I have a 2003 dodge Dakota with a 3.9 v6, it will foul the plugs in cylinders 3 and 5, I'll change the plug's and wire's and it will run like a bear, then about 60 day's or so I'll notice it does not have as much power and the gas mileage drops but it will go like this for a long time almost 30k, then out of the blue it will start spuddering and missing and no power, I change the plugs and wires and it run's like a bear, I want it to run like a bear, what do I do?

 

If you haven't already, cap + rotor. Prongs might be worn on those cylinders and not providing enough spark. Also, have you ever changed the crank position sensor? It (along with the cam sensor) control the timing. Crank sensor tells the PCM the engine speed to control spark, cam sensor tells the PCM where the distributor is to control fuel sync. When the crank sensor goes bad, it causes all sorts of weirdness from bad idle, fake misfire codes, sputtering, and even stalling. It mostly affects "bank 1" or the driver's side, cylinders 1, 3, and 5. Personal experience with a 3rd party crank sensor, my truck randomly threw fake 1, 3, 5 misfire codes when I drove over 65mph, sometimes idle was a bit rough, and couple times it flat out stalled. It or the cam sensor (plastic plate under the dist cap with a magnetic hall sensor) could be throwing off the spark or fuel sync. This might cause it to not burn correctly and allow carbon to deposit on the plugs, fouling them. You didn't say how the plugs were fouled, carbon or oil.

Cheapest test route (do first): Cap, Rotor, coil. These will resolve any "weak" spark conditions that'll cause fouling. Make sure you use a good 87 octane, if you're using a "cold" plug, like an autolite 3923, they might not be burning hot enough to self clean. Move up to the proper heat range plug, Autolite 5224 or better yet (what I run) NGK FR5-1 V-Power, it should be hot enough to self-clean and prevent fouling. Too high of an octane can do the same thing, if you don't ping/knock on 87, use that. ALWAYS check your gap, do not trust the pre-gap. Use feeler gauges and not the little cheap 'disc' gauge. If you cannot get feeler gauges and must use the disc, buy a new one (they're like, 97 cents) and center the line in the middle of the electrode, not the edge. Gap on the proper heat range should be .040.

If the above doesn't help: Crank/Cam sensors.
Crank sensor is found on the passenger side back of the block, in the bellhousing of the transmission. Take the passenger front wheel off, remove the fender liner, you'll see it behind the exhaust manifold. It should be 2 10mm bolts or you can use a hex key, personally I found it easiest with a 1/4" drive ratchet with 10mm socket. The plug is near the back of the distributor. ONLY USE A MOPAR SENSOR HERE. 3rd party ones do not read fast enough and cause issues. I resolved my issue with a junkyard one, told them it was a temp sensor, paid $4 for it. Make sure you do not push the rubber grommet in when re-inserting, else it will push the sensor into the flexplate and destroy the end.
Cam sensor is found in the distributor. Pull the distributor cap off, pull the rotor out, it's the plastic plate right under the rotor, extremely easy to remove. This particular sensor, I'm using a Standard brand sensor and it's done just fine, mopar is of course suggested, plug for it is in the same general region as the crank sensor.

Last ditch: Oil fouling might be happening, either your plenum gasket is blown, allowing it to draw oil in through the plenum into those cylinders... Or those two cylinder piston rings are failing, allowing oil blowby. I don't really see these being an issue, as they'd have a bit more tell-tell signs. You can always rent a cylinder pressure tester to check your rings. You'd also likely have blue-smoke out of the exhaust with any significant oil burning. As far as plenum gasket, take the air-hat off, open (or remove) the throttle body, and look down inside for pooling oil. Bad plenum can cause pinging, low mpg, loss of power, etc.

 

Skreelink,
Thank you, lots of good info you shared, I am not a pro and I am not sure what is making the plug's foul, I do know that truck does not burn any oil or leak any oil, and I do change the cap and rotor every time, they are too cheap not too, I do use bosch platinum plugs, I do not go for the most expensive wires but I do not buy the cheap stuff, I'll pay for good wires, I am not racing so I am not getting crazy with that.I have had speed sensors changed, I do not get any check engine lights, I did with the speed sensor, so I guess I'll try the crank positioning sensor. Thank you again.

