Sputtering/misfire, P0302 code and low compression?

Hey, guys.

Just bought a 140k miles '98/99 WJ w/ 4.0L (see https://www.naxja.org/forum/showthread.php?t=1165164) sight unseen. Not many miles into the 500 mile trip home (half of which pulling a 4k lbs trailer) CEL comes on and a sporadic sputtering presents itself. The sputtering appear as sudden bursts of power loss, primarily when cruising/coasting (and apparently never under load), but predominantly when idling (as a frequent 'poofing' from the tailpipe).

Other than the occasional sputtering, the car did not feel sluggish or low on power (I am calibrating that based on my experience with 800 lbs lighter 4-litre XJs with less power-absorptive transmissions), and I seemed to get normal gas mileage (22 MPG cruising/highway and 15 MPG with trailer). Oil and coolant both looks fine.

Only code it is throwing is P0302 (Cylinder #2 Misfire Detected). Not knowing whether I have the 0331 cylinder head (the oil filling cap is just spinning), I started the diagnostic process by checking plugs (they seemed to have som oilish sludge - see picture), and proceeding with cylinder compression, with the following readings (cold motor):

Cyl. #1: 149psi
Cyl. #2: 160psi
Cyl. #3: 156psi
Cyl. #4: 148psi
Cyl. #5: 162psi
Cyl. #6: 65psi (!)

Of course, I was pleased as punch until reaching the last cylinder. Thinking I maybe didn't get the rubber boot of the compression tester on properly (it's tight/crowded), I tried to verify multiple times.

So, what are we dealing with here? Head gasket? From what I understand, the infamous crack (almost always?) happens around cylinder 3 and 4? And why would it indicate cylinder 2 through the OBD?

Thanks in advance!

If it's intermittent, sticking valve?

Start with the usual suspects, spark plugs (Champion or Autolite APP985), coil failure, injector failure, coil or injector wiring issues, O2 sensors ( NTK, never Bosch or parts store generic), Coolant Temperatures Sensor, other OBD-II sensors.

The coil rail ignition system is very fussy about what spark plug it will work well on. The XP-985 and APP-985 are especially well proven on that engine.

The head should already be the more durable and crack resistant TUPY casting.

My oil fill cap was spinning I pried the top off and was able to unscrew the threaded part.

Best stick with the WJ forum and not cross-post!

N8N_99xj said:

If it's intermittent, sticking valve?

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Wouldn't that yield a distinct noise/exhaust note? There is no sound from the valvetrain that I can hear.

Tim_MN said:

Start with the usual suspects, spark plugs (Champion or Autolite APP985), coil failure, injector failure, coil or injector wiring issues, O2 sensors ( NTK, never Bosch or parts store generic), Coolant Temperatures Sensor, other OBD-II sensors.

The coil rail ignition system is very fussy about what spark plug it will work well on. The XP-985 and APP-985 are especially well proven on that engine.

The head should already be the more durable and crack resistant TUPY casting.

Click to expand...

I kind of skipped sensory stuff after having identified the compression issue.

The plugs are Champion RC12ECC (which I believe is the standard mnfg. recommendation?), and they were all sort of sooty/smudgy (and #6 a bit more so than the others).

I am guessing that with the distributorless coil-rail design ignition (i.e. one coil pr. two cylinders), I would also hav trouble (and fault codes) on cylinder #1 (as they share the same coil)?

jeep ride said:

My oil fill cap was spinning I pried the top off and was able to unscrew the threaded part.

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Yeah, but I don't want to mess with it until I have a replacement ready (these things aren't dime a dozen in the junkyards over here).

AFAIK, the TUPY head upgrade did not arrive until mid-2002?

RCP Phx said:

Best stick with the WJ forum and not cross-post!

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Didn't figure it mattered as this is XJ relevant 4.0L tech.

Tim_MN said:

Start with the usual suspects, spark plugs (Champion or Autolite APP985), coil failure, injector failure, coil or injector wiring issues, O2 sensors ( NTK, never Bosch or parts store generic), Coolant Temperatures Sensor, other OBD-II sensors.

The coil rail ignition system is very fussy about what spark plug it will work well on. The XP-985 and APP-985 are especially well proven on that engine.

The head should already be the more durable and crack resistant TUPY casting.

Click to expand...

WJ's didn't get the TUPY head until late 2001 production, he'll have an early 0331 head.

Carbon on the valves is a known problem, and there is a TSB about that.

Kittrell said:

WJ's didn't get the TUPY head until late 2001 production, he'll have an early 0331 head.

Click to expand...

Easy enough to figure out with just a good flashlight.

Tim_MN said:

Easy enough to figure out with just a good flashlight.

Click to expand...

If you had a MY 2002, absolutely, in his case unnecessary.