Misfire, need advice.

[baby#yota]

About a month ago my truck started acting like the torque converter was locking up and unlocking. Clunking and shuddering. I ended up serving the trans and a day later I get a misfire code. My fault on the poor diagnosis but not my trans has nice new fluid now so oh well. The miss got so bad it felt like I was driving over rocks when it would pop up. I went and bought a coil for the cyl3 (the cyl in question) and upon replacing it the miss went completely away. I thought I was in the clear. About a week later I started getting misses again. Not as bad as before and not enough to throw a code but enough I could see them on the monitors and feel them in my seat. I replaced the cyl 3 plug and they went away for a few days. Again I thought I was in the clear but then I started getting the occasional miss on 1 and 5 as well as most of them on 3. So I thought “great it’s my HG”. I ran compression, and pressurized the cooling system while looking in the cylinders with a bore scope. Held perfect pressure for 30 min and compression was perfect. A day later I got to looking at my harness and find a mouse chewed part way through my cyl 3 coil power wire. I fix this problem but the misses continue. They are not nearly as bad as they were but I get a few after I drive for 15-20 minutes or so. They mostly appear when I’m going up hill at lower rpm’s and the engine is lugging. I will press the pedal to change gears and I’ll feel the “thump, thump” of a miss. Not always but about 30% of the time. Can someone shed some light on what might be going on here?? I’ve checked every inch of the harness, and spent hours testing this and that on the truck with no luck. It purrs right along at idle without missing a beat.

05 v6 270k SR5

 

[baby#yota]

I also want to add that after I started to feel the first misfires I got a p0420 cat code. I assumed this was due to a misfiring engine and I had damaged my cat. I had a thought on my way home today and wanted to share. Today I had zero issues with the truck to or from work “about 15min”. I also noticed that the pending p0420 code is not appearing in my scan tool. Can this whole nonsense be caused by a failing o2 sensor? Just a thought. I would imagine that a bad sensor would screw up the air fuel mixture on bank 1 and cause some misses. Thoughts?

 

[Buckaroo]

We always start with...is it a cylinder specific misfire (P301-306) or a general misfire (P300)? Seems you may have addressed compression - spark, air and fuel are your other potential culprets....oh, and timing but not a typical suspect.

 

[baby#yota]

We always start with...is it a cylinder specific misfire (P301-306) or a general misfire (P300)? Seems you may have addressed compression - spark, air and fuel are your other potential culprets....oh, and timing but not a typical suspect.

I had a P0303 one time when the misfire got really bad and a p0420 from time to time.

 

[Buckaroo]

I had a P0303 one time when the misfire got really bad and a p0420 from time to time.

I see you replaced the coil and the plug on cyl 3. Have you had the P303 since? You mention cyl 5 & 6 - did you have P0305 & P0306 codes? Is your CEL on all the time? Seems like you are saying you are getting sporadic, isolated misfires that are not consistently setting codes - right? Agree that the CAT code is likely due to the engine missfire.

 

[baby#yota]

I see you replaced the coil and the plug on cyl 3. Have you had the P303 since? You mention cyl 5 & 6 - did you have P0305 & P0306 codes? Is your CEL on all the time? Seems like you are saying you are getting sporadic, isolated misfires that are not consistently setting codes - right? Agree that the CAT code is likely due to the engine missfire.

That is correct, I have never gotten a misfire code on any cylinder other than 3, and only one time. I see misses on my scan tool and feel them while driving. I’ve only logged 3 or 4 misses on 1 and 5 collectively but those cylinders never threw a code. My p0420 code has disappeared and so has my pending p0420 and the misses have stopped oddly.

 

[SurferUK]

My memory is hazy but I remember reading in a few places over the years that the P0420 code is notoriously unreliable, is that a post-cat code? It is usually something like an O2, MAF sensor, or a failed cat.

 

[Buckaroo]

My memory is hazy but I remember reading in a few places over the years that the P0420 code is notoriously unreliable, is that a post-cat code? It is usually something like an O2, MAF sensor, or a failed cat.

That P0420 is set when the downstream O2 sensor behind the CAT see's too much variability. The CAT should smooth it out if operating properly. So it typically is a bad downstream O2 sensor or a bad CAT. If the CAT gets overloaded due to misfires, it could set the code particularly if the CAT is weak. CAT's don't die on their own, they are murdered by continuing misfires that introduce unburned fuel. Also, if you have a hole in the exhaust above the downstream O2 sensor and it sucks in some air, that can set the code although it doesn't damage the CAT.

It is a two trip code meaning that the first time the condition is observed, it will set a pending code and not light the CEL. After key off, if it see's it again on the next trip, it sets the permanent code and lights the CEL.

 

[Buckaroo]

That is correct, I have never gotten a misfire code on any cylinder other than 3, and only one time. I see misses on my scan tool and feel them while driving. I’ve only logged 3 or 4 misses on 1 and 5 collectively but those cylinders never threw a code. My p0420 code has disappeared and so has my pending p0420 and the misses have stopped oddly.

Good that the P0420 went away. There is a threshhold number of misfires that need to occur before the misfire code is set so apparently you have not hit that but the one time on cylinder 3. Odd that you can feel them. It appears to have resolved itself for the moment which is not good for troubleshooting. Need to wait for it to get worse I guess. If it comes back, pay attention to presence of moisture. You might pull the coils and look at the boots to see if there is any tracking. If they look a little sketchy, you can replace them all relatively inexpensively. The NGK's are $5.75 each on Rockauto - if you replace any, I would do them all.

Ignition And Coil Flashover

 

[baby#yota]

Good that the P0420 went away. There is a threshhold number of misfires that need to occur before the misfire code is set so apparently you have not hit that but the one time on cylinder 3. Odd that you can feel them. It appears to have resolved itself for the moment which is not good for troubleshooting. Need to wait for it to get worse I guess. If it comes back, pay attention to presence of moisture. You might pull the coils and look at the boots to see if there is any tracking. If they look a little sketchy, you can replace them all relatively inexpensively. The NGK's are $5.75 each on Rockauto - if you replace any, I would do them all.

Ignition And Coil Flashover

I checked that article you linked and all my plugs had that “corona stain” mentioned in the article on them.

 

[Buckaroo]

I checked that article you linked and all my plugs had that “corona stain” mentioned in the article on them.

I would change the plugs and the boots.