Kalali

I have taken every possible step in trying to resolve a pesky single-cylinder (P0304) misfire. New head to replace a cracked 0331 head, compression is perfect across the board, new coil rail, switched injectors, traced injector wires through the harness to the CPU, checked and doublechecked the push rods and rocker arms, reindexed the cam sensor, all to no avail. I did read about the issue with coil springs on some new Clearwater heads but aside from that I don't know what else to check. The other key point is that I had the same exact misfire code right before the head was replaced.
So here are my questions: Could a faulty sensor cause a single-cylinder misfire? I know all my sensors; O2, CPS, TPS, MAP, etc., are original because I bought the Jeep new and they've never been replaced. I'm also wondering if its possible that the coolant leak from the cracked head - given the location of the crack near cylinders 3 and 4, has compromised the cam/rod bearings at or near cylinder 4. The only visible symptom is a very minor vibration/miss at idle more obvious if you watch the exhaust and can see a slight miss. Other than that the engine is quiet and very strong and smooth. I can reset the CEL and drive for a day or two before the light comes back only when the car idles behind a stop light and never while on the move. Its a 2000 XJ with 178K miles. The head was replaced at 172K miles.
Appreciate any help.

MichiganCherokee

Man! We have the same exact issue going on....except mine is on cylinder 1 and occasionally 6.
I just picked mine up from the dealer for a smoke test and they cured the large leak but now I'm getting a small leak code. Will be back in Wednesday for another smoke test(on their dime).
I also have no idea what to try next. Also swapped the 0331 head for a Clearwater but mine was not cracked before just did it for insurance. But I also had the misfire before the head swap.....

Jamie57

I tried everything until I ACTUALLY went and did a compression test. I had wasted so much time and money trying everything I could knowing I would have to do it anyways.
My problem was a burnt valve. Compression test showed it in all of 10 minutes. Could have used the money I spent on other things and put it toward the new head I put on anyways.

DO A COMPRESSION TEST!
It's the KISS principle. Keep it simple stupid!
I learned this way back when I first became an apprentice mechanic but I thought I had become so much smarter over the years.

Once you have eliminated or confirmed any MECHANICAL fault, you can now go on to the more complicated solutions.

For instance your P0304 code showing a misfire. a compression test will tell you if your valves are opening or closing properly or you have a ring problem with the piston and the cylinder wall. ONCE you have eliminated the hard mechanical problems then you start looking for the more complicated gremlins.

Having said all that I believe you have low compression in that cylinder. You had the same CEL code before you changed the head correct? You either have a cam lobe shaved off or a lifter not pumping up, or seized rings. It's the only possible things left after you have changed everything else.

MichiganCherokee

We both have checked compression. I did mine after the dealer did it just to make sure they weren't B.Sing me. Mine are all 150+/- a few.
We both have brand new heads with about 8k on them.

MichiganCherokee

Dealer checked rocker arms with the valve cover off all appeared to be rising at the same level.

MichiganCherokee

If I get time soon I'll recheck my push rods....perhaps they are worn and a touch shorter preventing full function of the valves but still giving a good compression reading?

Turbo X_J

Pushrods may bend, but won't wear.

Jamie57

Hmm well you have me stumped then. I found out that the PCM (or ECM) finds the misfire by where it expects to find the CPS when each cylinder fires. This happens in micro seconds of course but it is really pretty accurate.
Soooo the PCM is telling you that something is off on just that one cylinder.
It can't be affecting any other cylinders or you would get a P-300 code as well.
Try swapping the injector from #3 to #4. if your CEL code changes you may have found your culprit. Same with plugs. Just because they worked when the dealer had it doesn't mean they are still working properly.

opps reread you had already tried that.

Well it has to be something in just that cylinder anyways. Let us know if you do find it. it will help out the rest of us when it shows up again. And it will. Murphy is my guardian angel. BastaRD

Jamie57

Agreed. I've never seen a worn one in all my years of wrenching. Bent mangled and broken? Yes. Worn out? not yet.

MichiganCherokee

Okay thanks for the responses. I don't know about Kahali but I cleaned a few grounds and will try to clean all the others. When I swapped heads I rolled all my push rods on glass and were fine...
I will swap in injectors again.

Sometimes I get p0300 sometimes I don't. I know it's rare but what's the chances our ECUs are faulty?
My jeep runs great when it's off idle...at idle it misses, shutters.

Kalali

Thanks for all the input. Is it possible that a marginal sensor could cause misfire in only one cylinder? I checked the push rods (for cyl 4) and they were both straight. My gut feeling is telling me it has something to do with the coolant dribbling down around the push rods down to the lifters and cam, etc. I 'm pretty sure I didn't put many miles on the engine with the cracked head but I'm afraid I might have caught it late. The compression values are all 150+ and hot oil pressure is at ~15 PSI. At cold start up is sits between 40-50.

Jamie57

Hmmm A P0300? Have you tried the grounds near the PCM? A P0300 says that it's common across all the cylinders. How about the connection to the coil pack? Also check where the wire loom goes over the intake manifold just before it wraps back around the head. I have heard of a break or short in the wires there, where it may have sat on bolt. I found one that probably finally broke when I took it apart to check.

Funnily my engine ran great and I got pretty good mileage. It just kept popping the P0306 and ran crappy at idle. It also wouldn't complete the final checks on the cat according to my Torque Pro. All from a burnt valve.

Jamie57

I just can't see a faulty sensor only reading on 1 cylinder unless it's a sensor designed to check that one cylinder alone. That's why the PCM uses the CPS to tell if a cylinder is running optimally. It can tell from the cylinder "Push" if it's running correct and in the proper sequence.
It's really a creative way to use software to diagnose the engine using the least amount of sensors.

Here is another fun fact. The coil pack only has 3 coils in it. It fires the #'s 1 and 6 at the same time as well as 2-5 and 3-4. Another cheaper way of using the least amount of hardware to do the same job. On a Harley it's called a Loss fire ignition system. It fires both cylinders at the same time because one will be on an exhaust stroke while the other is on a compression stroke.
I have never liked these systems because Electricity is like water and will find the easiest path. Which spark plug will get the biggest spark, the one with no compression (exhaust valve is open) or the one under compression?

Have either of you replaced the Coil Pack? if not check it carefully for carbon tracks. it may be shorting out near that plug. A weak spark will not fire the mixture as well as a hot one. It's why race cars use a 70-80K volt ignition system and plug gaps around 50-60 thou. Bigger hotter sparks to ignite all the fuel.

MichiganCherokee

Bad fuel filter in tank for p0300 code? Has 168k don't know if it's ever been replaced. Just read a thread where a guy would have an issue with it running rough (debris got sucked up) shut off engine (released debris) started back up and ran fine before it got sucked up again. Sounds kinda what I'm dealing with. Now if I'm going to do this I want to drop my tank and clean it.....obviously run it til it's just about empty...undo gas cap..release pressure at fuel rail and unhook all fuel lines and electrical connections prior to dropping. What are good products to clean the tank...just warm water? ?

Kalali

New coil rail.