 

If you post pictures of the fouled plugs, the users here can inform you on how they're fouling. You can simply upload them to a site like imgur and post the link. You mentioning the bosch platinums makes me wonder if that's the problem. From everything I've studied, our trucks do NOT like bosch products, especially sensors. Also given they're platinums, might also be an issue. The reason platinum plugs last longer, is the platinum has a higher resistance than a standard copper core plug, which equates to less wear. However, this higher resistance, also makes the spark weaker, which would cause an incomplete burn, and cause a carbon buildup on the plug. Then it also wouldn't get hot enough to self-clean the carbon off.

That's why I also suggested a new coil, it might be degrading enough not to provide enough for the platinum plugs. Still a bit odd to mostly be those two cylinders. Good carriers like wires, cap, rotor, etc does no good if the push is weak, and coils are generally inexpensive unless you go with high output like MSD etc. The LA based magnum engines (3.9, 5.2, 5.9) run their best on a standard copper core plug. With how cheap they are, go ahead and give the Autolite copper core plugs a try, or a step up and grab the NGKs.

 

I am a little quick at throwing thing's like old plug's out, the truck does have 170k on it I am pretty sure coils are original, but what bothers me is that those 2 cylinders are the only bad ones, but crazier things have happened so I guess that wouldn't hurt to try, I have only put bosch in twice, I do not know what was in it before, I figure they last about 30k but when I pulled them out the other 4 looked like new, if I can I will pull the plug and see how they are, I changed them about 1500 miles ago when I did my tires would turn over just hitting the gas on take off, now it's the old slow mule.lol

 

Skreelink,
It's has been a little bit since you helped me, thank you, curious I see these E 3 spark plugs advertised and noticed they recommend there coils and wires, just wondering if the coil are a higher volts for more kick, what do you think, also what do you think of these throttle body spacers? and lastly 1.7 roller rockers. Thank you again.

 

Throttle body spacer does nothing on a port fuel injected engine. Some folks have gotten WORSE gas mileage with 'em installed. Save your money and spend on something that actually makes a difference.

Have you check the plenum gasket? Pull the air filter housing, crack the throttles wide open, (engine NOT running) and have a look, any oil pooling down in there?

Just standard copper plugs please. (manufacturer of your choice.) They work perfectly fine, and you don't get any better performance out of the fancy plugs. (and definitely skip the rare-earth fellers, platinum, and Iridium, our trucks just don't like 'em.)

Of course they are going to recommend their coils and wires. They want your money. Stock coil is perfectly fine for most applications. Just get GOOD quality wires, 8mm or better.

 

Do not fall for the hype of the E3 stuff, and as HeyYou stated, they're going to hype up their own product to make a sell. Also do check for pooling oil in the intake for plausible intake gasket. I'm pretty much repeating everything HeyYou put, it's good advice. I never really saw a point of the throttle body spacer, with the keg-style intake the magnum engines have, there's no benefit to any sort of air manipulation except something that feeds it more air, which the factory air hat is pretty free-breathing to begin with (and technically cold air, as it pulls from the fender instead of the engine bay). The air comes into the intake, then has to about-face up into the runners anyway, it would be more beneficial to remove the intake and cut the runners down to let them free-flow better. I've read good things about the 1.7 rockers, but it's also best with other supporting mods, such as the kegger mod to allow it to breathe better, and better flowing exhaust.

As far as plugs, good copper core, just not Champions. I know they're OEM, but me and everyone I know have never had luck with 'em. Co-worker has a 1999 Ram with a 5.2, he noticed rough running a bit after a tune-up, and with under 5k, his champion plugs had heat blistering and cracking on the insulators. He swapped to NGK V-Powers and has not had an issue since. Autolite, NGK, maybe single electrode Bosch (do not get the 2-4 prong things, they do no better) would be good options, my vote is with NGK. I'm no professional, or even a real mechanic, I just learn enough to keep it running right and dependably.

 

Probably not your issue ,, but.
i once had a Cherokee that two of the injectors had a corrupt spay pattern, they weren't fully clogged, but the corrupt pattern caused the fuel to burn inefficiently in those cylinders in comparison to the others.
incomplete fuel burning affected the temperature of the cylinder and resulted in a lot of sludge and carbon buildup and plug fouling in the affected cylinders.