That's true about the different metals expanding/contracting differently, but another factor contributing to the plenum leak is the plenum bolts themselves. The general consensus is that they are just a hair too long, so they bottom out in the boltholes and don't seal the plate tightly enough, most likely amplifying the effect of the aluminum/steel combo.

There are a couple of different options for these motors to eliminate the stock plenum altogether, but they are somewhat pricey at ~$500 or more.... a Hughes Airgap, and a Mopar M1 intake. Both of these are complete replacement intakes. A few guys here have one or the other, so I'm sure someone will chime in more info about them

 

I have the mopar M1 intake on my truck and can offer a little info on it for you. I only have one problem with this is that I never ran this intake on a stock motor. I think without certain modifications you will loose some bottom end power, not much but some. I have found that from 1900rpm and up is way better than stock. With this intake on the motor it makes changing the cap, rotor, wires sooo much easier. You can get it at any dealer, a very well made piece, It was built by mopar for our motors. All in all I have 0 complaints about this intake.


Mopar part numbers

P5007398AB Aluminum Single Plane Intake � 2-bbl MPI w/EGR (1992-97 5.2L/5.9L Magnum/Jeep� Engines) $ 639.95

P5007638 Intake Manifold Installation Kit (5.2L/5.9L Truck Magnum/Jeep� Engines) $ 69.95

P5007852 Aluminum Single Plane Intake � 2-bbl MPI w/o EGR (1998-03 5.2L/5.9L Magnum/Jeep� Engines) $ 499.95



* Note: P5007398AB & P5007852 allows use of AC and all factory accessories. Replacement for stock. Each may require modifications in some applications.

 

Sounds like several mechanics and dealer mechanics should find a new line of work because they are no good at their current one.
The dealer mechanics should know about the catalytic converters on these trucks.
The guy at Hughes Engines told me that the bolts bottoming out in the intake manifold is the reason for the plenum gasket failures. You can get the end of the bolt at the proper torque but the head of the bolt (where it matters) won't be.Airgap is the way to go. I don't have a plenum gasket any more so no worries about it blowing out.

 

Replaced whole exhaust front to back, what a difference. Just need to replace the plenum and hopefully this will fix the engine from running rough.
Bill

 

Don't wait too long, the new cat is slowly getting ruined again.