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A common problem with the 3.9, 5.2, and 5.9 chrysler/dodge magnum engines is the lower plenum gasket going bad. this gasket seals between the lower pan and the intake manifold. when the gasket goes bad, it allows oil from the lifter galley to be sucked up into the intake and burned. I’ve heard a few different reasons for why this happens. The 2 most common i hear is A: the bolts holding the lower pan are too long and bottom out before they can put sufficient pressure on the gasket and B: the gasket slowly gets torn apart due to different expanding rates between the pan and the intake (pan is steel, intake is aluminum). this problem can also lead to clogged cat, cracking heads, and fouling o2 sensors. There are kits that can be bought that use all new bolts, a new gasket, and a thicker aluminum pan.
Symptoms of this failure include: rough idle, lack of power, oil consumtion, heavy spark knock under load, random/multiple misfires (commonly seen on cylinder 1,3, and 8 ), poor mpg.
This problem can sometimes be tricky to diagnose if youre not familiar with this problem because it can act like other problems. On a scan tool it may look like an average vacuum leak. The easiest way to diagnose this intake leak is by looking into the intake through the throttle plates with a flashlight. if there is pooling oil in the corners of the intake, the gaskets are more than likely bad. If there is just a little bit of oil residue in the intake, its most likely the pcv valve, not the plenum. you can also check spark plugs (particularly #8 ) for excessive carbon build up.
Repairing the lower gasket is not a very hard job to do. it can be done with a basic set of hand tools and a torque wrench. If you dont own a torque wrench, you may have to rent or borrow a torque wrench to properly torque the intake bolts. The main thing to remember is to NOT reuse the plenum pan bolts. They are torque-to-yield bolts and will snap if you try to reuse them making the repair a waist of time and money. getting a chilton or haynes manual to help guide you and give you torque specs will make the job way easier.
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NOTE- For some reason Dodge used both metric and SAE sizes on this truck you’ll need a full set of sockets and open end/box wrenches to do the job. Be careful of that stud holding the fuel rail, driver side closest to the firewall I dropped mine and had to go to the junk yard as it is a discontinued part.
Dodge 3.9, 5.2, 5.9 Barrel Intake Manifold plenum issue and TSB 09-07-97
If you have had any of these issues
Symptoms of a blown plenum:
1. Excessive oil consumption
2. Heavy spark knock under load (sounds like a can of marbles when you have your foot in it)
3. Truck feels like it was castrated (loss of power)
4. Trouble getting up hills
http://dodgeforum.com/forum/2nd-gen-ram-tech/286307-end-all-be-all-plenum-thread.html (I don’t know about all the links on that page but the site is generally good.)
You probably need to do the Plenum. It explains there why it goes bad and I used the Hughes Engines kit since it eliminates the dissimilar metal problem and they give free tech help on the phone to customers. You get new plenum bolts and all the gaskets with the kit and new intake bolts are about $26 bucks. They link these instructions http://www.dodgeownerforums.com/smf/index.php?topic=9504.0 which leaves a LOT unsaid. I guess they assume you are able to figure things out if you’re able to tackle a Plenum gasket. Make sure you get an Inch Pound Torque Wrench for the plenum a foot lb one will snap your new bolts, (I know), and a foot Lb one for the final torqueing of the intake bolts to the block. The torque settings are pretty low you may be tempted to go harder, don’t. Expansion and contraction of aluminum may cause the tighter bolts to snap after you’ve put them in and started driving. Some auto parts stores loan tools for a deposit. I used Autozone for the Inch Pound torque wrench.
Some things stand out to me as I write You should remove the whole fan if you have a 3.9L (1 7/16” nut) as the shroud won’t come out over the fan clutch, there is a 7/8” slip nut on the back of the manifold under the EGR that has to be removed (you have to remove the oil sensor to d it and that requires a special one purpose socket) That nut should be the first thing you screw back into the manifold (not fully tight yet) as it is hard to start if you can’t move the manifold around. I had to remove the TB to get the TV cable bracket off, they are connected and I was cleaning the TB otherwise you can leave them together after taking off the accelerator cable.{EDIT; Sorry, the last statement is incorrect, the TB must come off to remove the bracket so you can get the intake manifold off the motor.) Use some masking tape and clearly write down the correct names of the things you unplug and from where.If you have had loud lifter/valve train like but not really lifter/valve train noises coming out of the motor especially under load you probably need to do the TSB 09-07-97 which is adding a timing chain tensioner. I bought a 32mm socket for the crank bolt because I read online that was the size, it felt loose even though it worked and I later found that some people use 1 ¼”. I did not check to see if that worked. Loosen it before you remove the belt. With a 3ft breaker bar (autozone loaner) I smacked it twice (counterclockwise) and it came right off the motor didn’t even move very much. The idea is to hit it hard enough so the nut moves first. I also used an autozone loaner steering wheel puller to get the balancer off. A regular gear puller that hooks on the outer rim will destroy it you’ll see why when you get it off. Screw the crank bolt sans washer back in and with the correct puller screw the pulling bolts into the crank pulley bolt holes.
You have to do the whole timing chain and there are many parts in the area under the Alt and AC which you have to remove, that should be replaced since you will probably never get back in there again unless you leave what’s in and it breaks. At least change the thermostat, bypass hose and the top radiator hose. I changed the two temp sensors since my truck is 20 years old.
That grenade pin looking deal on the tensioner is meant to hold the tensioner back so you can more easily install the chain. I’ve read where some people were able to install it just fine after removing it. Whatever you do don’t leave it in after the chain is on. Its simple as heck since it bolts right on in place of the piece of metal you remove for it. Actually installing the tensioner and chain is the easiest part of the whole job.I haven’t been able to get the engine flushed or the oil changed yet so I haven’t done a long test drive to let the PCM get to know its new parts but the short one I did do showed so much improvement it made the job very much worth it.
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