Earlier this week my rig developed a horrible misfire upon acceleration to the point where I had to have it towed home. The Jeep starts right up and idles like a dream (it does lobe out slightly from the cam) but it will not accelerate.

Specs on my Jeep that pertain to this: (1,400 miles on engine)

- 1994 Xj
- 4.0 L6
- ax-15 swap with stage 4 clutch
- Magnaflow 2.5" all the way back
- comp cams custom ground cam (has 6° advance)
- comp cams lifters
- rebuilt 0630 head decked 0.010" and ported
- Cloyes double roller timing chain set
- aftermarket header (forget brand)
- 99+ intake manifold
- cold air intake
- custom serpentine setup
- hesco water pump
- bored tstat housing
- 190° tstat
- 3 row radiator
- mech fan delete
- bored throttle body
- msd 8mm wires
- ngk plugs
- high flow oil pump
- 91 octane shell gasoline
- 703 injectors

New parts >100 miles:

- distributor
- coolant temp sensor
- oil pressure sensor

Recently replaced parts (between 500 and 5000 miles):

- ckp
- cmp
- tps
- iac
- iat
- map sensor
- o2 sensor
- msd wires and ngk plugs


Diagnostics run thus far:

COMPRESSION TEST (DRY):
1... 145psi
2... 135psi
3... 135psi
4... 135 psi
5... 140psi
6... 140psi

Being that these numbers were right in spec I didn't bother with a leak down test

TIMING CHECK:
I attempted to check timing with a timing light which showed that the timing fluctuates between 10° and 22° retarded but I am losing signal during the test. I have tested the timing light on a properly running vehicle which showed that the gun does indeed function properly.

FUEL PRESSURE CHECK:
- key on... 39psi
- idle... 39psi
- 2500rpm... 42psi

PLUG GAP:
I had my plugs gapped at .042 but I regapped them to the stock .035 for testing which did not improve anything so they are back at .042

DISTRIBUTOR TIMING:
Distributor is set correctly; rotor trailing #1 at TDC on the compression stroke

MISC FIDDLING:
- Unplugged o2 and map sensor wires to see if conditions changed. O2 sensor did nothing, unplugging map sensor made it run worse.
- reset pcm.... no change



Please help me figure this the hell out!!! I miss my baby lol

 

Will the engine rev up without missing in neutral or is this load related?

 

^^^^
this.
did you get the cpu done up. maybe the cpu can't make the correction for the new set-up, thus pulling timing sending the jeep into limp mode.

 

It misses revving up in nuetral aswell as with load applied

 

I drove 1400 trouble free miles on this setup. The other day driving to work this misfire started and the more I run the engine for testing the worse the misfire seems to get.

 

I'd put a vacuum gauge on it. An invaluable diagnostic tool. Maybe your exhaust is restricted/clogged.

http://www.classictruckshop.com/club...ts/vac/uum.htm

 

I'll see if I can pic one up today on my way over. If my exhaust is blocked we have a serious problem haha. I'll drop it anyway. Gives me an excuse to hear that cam lobe out haha

 

Idling shows 18". Returning to idle from 2k spikes to 22" and drops back to 18"

 

Well, that sounds about right.

 

Is the needle on the vacuum gauge steady or fluctuating? Could possibly be due
to the spark plugs themselves some engines are temper mental with certain plugs, try a different brand maybe? And you could always try running an injector
cleaner can make a world of difference.

 

yes. our motors don't like any other plugs except for copper. I noticed you were running ngks. Def check the plugs and plug wires. loose sparkplug wire etc.
Are throwing any codes? You might be down a cylinder or 2.

 

Back to 18 quickly with no jerkiness in the needle?

If so, your exhaust isn't clogged.

 

Alright, I know it'll sound crazy, but what about your alternator? GOTWAX had similar problems with his a while back and we thought it was bad injectors and other miscellaneous parts, but his is also a '99 so that might make a difference.
Just my 0.02

 

Needle drops steady with the rpms. I've been running ngk plugs for a little over 2 years without any issues. I don't see why they would run fine for this long then one day crap out on me all at once.

Edit: These plugs are new by the way. Replaced at the same time as the rebuild

 

Plugs are healthy and plug wires are all tight. Not throwing any codes besides the normal 12/55. All six cylinders are intermittently losing spark.