I have a case of the misfire's

[broncomitch]

01 jeep grand cherokee wj
Got this wj a month or so ago from my mom (was my dads before he passed) and about a week ago the CEL came on for multiple cylinder misfire for #5 &#6.
It ran fine so was hoping it needed plugs but that didn't fix it so installed reman ford 4hole injectors, coil pack, and still wont go away.

Once I got a cyl 1 misfire but its mainly 5 & 6

I dont see any damage to the wiring loom running behind the valve cover.

It seems to only do it past 3000rpm and if I hold the rpms there it will dip down to 2800rpm and the CEL will flash.

But, still seems to run fine.

I checked a few grounds and noticed my camshaft synchronizer is making a ticking noise but I don't think that would cause a misfire tho i did see some junk in there and cleaned it out.

Cleared the codes and went for a drive but have not driven it enough for it to come back on. (Usually will take 50 ish miles)

Curious if someone on here is smarter then me with the 4.0l xj's and can help me out.

It dose have a knocking noise at idle but goes away if you give it any throttle.

Wondering if it has collapsed lifters??

 

[DAA]

Similar symptoms on the 4.0 in my LJ were fixed with new precats. Was able to diagnose the plugged/bad cat using o2 data with the Torque app.

- DAA

 

[broncomitch]

Interesting.
I'll definitely look into my o2 readings tomorrow.

Did you replace it or gut the cats?

 

[DAA]

Replaced w/Magnaflow.

- DAA

 

[broncomitch]

I cant look at very much on the torque app.
The only thing it shows for the o2 sensors is voltage.
See attachments

I went to a farewell party last night and drove 115 miles round tip.
The way there it came on once and flashed then went away but the way back it came on and stayed on and would occasionally start flashing but it never really seemed to run much different..definitely wasn't running on only 3 cylinders it seamed like.
See attachments

Thats the first time I have gotten a cyl 4 misfire and evap leak.
I have gotten alot of #5&6 and a cyl 1 once

Guess I'll buy a temp gun and check the cats that way.

 

[DAA]

So I looked for the o2 data in Torque and didn't find it either. Until, a member here pointed me to the right place. You need to look in graphing. Then choose one bank at a time, both upstream and down stream and watch the graphs (look like a heartbeat monitor). If the downstream sensor is basically just shadowing the upstream sensor, it means the cat isn't doing anything. If the cat is in good shape, the upstream sensor will be all over the place as it's monitoring raw exhaust gas but the downstream will be pretty steady - as it's monitoring "converted" 02.

I wish I would have taken some screen shots of mine, before and after. But on mine at least, it was very, very obvious by looking at the o2 data in graphing.

The flashing CEL indicates a misfire that is potentially damaging the cats. Not necessarily that the cats themselves are bad (yet, misfires kill cats). In my case, one of the precats was completely broken up and when I got the intermittent flashing CEL I think it was when chunks in the pre cat were moved around blocking/plugging it. Not totally sure about that, but that's what I think.

Before the flashing CEL and finding the bad pre cats, I had a misfire due to a burned exhaust valve which I let go on for a long time before I finally pulled the head and had a valve job done. The flashing CEL and diagnosis of the pre cats happened only a few days after I got it all back together from the valve job. I'm sure the misfire I let go for so long is what killed the badly broken up pre cat. The other one was in pretty bad shape too though. Since replacing them, no codes, no stumbles, no misfires, no nothing but putting on more miles.

But if your precats aren't toast yet, they will be if you keep driving it with misfires. I really don't know if the pre cats are likely the cause of your misfires or not. Just that the symptoms you have now, I had too and replacing the pre cats fixed them. But if you have another cause of misfires, you really want to track that down and fix it before replacing your pre cats because the raw fuel getting dumped into them from the misfires will ruin them again.

Like I said, a burnt valve was the cause of my original misfire. I was able to positively diagnose that with a simple leak down test. Knew before I started tear down that I had a burnt #6 exhaust valve. Which, the machine shop said that is pretty common on 4.0's and usually if it's just one it's going to be #6 because the back of the 4.0 gets the least, hottest, dirtiest oil.

So, I think if I was you - and I basically was not very long ago - I'd do compression and leak down tests, as well as checking the o2 graphs. If the leak down goes good and the o2 graphs show bad cats, I think I'd just go ahead and replace them. But if the leak down reveals a valve issue, you'll need to address that first else new cats will get killed again.

Oh, and almost forgot... I had an intermittent loud knocking at idle too. It went away after the valve job - but the cam and lifters were pristine so it wasn't valve train noise. I'm not sure, at all, but my pet theory now is that was raw fuel detonating in the pre cat and destroying it.

- DAA

 

[DAA]

Slight correction to the above... Above, I said I knew before tear down I had a burnt #6 exhaust. That's not true. I knew I had a leaking #6 exhaust but not until tear down did I know if it was just a burnt valve, or a cam/lifter issue, or what. Thankfully, it was just a burnt valve.

- DAA

 

[broncomitch]

Well, crap.

Wish I read this sooner lol
I drove it alot sat-sun-mon
Witch shows from how many codes I have now.

I haven't spent much time playing around with the torque app but will definitely check that out, sounds like it's probably the issue.

Only thing I'm a little confused about is why there are so many different cylinder misfires. unless all of the exhaust valves are burned on those cylinders?
I checked the temperature readings on the cats last night.
Holding at 2k rpm
The pre cat for 1,2,3 was at 750ish in and 450-5 out. (Probably shot)

The pre cat for 4,5,6 was 450-500 in and 700 ish out
And the other cat was 400 ish on the in and outlet.
And the check engine light was flashing the entire time.