[MJTDI]

first, i have searched the forums for 4.7 misfire issues here and on JU and my regular forum jeepin.com. i am a mechanic/fabricator, not a DIY'er

i just bought an 01 WJ V8 ltd. it had no. 7 misfire and the person i got it from thought it needed an engine. i drove it 1/4mi to my shop and started diagnosing it immediately.
so, here is everything I have done to try to fix the misfire,
first I did a compression test, all 8 cyl were good, right around 115psi. then I threw away the autolite platinum plugs that were in it and put in some champ rc12yc's and a new no.7 coil pack. that made it run better, but still misfiring on 7.
I looked at the injectors and it had a mismatched one in #7. i swapped injectors between #5 and #7. that changed nothing. the chk eng light is simply a #7 misfire code. I was thinking my next step should be to check the voltage at the injector and coil, if there is no issue there I am not sure where to look after that. I did buy it knowing it could possibly need eng work or replacement. I am fairly convinced that the eng is fine. any ideas would be appreciated.

 

[dmr47]

RC12MCC4 are the correct plug.

 

[MJTDI]

yc's work fine

 

[buttnakedjeepin]

change the plt. plugs and stick with the copper cores..that may make a differense...what do you got to lose?

 

[MJTDI]

rc12yc's are copper. wow, guess i will stick to JU

 

[dmr47]

Possibly a bad driver for #7 injector in the ECM. Hard to guess exactly. 115 PSI seems low but if they are all reading in that area you are looking for differences, Cylinder leak down test? Wet and dry compression test? I think you are looking at a tired motor.
The 4.7 that year had a 9 to 1 compression ratio.

 

[MJTDI]

Possibly a bad driver for #7 injector in the ECM. Hard to guess exactly. 115 PSI seems low but if they are all reading in that area you are looking for differences, Cylinder leak down test? Wet and dry compression test? I think you are looking at a tired motor.
The 4.7 that year had a 9 to 1 compression ratio.

i am banking on the driver for 7 being bad, i wont know til tomorrow. as far as the motor being tired, it only has 107k on it and it pulls hard, gets 22/23 mpg on the hiway. not signs of a tired motor to me.

 

[buttnakedjeepin]

how about the grounding straps on the back of the motor? sometimes though can fall down and forgotten about when doing plugs? do those have anythign to do with the spark?...

my bad didn't read the part where you got rid of the platinum's..

 

[MJTDI]

i work on these things regularly and i am very familiar with WJ's, i just never had this much trouble fixing one.
no grounds were disconnected while doing the plugs, though they are all intact and not corroded.
i was thinking of getting another pcm, i found one for a 2000 V8 ltd for cheap, but i am not sure of the compatability. a friend of mine also has an 01 ltd that is identical to mine. i had thought about swapping pcm's, but i did not want to do it unless i was sure the pcms wouldnt be damaged in the process. automotive electronics are so sensitive and expensive, i dont want to take the risk of owing my friend a new one.

*edit. i did not mention that it only misfires under 1800rpm (every 3rd to 5th revolution until 1100rpm) below 1100 #7 does not fire at all.

 

[MJTDI]

after some more troubleshooting, i went to the dealer to talk to a mech i know over there and he said with conviction that i need a pair of valve springs. so i ordered a pair. guess we'll see what happens when i get them

 

[reward_1000]

I have had several of these come in with the rocker kicked out from under the cam shaft. I always replace the rocker and the lifter and everything works well. You might pull the valve cover off and see if it is kicked off. The compression will test fine if you are just relying on a compression test. I too have worked on several of the 4.7 engines and have not put a set of springs in one yet. Good luck with your issue.

 

[MJTDI]

if i pull it apart and thats the case, i will just have to order more parts i guess.

 

[MJTDI]

intake lobe on#7 was chewed up, i replaced lifters and rockers along w/ camshaft. all fixed and runs good now.

 

[reward_1000]

Sounds great. It is always something different with those 4.7's.

 

[bruckerben]

Lifters...

intake lobe on#7 was chewed up, i replaced lifters and rockers along w/ camshaft. all fixed and runs good now.

MJTDI,

How long did this take you to do? Any difference in performance after, other than running properly?

Thanks,