*all credit goes to Merc225hp*
“Plenum Gasket, what is it, where is it and what should I do about it” applies to 3.9, 5.2, 5.9 magnum motors.
When Dodge made the intake for these motors they made it in two pieces, cast aluminum for the top and a stamped steel plate for the bottom, with of course a gasket between the two. Over time this gasket fails for whatever reason (there are many opinions on this as to why it fails this is not a thread to discuss that).
Signs that the gasket has failed/failing, oil consumption goes up, pinging from the motor, possible blue smoke from the tail pipe, a miss in the motor (most times cylinder 8), rough idle, fouled spark plugs, general lack of performance.
Is there a test to see if it’s leaking? Not a real good one: Just opening the TB and looking down in the intake to see if oil is in there is not enough. The PCV system puts oil in the intake no way around that fact so that test is out. The idea of looking for oil pooling inside the intake at the back to me is also not a good test as there are vortexes inside the intake that will pick up the PCV oil and drop it elsewhere inside the intake.
What a leaking Plenum gasket can do to your motor: If left for too long, you will foul spark plugs, foul the O2 sensors, plug the catalytic converter (if the cat plugs and you run it that way you up the chance of cracking the already weak heads).
Can you test to see if it has been fixed before? Yes and no you can use a telescopic magnet put down the TB and see if it has a steel plate or an aftermarket aluminum plate. If it does not stick to the bottom it has been changed to the aluminum plate so you might be okay. If it does stick to the plate just start getting ready to do the job.
*notes from DD94: there was a fella somewhere that attempted this and his magnet was filled up with sludge, so depending on how nasty your intake is, this may or may not work. You may have to Seafoam it before you do this*
Fixes for this, from Dodge and other wise:
Dodge did a couple of TSB’s to try and deal with this issue, one is the routing of the spark plug wires (TSB 18-48-98), next is the Death Flash (TSB 18-34-97). Both of these TSB’s are bogus cover ups for the real problem. And TSB 09-05-00 where the job is done in full.
Aftermarket fixes: There are aluminum replacement plate’s out there, one on eBay and the other is Hughes engines, I believe both are full kits. This is the route many take to solve this problem. The other two options are replacement intakes one is the Mopar M1, two is the Hughes F1, and neither are cheap and both should be used with other supporting mods.
*notes from DD94: the eBay is just an aluminum plate with the plenum bolts, Hughes offers both the pan with bolts, as well as a full kit. Going the eBay route and then sourcing your supporting parts else where has been proven to be cheaper in most cases...the Mopar M1 is also unfortunately no longer in production, making them harder to find and they're climbing in price*
Can I fix this myself?: I have read and helped many a greenhorn do this job, some take longer than others but most can do it in a day. The biggest mistake I see is the use of wrong torque setting for the intake bolts, the torque setting is in inch lbs not foot lbs, so make sure you have a torque wrench that reads in inch lbs. The other obstacle is broken intake bolts, it happens, no doubt about it, and most can be removed with a pair of vise grips, however, some may end up requiring the replacement of the heads.
A good shop should be able to do this work in about 4-6hrs, some shops will know nothing of this problem so expect to hear "what are you talking about?"
All part numbers listed are Mopar parts readily available from your Dodge dealer. The P denotes performance parts, in some cases these parts are cheaper than stock parts, the intake bolts are the exact same as the stock part just cheaper.
53030541 Throttle Body Mounting Gasket
05017208AA Gasket, Intake Manifold Plenum Pan
P4876049 Intake Manifold Gasket Set $33.25
P4876772 Intake Manifold Attaching Bolts $7.75 set.
P5249862 Roller Lifters $117.50
P5007709 Timing chain tensioner part $30.00
P5249267 Double roller chain and sprocket set $58.00
Oil psi sending unit
Fel Pro 5.2/5.9 Intake Manifold Gaskets with belly pan gasket MS95392-1
*notes from DD94: You're never going to get a better chance to easily replace the cap and rotor, so do it while you've got the intake off.*
I hope this helps someone.
Big thanks to Merc for taking the time to ponder and write all this up for everyone!