I just got my 98 3.9 5 speed with 164,000 and when I drove it home I noticed it backfired a lot when I let off the gas in gear. I got it home and pulled the plugs and they looked like the original ones and the gap was, .085" twice what it should be, I replaced the plugs with the autolight 3923s that everyone recommends and I also did the wires, distributor cap, rotor, and ignition coil. Also I pulled the throttle body off and cleaned it and all of the sensors. The muffler was rusted in half so I replaced it with a 2.5" single chamber muffler, it sounds good now but I still get an annoying backfire when I let off of the gas, could I have another issue causing this or is that just the nature of the beast with the 3.9 and the muffler I have?

 

Maybe leaky injectors?

 

Could a bad intake manifold gasket cause this? I have read that it is common for them to go bad on the Magnum engines, what are another symptoms of a bad gasket?

 

That would be the plenum gasket. Symptoms include poor fuel economy, running rough, random misfires, and heavy oil consumption.

Backfires are excess fuel not getting burned in the combustion chamber though. Blown plenum gasket would tend to make the truck run lean. Think I would be tempted to check fuel sync (need a pretty good scanner for that.), and monitor fuel pressure. Maybe you have some leaking injectors.

 

It could be the injectors, when I let it sit overnight it losses fuel pressure and I have to cycle the key a couple times before it will start.

 

that's more the check valve in the fuel pump/regulator assembly.

It's probably fuel sync and possibly an O2 sensor. Have to watch your LTFT and STFT's to know for sure..i know the richer mine runs, the worse it gets. Tuner's been working with me to pull fuel away, tuning with a wideband. But mine is anything but the stereotypical setup.