Engine Misfire – Rough Idle On GM 4.8L, 5.3L, 6.0L Engines

Engine Misfire Intake Manifold
Engine Misfire Intake Manifold

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

Also, GM V8 engines are very prone to a rough idle and misfire when cold.

The check engine light may also come on, with codes P0300, P0171 and PO174.

But, as the engine warms up, the engine misfire or rough idle tends to go away, which seems confusing.

Armed with the knowledge of what can cause a engine misfire, rough idle condition and a specific testing strategy, you can easily find out what component is the one that has failed.

So, should the engine misfire, rough idle or (TDC) code point to an air leak in the intake manifold gasket, following the below directions.

A very common cause of a leaking intake manifold is the gaskets and bolts used from the factory.

As a result, the plastic intake manifold fails to seal and creates a vacuum leak.

General motors now blames the intake manifold gaskets as the problem and now sells a redesigned replacement.

What Are The Symptoms

Some owners may complain about:

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

What Is The Cause Of Engine Misfire – Rough Idle

The plastic intake manifold fails to seal and creates a vacuum leak. General motors cites the intake manifold gaskets as the problem and now sells a redesigned part.

The engine may be running too lean, due to a leaking intake manifold. Too lean means too much air and not enough fuel according to the power control module (PCM) calculation. When the engine is cold, not all sensors function. As the engine warms up the oxygen sensors report the lean condition. To make matter worse adding addition fuel.

The (L59) engine is calibrated to use 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.

What Is The Correction:

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

Green Gasket
Green Gasket

Important:

Above all, do not replace the upper intake manifold gaskets with the original orange gasket material, P/N 17113557.

Orange Gasket
Orange Gasket

Checking Intake for Warpage

So, the intake should be checked for excessive warpage that may cause an uneven clamping pressure of the seal. Also, a intake manifold with warpage in excess of 3 mm over a 200 mm area should be replaced.

This measurement is across only two of the intake runner port openings at a time. Consequently, measurements taken across the entire distance of all four intake runner ports will lead to unnecessary manifold replacements.

Always, replace the intake bolts as the seal under the head may leak.

Intake Bolts
Intake Bolts

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Engine Misfire Conclusion

Be aware that other engine mechanical conditions that can also cause misfires is when the engine is burning excessive amounts of engine oil. This causes Carbon to build up on the spark plug center electrode, eventually stopping spark. Finally, other engine mechanical problems are: vacuum leaks, dirty throttle plate and throttle bore.

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