Rough Engine Idle and Misfires When Cold on GM 4.8L, 5.3L, 6.0L engines is common, and getting harder to diagnose.
As, GM V8 engines are prone to a rough idle, and engine misfire when cold.
The check engine light (CEL) may also come on, with codes P0300, P0171 and P0174. 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 and rough idle when cold condition. And, a specific testing strategy, so you can easily find out what component 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.
First, 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.
GM, now blames the intake manifold gaskets, and bolt seals as the problem. And, now sells redesigned replacement parts.
So, What Are The Symptoms
Some Owners May Complain About Things Like:
- A Rough Engine Idle
- One or More Engine Misfires
- Check Engine Light (CEL) Comes On
- A Stored Diagnostic Trouble Code (DTC) P0300
And, What Is The Cause
The plastic intake manifold fails to seal, and creates a vacuum leak. GM cites, the intake manifold gaskets as the problem, and now sells a redesigned part.
So, 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.
Also, when the engine is cold, not all sensors function. But, as the engine warms up, the oxygen sensors (O2), 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. And, 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. So, follow the instructions on the manifold information, to determine if replacement needs to happen.
What Is The Correction For Engine Misfire And Rough Idle When Cold
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. As, you don’t want the engine misfire to return.
Checking Intake Manifold For Warpage
Above all, excessive intake manifold warpage is a big problem. As, that may cause an uneven clamping pressure of the seal. So, look for any warpage, in excess of 3 mm over a 200 mm area. And, replace anything over that amount.
This measurement is taken across only two of the intake runner port openings at a time. As, measurements taken across the entire distance of all four intake runner ports, will lead to unnecessary manifold replacements.
Always, replace the intake manifold bolts, and washers.
Conclusion
Recap: So, a truck has a rough idle or misfire, when the engine is cold. But, seems to clear up, as the engine warms. Then, a common cause of failure is the intake manifold gasket, and the intake manifold bolt seals. The manifold gasket and the bolt’s rubber crush washers can fail, and cause a vacuum leak.
Also, 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. And, other engine mechanical problems can be vacuum leaks, dirty throttle plate and throttle bores.
BY DANNY BENDER
