Multiple misfire Issue. P0300, P0301, P0302

[Lost_Wrench]

Lots of related threads on this, and likely culprits, so here's my misfire thread. I promise to follow it through with my eventual solve as well.

1996 XJ 4.0 that I've done a lot of work to in the last year, these parts shouldn't be ruled out, but are probably not likely due to recently being replaced...
-New (refurbished and matched) Siemens fuel Injectors
-Champion Plugs, wires, distributor cap
-Head Gasket (old one wasn't bad)
-Crown Automotive Fuel Pump and Fuel Filter
-O2 sensors (both)
-Battery is new and terminals are clean and tight.


All I have done so far is checked the spark plugs in cylinders 1 and 2. They look fine and are properly gapped at .035. I replaced them anyway. No change.

Symptoms: I've owned this Jeep for about 2 years now and it's always had a small "pop" in the exhaust so I knew there was a slight misfire issue but now it's getting worse and threw the codes the other day. I will be testing fuel pressure and swapping the injectors to other cylinders to see if the codes follow them, but I kind of doubt they are the problem with how new my fuel delivery components are now. I recently did my head gasket last winter too, and don't have any overheating issues so I don't think I have bad compression, but that test will be done as well before I start shooting the parts cannon at various things. Anyone have input or other likely (or simple) culprits to check before I get started with serious diagnostics? Thanks!

 

[Unclewolverine]

Check the compression. Sounds like burnt valves, rounded cam or bad lifters to me.

 

[Lost_Wrench]

Thanks I'll keep that in mind, especially when I get to the compression tests.

So far I've tested fuel pressure at the rail and it's a good 49 psi. I swapped injector #2 which is the code that comes up most frequently and no change so I don't believe it's an injector problem unless it's in the injectors wiring. I cleaned the throttle bottle and idle air control sensor - which was completely covered in carbon. I had a brand new TPS in the garage so I threw it on. Still no changes. It has a strong blue spark at the plugs. I guess compression test is next.

One strange symptom is that it idles nicely at 800 rpm when in park, but as soon as I shift it into drive or reverse, the misfiring is very noticeable and the rpm's drop to around 600 and I can feel the Jeep shaking/stumbling until I start driving it. Anyone have thoughts on that symptom as it relates to misfires. Otherwise it seems to run fine. I drove it up a mountain pass skiing yesterday.

 

[Lost_Wrench]

I should also mention that while cylinder 2 is the most common misfire code, I have had all 6 cylinders come up with misfire codes at once which leads me to believe it's not an isolated cylinder issue.

 

[Lost_Wrench]

Finally got around to renting a compression test kit from the auto parts store. Here are the results which obviously explains the persistent cylinder 2 misfire code.

Cylinder 1 = 110
Cylinder 2 = 60
Cylinder 3 = 135
Cylinder 4 = 120
Cylinder 5 = 120
Cylinder 6 = 112

As far as I can tell my oil and coolant both look fine and aren't mixing. Not sure what the next step from here is so suggestions welcome.

 

[lawsoncl]

I think I'd start by pulling the valve cover and inspect, then probably the head next or at least a borescope next if you don't see anything obvious.

 

[Lost_Wrench]

UPDATE:

I just did a leakdown test and when I put compressed air into cylinder 2 it all came rushing out the throttle body. So bad valve? I didn't see any air bubbles in the coolant, but there was some slight air movement that I could feel coming out of the valve cover, but I heard that was normal in a leakdown test? I'm trying to eliminate the possibility of the piston rings being bad because I'm willing to put a new cylinder head on but I'm not sure I would do an entire engine replacement on this particular Jeep. None of the spark plugs had any oil on them when I pulled them, nor does the engine seem to burn oil or have smoke out of the exhaust so at this point I think the piston rings are ok. Should I go forward with a refurbished head or are there still other tests I should do first?

 

[Lost_Wrench]

Well I've about concluded all the tests I'm going to do for now. I did pull the valve cover and inspected, nothing looked bent, broken or out of place, and the valves all seemed to move up and down the same, nothing looked bad at all. After doing a dry/wet compression test on Cylinder 2 it seems likely a problem with the piston rings. I got 60 psi and then after adding a tablespoon of oil I got a reading of 75 psi. I changed the oil and added a can of Restore but haven't noticed any changes. This Jeep is mostly a camping/hunting rig so I'm not going to be replacing the motor. It still runs like a top really, the only noticeable symptom is a slightly rough idle so I guess we'll see how far she goes before she blows! Thanks to the few who responded!

 

[HRDROKN]

Unless I'm missing something....

If your leak down test resulted in air escaping out the intake... then that's a problem. Not sure that 15 psi on a wet/dry compression test indicates a problem. Does a wet/dry test on another cylinder produce similar results? Just to make a point.... 75psi is still a problem cylinder.

I would pull the head for a rebuild, look at the cylinders for wall scoring (broken ring) and if all looks good.... put the new head on.

Another option if cylinders look good... is to drop the oil pan, pull piston rod, install a new set of rings, maybe a set of rod bearings and put it all back together.