Hey guys,

I own a 2002 Ram 1500 rwd, 5.9L V8. I went to a transmission shop today to get it checked out, and on the test drive we noticed error code P0306 come up, meaning cylinder #6 is misfiring.

Based on some quick google searching, it sounds like a common symptom for this is a bad spark plug. The spark plugs and cables/wires have all been replaced recently-ish (March 2023), and they may have around 20k miles on them (NGK nickel plugs).
I've also seen people say it could be a bad fuel injector, O2 sensor, bent valve, or bad ECM, but I'm really hoping it's none of those. One individual said it could be from incorrect plug wiring, but I'm pretty confident they are correct.

Some symptoms I've been having:
1. Engine can be pretty loud and little sluggish when going from stop to around 30mph. After that the sounds goes away. I experience this probably every 4th time I start the truck in the morning (25% of the time). I did find out today that the truck skips first gear and goes directly into 2nd, but the loud sound is not consistent with this so it makes me think it's something else.
2. There is some shaking/shuttering I experience at all speeds. Could just be my worn out 46RE tranny. We did find out today that the governor pressure is well below what it needs to be (at 50 mph, the desired pressure is 126.8 psi and I was only getting 41.0 psi).
3. Rough idle. I've replaced the MAP sensor, cleaned the throttle body, and cleaned the IAC valve. I checked out the TPS as well, but didn't look to be in bad shape.
4. If I leave the truck parked for a few days, cranking usually takes much longer the next time I start it. Just when you think the engine won't turn over it will at the last second.

Hopefully that information is helpful. I'd appreciate some expert advice/opinions on what you think the problem likely is. Thanks!

 

The #6 spark plug may have cracked during the installation. I'd replace it, along with checking the #6 spark plug wire (ohm test) to ensure it's properly conducting current. You might also want to check the condition of the distributor cap and rotor, too.

 

How is your plenum gasket?

 

I would star by doing a compression test, Dry and then wet. Also if you can do a leak down test. You could also switch the spark plug to cylinder number 1 and move the ignition wire to cylinder number 4. Does the misfire follow one of the those cylinders or stays at number 6?

 

Update. I swapped out the cylinder #6 coil with a spare from a set I inherited. The misfire continued, so I know it's not the coil. I then swapped the spark plugs on #6 and #4, and the misfire still resided on cylinder 6...so it's not the plug. Spark plugs overall show the same wear and tear, but the white powdery look and slightly eroded electrode seems to tell me they're being damaged from the engine running lean or them not having the correct heat rating for this engine.

Today I grabbed my stethoscope and tested each fuel injector to make sure they're all functional. Every single one had the nice "tap tap" noise, so I know they're not the issue.

I then proceeded to do a dry compression test. I got the engine to running temp but this being my first time it took me probably 1.5 to 2 hours until I got the test started (warm engine, but not HOT). I pulled all coils and plugs, removed the fuel pump fuse, and put the throttle at full open. See the results below. The order of testing went 8-6-4-2-1-3-5-7. Four of my cylinders (2, 3, 4, 7) read 180 psi, while 3 of them (1, 5, 8) read 165 psi. Cylinder number 6 (the problem child) read a little low at 145 psi. This one I decided to wet test by adding about a teaspoon of oil and retesting. This test brought it up to 180 psi. Something to note: I did not realize how long it would take each cylinder to get to it's final pressure reading. People said it usually takes about 4 or 5 "puffs" until it levels out...mine took about 20 puffs on average. Is that normal?? I made a note that cylinder #8 may have been 180 psi instead of 165 because I was concerned I did not allow enough puffs (since it was the first cylinder I've ever tested). But after cylinders 1 and 5 also read 165 psi, I chose not to retest 8 and believe 165 psi to probably be accurate. So I'm going to need some help interpreting the results. From what I've seen on the web, I should need a minimum of 100 psi. Check. I've also seen that you don't want more than 10% difference between highest and lowest cylinders...so 180 to 165 seems acceptable, but definitely not 145. Since #6 increased quite a bit after adding the oil, does that mean my piston ring is bad? How serious of an issue is this? I'm just surprised that I'm still getting a misfire. Would love to hear what you all think the likely problem is.

 

It still could be the fuel injector for #6. Even if the injector seems fine from an open/close perspective, it could still be possibly clogged or not spraying fuel properly.

 

I know many members are not going to like what I'm about to say. I have been with dodge dealers for 30 years. The 3.9, 5.2 and 5.9 had a defect on the cylinder casting. The trucks with high mileage would have a crack between the valves, causing a misfire. You can change all the plugs, wires cap and rotor. And the plenum gasket under the intake manifold. And you may still end up with a misfire. I would recommend to replace both cylinder heads with Reman's. .