Engine Misfire or Rough Engine Idle on GM 4.8L, 5.3L, 6.0L is becoming, more common and getting harder to diagnose.
Because, GM V8 engines are very prone, to a rough idle and engine 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.
Now, lets get armed with the knowledge of what can cause, a engine misfire or rough engine idle condition. And, a specific testing strategy, so you can easily find out what component, is the one that has failed. So, should the engine misfire, rough idle or (TDC), point to an air leak in the intake manifold gasket. Then, you should following the below directions.
Because, 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.
So, What Are The Symptoms
Some owners may complain about:
- Rough engine idle
- Engine misfire
- MIL illuminated
- A stored DTC P0300.
And, What Is The Cause Of Engine Misfire – Rough Engine Idle Condition
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. So, when the engine is cold, not all sensors function. But, as the engine warms up, the oxygen sensors, report the lean condition. Then, to make matter worse, it starts adding additional fuel.
So, the (L59) engine, is calibrated to use, ethanol fuel (E85). But, 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. Consequently, 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 For The Engine Misfire or Rough Engine Idle
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
Important: Above all, do not replace the upper intake manifold gaskets with the, original orange gasket material, P/N 17113557. Because, you don’t want the engine misfire to return.
Checking Intake Manifold For Warpage
So, excessive intake manifold warpage is a big problem. Because, that may cause, an uneven clamping pressure of the seal. Therefore, look for any warpage, in excess of 3 mm over a 200 mm area. Finally, replace anything over that amount.
This measurement is, across only two of the intake runner port openings at a time. Because, measurements taken across the entire distance of all four intake runner ports, will lead to unnecessary manifold replacements.
Always, replace the intake manifold bolts, as the seal under the head may leak.
So, be aware that other engine mechanical conditions, can also cause a engine misfire. A common one 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 can be, vacuum leaks, dirty throttle plate and throttle bores.
Thank You !