I did this fix on my '95 ZJ last year and documented as I went to hopefully help those out where are facing this themselves. I just now got off my lazy butt and put together this writeup.
Dodge/Jeep Magnum engines all use the 'Barrel' intake manifold that has a bottom cover and a gasket. Over the years the gasket may fail causing the vacuum in the intake to suck oil from the lifter gallery area.
Hate to say it but it is a pretty common problem if you have a 5.2L or a 5.9L Jeep or Dodge truck for that matter. The symptoms are most noticeably is pinging under acceleration (worse when hot), increased oil consumption, poor performance, mileage, and a poor idle. If you think your ZJ may be suffering from a bad plenum gasket according to these symptoms here is a couple checks.
Remove the air bonnet from the throttle body and open the throttle plates. Peer down the throat of the throttle body with a flashlight and look for gummy grimy oil deposits. Also, sniff for a nasty oil smell. Good chance you need the plenum gasket fix done.
You may ask: "Is this a HAVE TO fix?" The answer is YES if you expect your ZJ to live a long life. When you get oil in your air/fuel mixture the, the drastically lowers the octane rating of the mixture by making it more 'unstable'. That is the pinging/knocking when you accelerate. Prolonged detonation/pre-ignition/pinging/knocking, whatever name you call it will damage rod and main bearings. So one day you are driving down the road and your Jeep slings a rod. Not fun, it died of neglect.
Lets get started. If you are going to attempt this you have to have a descent set of tools. Metric and SAE, your Jeep has both.
Get your tools together and pull your Jeep into your work area.
Open the hood and take a good look at what has to come off to get this job done. That's right, everything! The basics first. Disconnect the battery. Remove the fan, fan shroud, radiator and upper and lower hoses and accessory belt. With those out of the way we can eyeball the accessories and see how they mount.
With the big aluminum bracket the accessories bolt to, they can all be removed together and set aside.
Unbolt the dipstick. Be careful, it breaks pretty easy.
You have to wrestle this tube a bit to get it out.
Unbolt the power steering pump.
This little trick makes the job go much quicker. You see what I did was lay a old floor mat over the battery, and used a bungee cord to keep the whole thing from flopping over. You AC hoses will allow you do this, but go easy, they can't be stressed too much.
Now we can see the whole intake. Doesn't look too tough now does it?
Take this time to familiarize yourself with tube routing, the wiring harness, and how things hook up in general. You may want to mark tube or wiring to keep the goofs down to a minimum.
Take a look at this jewel. This hose is not leaking, but it doesn't look so hot. It is worn out and could fail any minute. I did have enough sense to buy a replacement because this is a time consuming tear down. Can't say the same for the water pump. I should have replaced that too even though it didn't have a problem. It was the factory piece, it was 10 years old and I ignored it. Yep, and it started leaking a week after I got everything all back together. Save yourself a lot of heartache and replace the old stuff while you have it apart and be DONE with it.
Time to start removing hoses and wires. BE CAREFUL because the rubber/plastic parts are old and brittle. You may find like I did some parts just break off in your hand. Have some of the correct size ID tubing to replace the rotten stuff. Take your time here. The tube that broke on me has a 1 way valve in it. Keep things in order for reassembly. Take a few pictures if you have to for reference.
You need a special tool here. Don't try to pry it, you will just get frustrated for no reason.
Don't forget to release any pressure in the fuel system at the test fitting on the fuel rail.
Slip the tool into the coupler and press HARD. You might have to move the coupler around a bit before you successfully get it off.
Continued in second post!