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I bought this 1999 dodge sport with a 5.9 about a month ago it ran prefect the first 2 weeks i want to get supplys for a project and it started raining that day water got up to knee high it sits on 32 that sticks out a bit and on the way home that night it started to back fire through the throdle body and the exhaust and ounce you start it it runs fine for a few minutes then itll start to backfire but if you barley let your foot off the gas itll clear up for a few minutes then after awhile itll start backfire again but i found out if you shut the truck off and restart it itll be ok untill it starts again ive replaced the plugs wires distributor cap coil coil wire and just most of the sensors on it and it dont have a check engine and dosnt throw any codes if anyone has a idea on what maybe wrong please help cause im at my wits end with it thank and sorry if this seems off its my first time posting here thanks again
 

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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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Drifter99
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I really like maxwellp's explanation but also check the catalytic converter is not clogged. My 96 2500 5.9 from years ago had similar symptoms. Because the cat was clogged it also ruined my EGR system and melted the EGR Transducer. I ended up replacing the cat and the EGR/Transducer assembly and eveything cleared and ran well.
Also, be aware that the catalytic assemblies aint cheap. CARB compliant units are upwards of 4-500 bucks. Fed compliant units are around $300. Best wishes to you and this issue and please post up the resolution when possible.
 
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