Misfire throughout RPM band, rough idle. Please help! - JeepForum.com
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Unread 07-15-2014, 11:22 AM   #1
Jesse86
Registered User
1997 ZJ 
 
Join Date: Mar 2010
Location: Clearwater, Florida
Posts: 4
FIXED!!!!!!!! Misfire throughout RPM band, rough idle. Please help!

I've been lurking around here for years, but this is my first post. Grab some popcorn it's going to be a long one...

I have a misfire that's driving me crazy. I've thrown a lot of money at my ZJ trying to fix it with no luck...

It's a 1997 ZJ with around 200,000 miles, 4L and an auto tranny (42RE I believe). Sitting on 4 inches of Rough Country lift with 31x10.50 A/Ts and stock gears. The transfer case was acting up so, long story short, it's now a 2x4 ZJ.

It's been my daily driver for a year. No problems at all, it has run great.

About a month ago it started missing intermittently. No problems starting or anything like that. It fired right up and would drive fine. Out of nowhere, (normally, but not always, at a stoplight) it would start to run very rough (sounds like a small block v8 with a big cam), lose power and have a very bad exhaust smell.

I'm assuming what ever the problem is is screwing with the spark.

I drove it like that for a month, but now the problem is getting worse to the point of being unsafe to drive.

It always starts right up, but instantly starts missing and spitting unburnt gas out the tail pipe. It will drive around okay but has a huge loss of power and stalls randomly. The miss is throughout the whole RPM band. After it stalls it's hard to re-start. I normally have to crank it for awhile to re-start it when it does die. It will randomly run fine for about 30 seconds, then act up again.

No CEL or any codes...

Here is a list of what I've done so far:

-Checked all electrical connections and vacuum hoses.
-New plugs
-Wires
-Distributor cap
-Rotor
-Ignition coil
-CPS

Bench checked the following sensors:
-IAC
-TPS,
-MAP
All checked good.

I also removed and deep cleaned the throttle body, and know for a fact that it's not a clogged cat.

I don't have a gauge to check the fuel rail pressure but I'm assuming it's fine considering I have raw fuel coming out the tail pipe, and the miss happens throughout the RPM band as-well as at idle. I think that MOSTLY rules out a fuel problem.

I'm thinking upstream o2 sensor or the PCM...

here is a video of the engine running
?start=" frameborder="0" allowfullscreen>

Thanks in advance for any help!


Last edited by Jesse86; 07-17-2014 at 09:45 AM..
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Unread 07-15-2014, 12:32 PM   #2
Candymancan
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1998 ZJ 
 
Join Date: Aug 2010
Location: Virginia
Posts: 3,852
Check for a broken valve spring.. Take the valve cover off.. and look at all the valve springs.. I bet you one of them is broken... Our 4 .0 had the same problem ran extremely rough and was missing like crazy and it turned out to be the #3 valve spring was broken
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Unread 07-15-2014, 01:51 PM   #3
Jesse86
Registered User
1997 ZJ 
 
Join Date: Mar 2010
Location: Clearwater, Florida
Posts: 4
Thanks for the reply! I didn't even think of that. I'll pull the valve cover first thing tomorrow.
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Unread 07-15-2014, 01:53 PM   #4
Jesse86
Registered User
1997 ZJ 
 
Join Date: Mar 2010
Location: Clearwater, Florida
Posts: 4
Here is the video of it running.
YouTube (Short URL)
?start=" frameborder="0" allowfullscreen>

Last edited by Jesse86; 07-15-2014 at 04:11 PM..
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Unread 07-15-2014, 01:56 PM   #5
zj97ltd
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1993 ZJ 
 
Join Date: Apr 2013
Location: Cape Coral, FLORIDA
Posts: 1,078
Maybe you have an injector sticking open, dumping extra fuel into a cylinder.
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Unread 07-15-2014, 07:26 PM   #6
ZeeJay1997
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1997 ZJ 
 
Join Date: Dec 2008
Location: Sweet Home, Alabama
Posts: 6,889
disconnect the front o2 sensor and see if it runs better. also check the wiring o2 wiring for damage.

if it runs better with the o2 sensor disconnected, replace it with an NTK, do not use a Bosch sensor.
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Unread 07-17-2014, 09:49 AM   #7
Jesse86
Registered User
1997 ZJ 
 
Join Date: Mar 2010
Location: Clearwater, Florida
Posts: 4
Quote:
Originally Posted by ZeeJay1997 View Post
disconnect the front o2 sensor and see if it runs better. also check the wiring o2 wiring for damage.

if it runs better with the o2 sensor disconnected, replace it with an NTK, do not use a Bosch sensor.
ZeeJay you are the man!

Unplugged the upstream o2 sensor and I ran better. Swapped it out with a new one and now It's good to go!

It's strange, no codes at all....

Thanks again!!
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