Recommend Eng Upgrades 5.9l Gas
The plenum is just the belly pan gasket of the intake manifold. I don't think it's near the issue on 3rd gen 5.9s as it was on the 2nd gens. IIRC, you all had some advancements in the tuning that kept it from being too much of an issue.
In short, the problem is the fact that the intake manifold on the 5.2 and 5.9 magnum motor is made from aluminum, but Dodge in their all mighty wisdom thought it would be cool to use a steel belly pan. Any of you that have had a lick of chemistry, or basic knowledge of metals can already see this is a bad idea. When you apply heat to steel and aluminum, they both expand, BUT the issue is they expand at different rates. This causes the gasket in between the two to get pushed out.
Most common problems this causes:
1. Lack of power
2. Oil consumption
3. Detonation under acceleration
4. Usually takes the catalytic converter along with it when it fails.
5. Random cylinder misfires.
There are a lot of different ways that people think is best to check to see if this is leaking. IMO, the best way is to just pull the manifold off and have a look see.
Other acceptable methods are to watch your oil consumption, and sometimes you can get away with peering down the throttle body blades to see if you can see oil pooling. If there is just a thin film on the bottom of the manifold, you have a leak.
Like I said in a post earlier, you basically have two options for a fix. You can pull the manifold and replace the steel belly pan with an aluminum one, or you can install either a Summit or MOPAR Performance M1 intake that eliminates the belly pan and it's gasket all together.
