Modified 4.0 Misfire
#1
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Year: 1994
Model: Cherokee
Engine: 4.0L HO
Modified 4.0 Misfire
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
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
#2
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Year: 1990
Model: Cherokee (XJ)
Engine: 4.0
Will the engine rev up without missing in neutral or is this load related?
#3
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Year: 1999
Model: Cherokee
Engine: 4.0
^^^^
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.
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.
#5
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Year: 1994
Model: Cherokee
Engine: 4.0L HO
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.
#6
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Year: 1990
Model: Cherokee (XJ)
Engine: 4.0
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
http://www.classictruckshop.com/club...ts/vac/uum.htm
#7
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Year: 1994
Model: Cherokee
Engine: 4.0L HO
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
http://www.classictruckshop.com/club...ts/vac/uum.htm
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#9
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Year: 1990
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Well, that sounds about right.
#10
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Year: 1995
Model: Cherokee
Engine: 4.0
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.
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.
#11
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Year: 1999
Model: Cherokee
Engine: 4.0
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.
Are throwing any codes? You might be down a cylinder or 2.
#12
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Year: 1990
Model: Cherokee (XJ)
Engine: 4.0
#13
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Year: 1990
Model: Cherokee
Engine: L6 242cid FI I-6
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
Just my 0.02
#14
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Year: 1994
Model: Cherokee
Engine: 4.0L HO
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.
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.
Edit: These plugs are new by the way. Replaced at the same time as the rebuild
Last edited by XJeepWerks; 04-10-2013 at 10:10 AM.
#15
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Year: 1994
Model: Cherokee
Engine: 4.0L HO
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.