After a few years, modern GM V8 engines are very prone to a rough idle and misfire when cold. The check engine light may also come on, with codesP0300, P0171 and PO174. Strangely, as the engine warms up, the roughness tends to go away, which seems confusing.
Diagnostic trouble codes (DTC) P0171 and P0174 indicate the fuel air mixture is too lean
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’s calculation. When the engine is cold, not all sensors function. As the engine warms up the oxygen sensors report the lean condition and additional fuel is added to compensate. This is known as fuel trim. When fuel trim approaches 25%, the check engine light is turned on.
A very common cause of a leaking intake manifold is the gaskets used and the type of fasteners employed. 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 The original red/orange gasket has been replaced by a teal green colored material.
Replacing the gaskets will involve removing the throttle body and intake manifold. Once the manifold is removed the cylinder head ports and lifter valley cover are visible.
These areas need to be thoroughly but very carefully cleaned. The cylinder heads are aluminum and can easily be damaged by scraping or buffing with abrasive wheels. Great care should also be taken to avoid anything falling into the open intake ports.
The 4.8L, 5.3L and 6.0L engine also had considerable problems with moisture leaking into the knock sensors. GM advises inspecting the sensors and replacing any that show signs of corrosion or moisture damage. A further precaution is to build a dam with silicone sealant to prevent moisture from running into the knock sensor (2) areas.
The intake should also be checked for flatness with a straight edge. If the manifold is warped, it will have to be replaced to correct the problem. The fasteners that hold the manifold in place are also a bit problematic. The bolt tightens against a metal sleeve and only the rubber cushion on top holds the manifold. In our experience these bolts should NEVER be reused. The cushions compress over time and will not hold the manifold properly.
Many times, we have had to re-repair these vehicles because a previous repair shop failed to replace these fasteners. The ten (10) bolts can be obtained from a GM dealer. Don’t be surprised if they do not have them in stock as we find many dealers do not replace them.
Torque should be applied to intake manifold bolts in two steps and a specific sequence. First tighten all bolts to 44 inch-pounds in the order listed above. Then repeat the sequence with 89 inch-pounds of torque in the prescribed sequence. Reassemble the engine and clear the diagnostic trouble codes. If everything is properly assembled, the engine will idle better than it has in a long time. If you prefer, AGCO can perform this work for you. We keep all parts in stock and can have you in and out in a few hours.