I have a '98 Dodge Ram 5.9. My check engine light comes on. The code P0305. I have replaced cap and rotor, wires, spark plugs and #5 fuel injector. Also replaced o2 sensor. Still misfires. I noticed a light coating of oil in the intake, could the plenium gasket cause the single cylinder misfire? Any suggestions?

 

There's a test for the plenum gasket failure in the TSB. If that test says your gasket is good, it's good.

Have you routed your plug wires in accordance with the TSB?

Some small amount of oil on the intake belly pan is perfectly normal and comes via the PCV valve. It's an often overlooked tune-up part that can cause all manner of grief when it fails, but I wouldn't expect a single cylinder misfire to come from it even though it is pretty darn close to the #5 intake runner.

So, if you haven't yet, reroute those plug wires and get back to us.

 

If those things don't fix it, do a compression test and figure out if you need to find lots of free time :/

 

man- talk about making something simple look like alchemy.. that TSB for the routing came straight out of a boy scout manual..

".... after the bimini twist, and the double prusik with overhand lash, and whilst carefully balancing 'yern beer atop the radiator cap- attempt anchoring the running end with a bowline as required (you know, the rabbit went around the tree and back down the hole).. Now put the tip of your tongue on the third molar and balance 'yern self with 'yern right hand pinky finger on the power steering reservoirs cap, and half hitch the coil wire to the third blade of the clutch fan leaving enough loop to make it some distance without having to get out and unwind it...."

holy Moses... what happened to sensibility and nice flashy little routing harnesses? Do 'yall get to figurin' sometimes we make things way to difficult too?

I'm figurin' the OP is having some crossin' too.. Instead of routing that will make a boy scout cry though, I'm thinkin' some of that there heat shield flex tube will solve a lot.. just sayin'... 0304 issues on my rig were solved by such..

 

Ignition systems used to generate about half the voltage our trucks make, which is about half what the newer vehicles make. That's what happened to nice flashy little routing harnesses. Just so ya know.

Way back in the day I had slickass chrome looms sticking up off of my valve cover bolts... they matched the winged valve cover hold-downs that were handy if you had to adjust your rockers atop solid lifters all the time. These days those chrome looms would be a no-no.

That there heat shield flex tube is known as convolute, just so ya know.

It's always been considered best practice to avoid long parallel runs in high voltage ignition wiring, but it didn't used to be so critical as it's gotten to be. Way back when, we'd buy whatever plug wires were cheap, drape 'em around or route 'em up nice, and either way they worked just fine. Those days are gone unless you're running a concours classic with the wires routed as they came from the factory, with long parallel runs that looked purty and didn't hurt much if at all.

For my money, though, if you're going to run cheap wires, route them in accordance with the TSB. Rerouting takes 20 minutes if you're slow, and brings plenty of peace of mind.

 

Unreg, are you trying to chop down my shade tree? :-)

 

Oh hell no! Toss another lawn chair under there for me. I figure I might talk you out of a second drink if I take long enough rerouting your plug wires.

 

A plenum leak right next to the intake port for the #5 cylinder could be a likely culprit as well. Extra unmetered air + engine oil is a recipe for hot cylinder and excess carbon buildup.

 

i know i should do a compression test on all cylinders but i lacked the time so i just did the 1 cylinder that was misfiring out of curiousity quick cause it was buggin me. not even sure i did it correctly but here goes... i removed the coil wire so it wouldnt fire, put the tester on the five cyl. and cranked the key for about 2-3 sec. , the guage pulsed on every stroke, the first pulse was about 100 psi and 2nd jumped to 145 psi. the tester i used had a valve in it so it wouldn't bleed out... i removed the valve and did it again and was consistently at 120 psi. is this okay? should i do it differently? or was that okay? i just cant figure out why the po305 misfire. frustrating. i was completely ready to tear the heads off this thing but now not sure if i need to... any input greatly appreciated thanks!!!!!!!!

 

The first attempt used the correct method, the second was incorrect. Compression is fine on that cylinder.

Have you rerouted your spark plug wires IAW the TSB?