Severe Misfire
- Thread starter Tx99tj
- Start date
OP
OP
it was equally crappy on all plugs. However - I should restate my comment "lots of carbon". As I look on the interwebs at pics of sparkplugs, relatively speaking my plugs were pretty normal. They may have been original plugs. I'm at 103k miles. There were carbon deposits but after looking around online, I guess it wasn't too bad. Compared to the new plugs of course it was night and day. of course, in hindsight, I should have taken pics.
Yes, pics would help a lot. Almost feels like a fueling issue but without hearing the "miss" or seeing the plugs, it's all just shots in the dark. A plugged cat would certainly kill power but I think a miss would be unlikely unless it fowled the plugs which you changed.
A plugged catalytic converter can absolutely cause a misfire, though it most often appears during high RPM or high throttle usage, and less often at low throttle/low RPM or idle.
A plugged catalytic converter creates significant back pressure in the exhaust. The pressure increases proportionally with increasing exhaust flow, so the higher the air/fuel consumption, the higher the back pressure.
As back pressure increases, more exhaust remains in the cylinder on the next pair of strokes, reducing the amount of oxygen the cylinder has to burn. Not enough oxygen either results in a rich condition, resulting in carbon buildup, or a misfire if the resulting mixture is not easily ignited.
It’s like trying to light a wood stove or fireplace with the damper closed and only a door vent open. Sure, you could build a tiny fire, and maybe it will be a bit smoky, but if you try and light something big it’s just going to smolder and go out.
A plugged catalytic converter creates significant back pressure in the exhaust. The pressure increases proportionally with increasing exhaust flow, so the higher the air/fuel consumption, the higher the back pressure.
As back pressure increases, more exhaust remains in the cylinder on the next pair of strokes, reducing the amount of oxygen the cylinder has to burn. Not enough oxygen either results in a rich condition, resulting in carbon buildup, or a misfire if the resulting mixture is not easily ignited.
It’s like trying to light a wood stove or fireplace with the damper closed and only a door vent open. Sure, you could build a tiny fire, and maybe it will be a bit smoky, but if you try and light something big it’s just going to smolder and go out.
When my cat went it was only one bank so there was a clear difference when looking at the plugs.
OP
Yeah I messed up not taking pics.
Jeep seems to run fine until it warms up and then begins to miss at higher rpms and more load. As it continues to run it just gets worse to the point its missing across the rpm/power band.
While the hypotheticals and theoreticals in there sound good, the normal real world presentation of a plugged cat is different. While it's certainly possible to cause a miss with a plugged cat, is usually a loss of power and inability to rev that gets noticed. You are right about the back pressure, but in an n/a application, remember that it's a system and the increased back pressure on the exhaust side will lead to decrease vacuum on the intake side which the map sensor will see and fuel accordingly. Hence you usually see the noticable loss of power before the miss ever becomes evident, and OP didn't mention that. But yes, you can eventually fowl the plugs driving like that and that's why I asked questions first (about the plugs for example) instead of projecting my own previos engine issues on to this dude and suggesting parts to throw at it.
Exactly why I asked what his plugs looked like, but remember, you have 2 pre-cats so half of your engine ran fine while OP only has one main kitty.
I wouldn’t recommend that the original poster replace the cats without looking at them. However, I do recommend looking inside to check on their condition. That way, that issue can be ruled out without buying any parts.
OP
Thanks guys - I'm going to remove cats this week. More to come. I appreciate the help!
Just want to confirm, it will run normally while it's still cool, even at higher rpm and load? Will it free rev to say 4k when it's hot with no load?
OP
When its cool it does not miss nearly as frequently as it does when its at temp. At a higher rpm and load it will miss when its cool.
It will free rev to 4 when it is hot with no load
OP
Found the culprit. Crankshaft position sensor. She’s running like a champ now
