket


  • Furthermore do not replace the upper intake manifold gaskets with the original orange gasket material, P/N 17113557. Inspect Intake Manifold to Cylinder head Deck for Warpage


  • The intake plenum should also be checked for excessive warpage that may cause an uneven clamping pressure of the seal. A intake manifold with warpage in excess of 3 mm over a 200 mm area should be replaced. Intake warpage is measured at the gasket sealing surfaces and should be measured within a 200 mm (7.87 in) area. This measurement is across only two of the intake runner port openings. Consequently measurements taken across the entire distance of all four intake runner ports will lead to unnecessary manifold replacements.
Engine Misfire
Engine Misfire


1. Locate a straight edge across the intake manifold cylinder head deck surface. Position the straight edge across two runner port openings.

2. Insert a feeler gauge between the intake manifold and the straight edge. Do not replace intake manifolds with warpage that measures less than 3 mm over a 200 mm. Part Number Description 89017589 Gaskets, Upper Intake Manifold (Teal Green)


About Danny Bender 180 Articles
I have been into cars for as long as I can remember but the engine is what interested me the most. Now with over 40 years in the engine rebuilding and machine shop industry I have retired. I will be spending my time sharing what I have learned while keeping up with the latest Technology - Helping Solve Engine Problems !

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