Lean conditions can be either caused by too much air in the mix (vacuum leak) or not enough fuel (fuel delivery). Next step is to determine which is the case. If the system goes lean at high fuel demand then the problem is with fuel delivery. Easiest way to determine a problem there is to drive the truck at road speed and accelerate hard. Notice if the truck loses power and record O2 activity. In this case the truck had great acceleration and O2 for both banks showed plenty of fuel in the mix under high demand.
The other side of the coin would be a vacuum leak. A vacuum leak would drive the mix lean at low rpm, low demand and at the present time, with the warm engine. I wasn't seeing a problem there either.
The thing is though, I happen to know these engines have a common problem with intake manifold gaskets allowing vacuum leaks when the engine is cold and sealing once the engine is warm. I could let the truck sit overnight and look for vacuum leaks on the cold engine. Usually you can monitor short term fuel trim while spraying carb cleaner at the manifold base and if the spray gets pulled in the fuel trim will drop dramatically.
There is another option though. Freeze frame data is stored when a code sets and captures data that was present at the time. So if we look at freeze frame and see a cold engine, low load and low rpm that means vacuum leak. It would also indicate the vacuum leak was the intake gasket problem.
That is enough evidence to warrant removing the intake manifold. Definitely bad intake gaskets.
Hopefully this will also help with the poor running complaint. At any rate, the vacuum leak is fixed.
Thanks!
Kenny@ggauto.repair
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