Miss at idle

[Quikstang]

hey guys! long time no hear, been working 6 days a week and haven't had time for much. Any way, reason I'm here is I have a new addition to the family, a 99 jeep grand cherokee larado, picked it up for a very reasonable price and found out why when I got it home....needed a new motor. So, with the new motor comes a problem I cannot figure out and hoping someone here can help.

Like I said, it's a 99 jeep grand cherokee with a 4.7l V8, once the motor was in it ran fine for the first 50 miles or so then developed a miss at idle, check engine light code is a missfire in cylinder 4.

things I've tried:

spark plugs - new
injector - replaced
coil pack - replaced
swapped all around to try and make the miss move
there is voltage at both
there is pulse voltage at both

I've read that it could be either or both the crankshaft possition sensor and camshaft possition sensor.

the motor manufacturer suggested taking it in and putting it on the diag machine.

The miss appears to go away once you accelerate.

Looking to see if any of you guys could help steer me in the right direction before I start pissing any more money away.

Thanks for reading

[socalmach]

You've already trouble shot what I would have. I'm stumped, good luck.

No warranty on the new motor I guess?

[Quikstang]

warranty = 3yrs/unlimited mileage

but I have to rule out any thing that is not internal before the warranty kicks in.

[Quikstang]

Well....shop can't figure it out either. When they put it on the machine it was indicating that the miss was jumping all around and for some reason that the onboard computer was only showing cylinder number 4. jeep's still there, they wanted to keep it over night for observation (haha), they weren't done with it.

I did have to school the tech, when I replaced the motor I used wire markers to help me remember where stuff went back and I labeled the cylinder wires like my mustang when actually 1,3,5,7 are on the drivers side and 2,4,6,8 are on the passenger side, when I told him that he defensive started to argue with me....the wires have markings on them that came from the factory, it couldn't possibly be the way you described....I told him to look at the stamping on the intake.....he said......oh...

I'll post findings.

[Quikstang]

Well, I picked it up day before yesterday IMO with the the miss stil there. I was assured that once I got it on the interstate it may clear up. Left it with the wife and yesterday she called me at lunch time to inform me the check engine light is back on.....I kinda figured it would but hoped for the best. So today it's back down there, I don't feel so bad now that I know that a $10,000 diagnostic machine can't figure it out. They are at a point now, after puttsing with it all day, where they are going to try and get a used computer, have it flashed and see if that clears it up.

Still frustrating me not knowing what is wrong, I can only imagine what it's doing to 4 mechics with the best equipment on hand.

[Jazz]

Dang!

[socalmach]

Hopefully the new computer will fix it or not, depending on how you look at it. A new computer isn't cheap.

Good luck

[Quikstang]

Update:

Never got to the computer part, during my conversation with the mechanic and (in my mind ruling out an ignition problem with everything he told me the machine was saying) the subject of a compression test was brought up (kinda feel dumb for not thinking of that in the first place), well how does 180 in all cylinders except number 4 which was 100. Bingo! internal problem, called the warranty company and they have authorized the shop to perform the necessary repairs, even with a "self installed" motor.

[novajoe]

Glad to hear you got it figured out. I feel dumb not thinking of a compresion test my self!

[socalmach]

Good news, they finally got the bad cylinder. Strange that the diagnostics showed the miss jumping though.