4.0L runs good for 30sec then rough - JeepForum.com
LinkBack Thread Tools
post #1 of 4 Old 08-24-2013, 07:59 PM Thread Starter
Registered User
Join Date: Nov 2011
Location: freeport
Posts: 16
4.0L runs good for 30sec then rough

I have a 99 4.0L with 96k. All of the sudden it has been acting strange. It will start and idle good for 30sec. Then it doesn't want to rev and will backfire through the tb and sputter but won't die just idle rough and won't rev. It will do this for about 1min then it's fine for the rest of the trip. If it sits not running for longer than 30mins it will do it all over again. Sometimes the revs will come down real slow. And also there is no check engine light or codes.

Today I pulled the tb and cleaned the iac and installed a new tps. When I was in the driveway it was idling rough and I could hear the tb sucking air like crazy. And if I remember correctly my Ram with a bad plenum gasket did the same thing. So do I have a vacuum leak or is it something else? I don't know if its connected but my ac will switch on its own from vents to defrost and then back to vents. Kind of weird and one of my WJ's quirks.

Last edited by bergoff; 08-24-2013 at 08:28 PM.
bergoff is offline  
Sponsored Links
post #2 of 4 Old 10-27-2013, 07:55 PM
Registered User
1997 TJ Wrangler 
Join Date: Nov 2008
Location: Florence, KY
Posts: 12
I am having the same issue with my 2000 Gr Cherokee 4.0. It will start up and run perfectly, once it warms up a little and sometimes it happens once it warms up all the way, but it happens everytime I drive it. It bucks and backfires, and misses. It then stops all together after giving it some gas, and then its fine the rest of the time until it gets cold again and I start it and drive it again.

Im just starting to diagnose, was wondering if you came up with anything, or if anyone knows where to start. I'm thinking intake leak. Faulty fuel pump. Faulty fuel relay.
It has new plugs, new air filter, it has 177,000 miles on it. It's definatley weird, that it only does it once it's warm, and only once at idle , but then at idle for the rest of the time it's running, no issue.
str8six is offline  
post #3 of 4 Old 10-28-2013, 10:53 AM
Registered User
Join Date: Jun 2013
Posts: 168
Bad TPS can cause running issues…just a thought. Easy to test with a multi metre
rockatansky666 is offline  
post #4 of 4 Old 10-28-2013, 01:19 PM
Registered User
1997 TJ Wrangler 
Join Date: Nov 2008
Location: Florence, KY
Posts: 12
I fixed the issue. It was the front O2 sensor. It had a faulty heater circuit. It wouldn't heat up as fast as the car or something like that, so as soon as the jeep got to operating temp, the front 02 sensor gave the computer the wrong reading, causing it to run very lean. But new O2 sensor and all is good.
str8six is offline  

Quick Reply

Register Now

In order to be able to post messages on the JeepForum.com forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Please enter a password for your user account. Note that passwords are case-sensitive.


Confirm Password:
Email Address
Please enter a valid e-mail address for yourself.

Email Address:


Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page

Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On

For the best viewing experience please update your browser to Google Chrome