engine being funny - Jeep Wrangler Forum
Jeep Wrangler Forum

Go Back   Jeep Wrangler Forum > TJ Jeep Wrangler Forum > TJ Tech Forum

Join Wrangler Forum Today


Reply
 
Thread Tools

Please support our sponsors and let them know you heard about them on WranglerForum.com
Old 05-06-2013, 08:10 AM   #1
Jeeper
 
Join Date: Mar 2011
Posts: 961
engine being funny

Hey this has been going on since I can remember, we bought the Jeep 3 years ago. '04 sport with an inline 6 in it. All stock, and we offroad it somewhat heavily in spring and summer. Well there's a number of things I notice but it's not consistent, just thought I'd ask and see what people think:

1). After driving on the highway for like an hour or 2 then stopping and turning the engine off for 5 or 10 minutes then starting again it idles extremely rough. It shakes and sounds awful. I turn it off then turn it back on and she's smooth as usual!

2). Turn it on move it for a few minutes then turn it off (repositioning in my tiny driveway). Then come back later and turn it on, it can't start up immediately, it turns over for a number of seconds before starting up. (normally it will start right up even in cold weather)

It doesn't seem to run badly at all, just curious what might be the cause of these things.

techflork is offline   Quote
Old 05-06-2013, 08:37 AM   #2
Jeeper
 
Rubi513's Avatar
 
Join Date: Apr 2013
Location: Conroe, Texas
Posts: 1,581
If you ever figure it out, please let me know. Mine does the same thing. Most folks will tell you that you need to take out your IAC and clean it. I have taken my throttle body off and thoroughly cleaned every part including the iac. Still does the same thing. I guess the next step is to change throttle position sensor or iac sensor. Good luck and please do let me know if you figure it out.

__________________
"I have not failed. I've just found 10,000 ways that won't work."
Rubi513 is offline   Quote
Old 05-06-2013, 09:33 AM   #3
Jeeper
 
Yellow Monster TJ's Avatar
 
Join Date: Jun 2012
Location: Noblesville, IN
Posts: 129
I have had the same issues, and some buddies of mine the same thing. Most of the people I have talked to just say its common and don't know what the fix is.
GOOD LUCK!!!
Yellow Monster TJ is offline   Quote
Old 05-06-2013, 10:19 AM   #4
Jeeper
 
UnlimitedLJ04's Avatar
 
Join Date: Aug 2010
Posts: 8,242
all vehicles have little idiosyncrasies. those just happen to be two of the Jeep 4.0L's.

Quote:
Originally Posted by techflork View Post
1). After driving on the highway for like an hour or 2 then stopping and turning the engine off for 5 or 10 minutes then starting again it idles extremely rough. It shakes and sounds awful. I turn it off then turn it back on and she's smooth as usual!
vapor lock. many 4.0L's have a bit of a design flaw around this...hot exhaust manifold heat soaks the aluminum intake manifold, in turn heat soaks the fuel injectors and the fuel rail, enough to boil the fuel in the rail into vapor.

the injectors can't spit vapor, and it confuses the computer. you don't have to restart it to fix this, just punch the throttle for a minute and clear the vapor out.

there was actually a TSB about this around 2000-ish...Jeep's "fix" was to put some heat insulation around injector #3....worked for some, not so much for others.

the root of this problem (IMHO), is the fact that the exhaust manifold and intake manifold are on the same side of the head. many newer motor designs have the exhaust and intake runners on opposite sides of the head...but the 4.0L is basically 1950s-1960s technology.

Quote:
Originally Posted by techflork View Post
2). Turn it on move it for a few minutes then turn it off (repositioning in my tiny driveway). Then come back later and turn it on, it can't start up immediately, it turns over for a number of seconds before starting up. (normally it will start right up even in cold weather)
you turned the jeep off before it had a chance leave open loop and warm up to closed loop. if you think back to carb days, you basically flooded the engine. to somewhat prevent this, rev the throttle before shutting it off, or let the engine warm up above 100°F or so.
UnlimitedLJ04 is offline   Quote
Old 06-01-2013, 08:02 PM   #5
Jeeper
 
Join Date: Mar 2011
Posts: 961
Thanks man! sorry for the late post, that was pretty helpful. Reassuring too!
techflork is offline   Quote
Reply

Thread Tools

Posting Rules
You may not post new threads
You may not 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 Off
Refbacks are Off




Download our Mobile App

» Featured Product

All times are GMT -5. The time now is 05:58 AM.


User Alert System provided by Advanced User Tagging v3.1.0 (Pro) - vBulletin Mods & Addons Copyright © 2015 DragonByte Technologies Ltd.

Jeep®, Wrangler, Liberty, Wagoneer, Cherokee, and Grand Cherokee are copyrighted and trademarked to Chrysler Motors LLC.
Wranglerforum.com is not in any way associated with the Chrysler Motors LLC