I have a 99 Ram 4x4 1500,Auto and a 360.
Some back ground here...

About a month ago the truck shut down totally on my way home from filling the tank. I managed to have enough speed to pull into a shopping center to get it off the road, and call AAA.
While waiting I checked the FP fuse and swapped the fp relay with the ASD relay because I could not hear the FP running, with the key on. I was convinced then the FP was gone after 133K miles. Managed to get the truck onto my lift and replaced the FP ( arggg had to drain a full tank).
Anyway the truck ran fine after the pump replacement. About a week or so later, my wife took the truck out (about a 60 mile round trip) and when I went to load up for a firewood run, I noticed the MIL was on. This was not real unusal as I would have this engine check lite on from time to time and it always seems after my wife drove the truck. I think it had to do with the OD being engaged in stop and go low speed driving as I disengaged OD unless on the highway. Usually the MIL light would go out after I drove the truck a bit.
SO on my trip to cut wood, going up a long grade, the truck started to miss and buck and run rough. It was so bad I turned around and had to baby it back home (20 miles) and back into the garage.
Starting with the basics, I pulled plugs.. Cyl 1,8,2 had real issues. Cyl 1 had fused enough the contacts were directly shorted. Cyl 2 had the copper electrode gone and cyl 8 had the same condition with a good coat carbon looking foulling. All other cylinders were perfect and the plugs (Champion stockers) looked new. ( I had replaced plugs very recently, before this problem).
Note the 2,1,8 cyls are in that firing order. I suspect the long drive home really exasterbated the condition and blew these plugs.
Next Phase I replaced Wires, Cap, rotar, new Dist and CPS. (wire routing is correct).
Verfied CPS set to No 1 TDC on the dist install. (I marked the CPS with blaze orange on No1 cyl to make things easier to see down these..)
RESULT:Engine still runs rough and on a test drive and watching real data the MIL light would flash, truck down on power and I cold feel the miss . Codes were P0300,P0301, and P0308.

I next pulled the PCV hose from the intake and blocked off the hose and port. Pulled the Hose from the Hat and ran the RPMS up a little. I expected there should be no VAC on the hose to the hat, but I could feel VAC. I think this was the correct to test for a Plenum leak
and I am ordering a Hughs kit and bolts today. Was this the correct proceedure?
Given the fouling I saw on #8, this makes sense to me, but wondering what you guys thought. I see no oil in the Plenum but can not see the whole thing, or even into the back.
I am refreshing the CKS sensor soon with a Mopar unit and I went ahead and replaced the upstream O2 as well. ( no issue with the old sensor noted and it looked clean, but I decided to do some preventmaint since the truck has a lot of miles on the old stuff.
I did notice after the New sensor, the down stream stated to throw a high voltage code
out of the blue, but I am not to worried with that code at the moment since upstream has input into A/F rules.
Sorry for the long post....

 

It is hard to take a guess. Initially I would think valve seals if the plenum is clean expecially because only 3 cylinders have bad plugs and the rest are like new. The oil is comming from somewhere. If it were the plenum, I would expect them all to have a light film or discoloration. Here is a pic of mine when I did my plenum. These plugs were 2 years old.



That said... since I re-read your post and saw that even after a full tune-up the truck still ran rough (assuming the wires are in the correct spot), this makes me wonder if you are a victim of "cracked heads". Even if your problem was the plenum, with a new set of plugs you should have been good to go until they re-fouled again. One other thing you can try is to swap those problem cylinder injectors so rule those out as well. The two cyl that were melted may be running lean? (injectors suggested by dodgeboy7) Wish ya luck and keep us posted.

 

I managed to finish the Plenum install and found the gasket was blown out by cyl 6 and 8 by about 2 inches and also breached around the bolt holes near cyl 1.
It took a while to do the project since I could only work a little at a time , plus I am kinda **** about labeling everything and bagging parts with bolt sizes and torqe specs so I don't have to keep gping back to the books. Cleaning took most of the time.
Anyway, I fired the truck up yesterday and it runs with out a noticable miss and very smooth at idle. In gear it seems to run just a tad rough at times, but good on power and a world apart from what it had been doing. Running with a live scanner hooked up produced no codes or error indications. I need to take several trips to really get a feel for whats going on, plus a few heat cycles to let things settle down.

Only thing I have not done yet is replace the CPS which I have a New MOPAR one already.
What is the best way to access the sensor? Top side, bottom? I have a lift and its not visibale from under the truck but can see the connector.

I don't think the old one is bad but Is the last component to swap out for pease of mind. Besides, a code is not always thrown if the sensor is defective.

 

Only thing I have not done yet is replace the CPS which I have a New MOPAR one already.
What is the best way to access the sensor? Top side, bottom? I have a lift and its not visibale from under the truck but can see the connector

Just did this last week with one from Advance on a friends truck and it started right up. We got it from the top side by following the tranny dipstick tube. 1/2" socket, 6" ext and a snapon bottleneck rachet. 15MIN was removed. Oh and the connector/pigtail run's behind the dizzy.

Got pics of the plenum repair/intake engine?

 

Man, I gotta say it- you're symptom also sounds like a clogging catalytic converter.. If your plenum gasket has been leaking a while its not far out of the equation anyway.. I've seen it myself, and heard countless times of chunks shifting around in there and sometimes providing enough block to choke up the engine.. but sometimes be in a position to allow the engine breath when it's idling or not under load..

I'd be tappin' that rascal with a mallet if it were mine, to listen for chunks clattering about..

Good luck man!

 

+1 on the clogged cat, unscrew the precat o2 temporarily and see if anything changes.

 

Thanks Tweeker909 and guys,

I should have had the insight to nail the CPS when I had the Keg off. (Note to self)...
I didn't take any pics of the OLD plenum with the damage but it also did not have lats of oil on the inside of the plate. I was also surprised of the lack of any gunk or build up in the valley. It looked pretty clean.

I will pull the front O2 and see if I detect any changes as well. Check for any noise in the Cat. I put on a new muffler yesterday, since the old one had seen better days, and did not note any lack of exhaust flow after the replacement. Maybe as the cat heats up, it could cause an issue.

 

Update.

I replaced the CPS and still get a misfire randonm on CYL1 and 8. MIL lamp does light after driving the truck a bit and just doesn't like iy when its in overdrive. If I accelerate, things smooth out and the MIL will flash. I also see a HIGH VOLTAGE code on the aft O2 sensor. Real time scanner data shows it spike over 1 volt occasionally.
I picked up a Magnflow cat to install and when I cut out the OLD stocker out, it was totally OPEN with no honeycomb material in the CAT. Straight through flow end to end.
I will weld in the NEW cat this weekend and see what gives.
With all the parts I've changed, I think it may need a timing resync from Dodge next, but I will see after I finish the cat install.