So, as I continue to combat misfires, I'm considering deleting my catalytic converter. If you've read any of my previous threads, you'll know I've done quite a bit of work and made reasonable progress against the misfires.

The truck had numerous misfiring problems for an unknown amount of time prior to me buying it. #10 had cam damage, so I replaced all 30 lifters and followers, along with the Driver side cam shaft. I replaced all plugs and COPs, I replaced a dead injector on #4, and I cleaned the rest. Replaced the battery due to it being older and somewhat under voltage. Even replaced some COP plug connectors that wouldn't snap on anymore.

The truck runs great, but often during startup/cold it misfires a bit. Feels like one cylinder, but the computer only gives a multiple misfire code. I'm wondering if the CAT might be plugged up, and if that could be contributing?

If I clear the code once it warms up, it usually stays gone until next cold start.

I know these things have downstream O2 sensors, is this something I can turn off relatively cheap? Muffler shop quoted me $100 to cut it out and pipe the gap.

Thanks!

 

Problems sound like nothing to do with the cat.

There is a vacuum test that will help detect a plugged cat. You can search for that, as I can't at the moment.

If it "runs great", it's not the cat.

 

Yeah, I wasn't sure, but running out of ideas as to why it's still misfiring. Especially since it seems to be somewhat inconsistent. Thought maybe if it was plugged up a bit, being cold might restrict exhaust, but as it warms maybe expands enough to just barely prevent enough misfires to register?

I'm just grasping at straws I'm sure.

Not really sure what else to move to next.

However, being that it has had a misfire for who knows how long, is it likely that the cat is still in good shape?

 

Two things off the top of my head (well, maybe more):

1) Clean the MAF yet?
2) Check the throttle body for carbon
3) Does it have an EGR? (I don't think so, but I have to ask).
4) Have you gone over the ENTIRE vacuum system and checked for leaks? Even a leaky brake booster one-way valve that causes the booster to loose vacuum? (that would pull a lot of vacuum on initial start and cause it to run rough)

 

MAF was cleaned, intake filter inspected.
Throttle body has carbon, but I wouldn't say it's significant. Any way to suggest how much is too much?
No idea about EGR, I don't think so.

I haven't done much vacuum checking other than the two lines running to each valve cover, which seem to be in tact though the insulation material is not the best looking anymore.

 

If you want to rule out the cat remove the oxygen sensors before the cat and you should feel a bump in acceleration.

Plugged cat will feel extremely underpowered.

 

Hm, well to be honest I've never driven a V10 prior to this one, so I wouldn't exactly know what underpowered would feel like. I've driven my fathers 6.0 and I don't imagine the two would feel totally similar.

IMO it feels like it has good power, and it lands the usual 10-13mpg if that matters. It just has a minor misfire issue now that I can't iron out.

I *think* it has a minor exhaust leak as well, but I don't think that would cause a misfire.

 

What sort of misfire codes are you receiving---what DTC code numbers?

I'm wondering if there isn't an issue with the timing chains or tensioners? Has oil pressure at the sending unit been checked at cold start up?

 

Johnfist I haven’t followed your story so I have to ask the “dumb” questions -
have you changed the spark plugs and boots?
If not changed; have you at least looked into the spark plug wells (for accumulation of water or oil)?

Regarding removing the cat(s); Ive heard it’s easy to fool the o2 sensors by covering most of its element with a piece of copper tubing and then simply reinstalling. I plan to do that very soon. Google it

throttle body is pretty easy to remove. Do that and then buy the special cleaner. It’ll be a 60-90 minute project that will make you feel like a man, and maybe make the truck run better. I remember a few times here on FTE people claiming noticeable improvements from the same. Doesn’t seem logical to me but hey.

 

It's just a normal p0300 code. Sometimes I'll get a p0304 but that's because that plug sometimes has moisture in the hole, for reasons I haven't quite figured out. That's an issue on it's own that is inconsistent as well. Sometimes there's water (possibly coolant) sometimes there's not. Doesn't seem to be reliant on amount of driving or anything.

I recently had the valve covers off when I did the followers, lifters and a cam. Chain and tensioners didn't appear to have excess slack or wear. Though the view isn't the best from up top. Have not checked oil at sending unit during cold start.

Changed everything with new, plugs, cops/boots.

I may have to consider the throttle body at some point as well. Can't hurt I guess.

 

Any other suggestions before I consider tearing into parts of this engine again?

 

i chased a misfire (#5) for months. I changed my 300,000 mile original spark plugs, fully expecting the misfire to be resolved. But nay.
I swapped coils, spark plugs, boots, fuel injectors- and drove it to see if the misfire would follow a part. But always #5.
I noticed it would crank slowly on the first crank after sitting for a while and getting cold. I also noticed it was spittling oil/coolant from the broken exhaust stud #5 cylinder.

I bought a cheap borescope and found coolant in that cylinder.
We Removed the head (while still in the van! It’s possible) and found the stupidest little random pin hole in the combustion chamber.

A $1,000+ and dozens of hours later; its running. With no misfires.

 

Here's hoping I don't have to remove the head(s) for any reason. :/ Thanks!