Jeep 4.0l idle

paradisePWoffrd

paradisePWoffrd

Recovering Project Junkie
Joined
Nov 5, 2005
Location
Newton, NC
Ive got a 98 ZJ with a 4.0l and ~130k on the clock. It has was seems like a slight miss at idle, when in park. When you put it in gear, the idle is rough. I cant completely tell if it is low RPM or missing. No codes.

For more info/back story.
Jeep ran great with no issues. Overheated one day on the way home. Through some troubleshooting I determined it was a bad head gasket. No milky oil, but exhaust and oil in the coolant. Replaced head gasket. Checked head with straight edge. Didnt notice an cracks. Replaced thermostat, water pump, radiator, fan clutch, plug wires while in there. Before the head gasket it was running rough, after overheating. After it runs much better other than this slight miss. Pulled the throttle body and cleaned it and the IAC, with no change. If I pull the plugs on the injectors while idling, one by one, you can tell it starts to miss more until I connect them back. When pulling #2, it doesnt run as bad as it does when pulling the others. If I pull the #2 plug wire vs the injector, it runs worse than pulling the injector.

I am currently at a loss, not wanting to throw more parts at it. The miss at idle, in park, is enough that it doesnt run smooth, but barely noticable. In gear, it is much worse. Other than idle, it runs and drives as good as ever, until you come to a stop, and it wants to die. Seems like the idle is dropping really low.

Any suggestions?

I have uploaded a video of it in park and in gear. It will run much worse in gear.


 
Last edited:
Sounds like #2 injector might be partially stuck open. You can back flush it with a 9v and carb cleaner or swap the injector around and see if your miss at idle follows it.

Fyi, a good check for turning off cylinders is to put a short lead of vac. Or fuel line from the end of the plug lead to the back of the plug. The spark will jump the gap under normal conditions, if you run a test lead from the negative battery terminal to the hose, the spark will ground through the hose and shut off the cylinder.

I was told pulling plug wires can be a good way to cook an ICU if you aren't careful.
 
Jody Treadway said:
Hold a dollar bill at the tailpipe at idle. See if it occasionally gets sucked in. If so, you have a burnt exhaust valve. Pretty common on overheated 4.0s.
That's exactly where I would start with the diagnosis
Click to expand...

1stgenxxx said:
Is it sucking in dollars like a new girl at the strip club?
Click to expand...

I had some time this morning to look at it. Pretty sure the answer is yes, but I took a video for someone smarter than me to look at.

youtube.com

jeep zj burnt valve test

youtube.com youtube.com


And if so, my only option to pull the head again?
 
Jody Treadway said:
Hold a dollar bill at the tailpipe at idle. See if it occasionally gets sucked in. If so, you have a burnt exhaust valve. Pretty common on overheated 4.0s.
That's exactly where I would start with the diagnosis
Click to expand...
I have a 4.0 with similar problems. Is there a way to tell this when the head is off the engine?
 
I pulled it apart and snapped some pics, incase there is anymore feedback.

20220422_210921.jpg
20220422_210852.jpg
20220422_210917.jpg
20220422_210832.jpg
20220422_210907.jpg
 
To update...

Pulled the head, and the valves out to take a look. All of the valves and seats had carbon build up and alot of pitting. Decided to get the head rebuilt. While it was off I also cleaned the injectors. Put it all back together and it still has the same rough idle/miss.

When it first cranks up it seems to run smooth for ~30sec, then seems to settle into the rough idle. Took it for a drive, and it drives great, but when you stop it shakes bad and seems to be idling low. I tried unplugging the IAC to see if it changed anything, and didnt seem to matter. I have taken it off and cleaned the carbon out of it, but could it be bad?
I have an OBD reader and the Torque app, any chance I can see anything on that to tell?

Here are some more videos of it, if they help.
In park

In gear
 
Daughters TJ had an IAC problem that couldn't be fixed with cleaning. Slapped in a new one and issue has gone away entirely. Her issue was a really rough idle and an occasional no start without pumping the hell out of the gas. Once started you couldn't let it drop to idle until it was warmed up.

The torque app won't really show you what's going on there.
 
willness33 said:
Daughters TJ had an IAC problem that couldn't be fixed with cleaning. Slapped in a new one and issue has gone away entirely. Her issue was a really rough idle and an occasional no start without pumping the hell out of the gas. Once started you couldn't let it drop to idle until it was warmed up.

The torque app won't really show you what's going on there.
Click to expand...
That's what mine did about 15 years ago when I replaced the IAC. Problem solved.
 
To give an update...

A friend had a "good" IAC that i swapped in, with no change. Pulled the dizzy cap to check the shaft play, and cap and rotor were toast. Not sure how it was running at all.

20220515_202249.jpg

20220514_162811.jpg


Changing those and rechecking the plugs didnt fix anything.

After more diagnosis and checking, took a gamble on the Crank sensor. When snapping the throttle open from idle, it would backfire through the intake, which indicated that the timing was off. Changing the Crank sensor was a gamble, but it fixed the problem.
 
Last edited:
Fucking XJs and their crank sensors...

I had to change one in the middle of a supermarket parking lot after it shat the bed on me once.



Glad you got it figured out !
 
Bebop said:
Fucking XJs and their crank sensors...

I had to change one in the middle of a supermarket parking lot after it shat the bed on me once.



Glad you got it figured out !
Click to expand...
most of the time I have seen/heard about them dying from the crank sensor. That is why I didnt think to look at it before. After checking everything else that could effect timing, the CPS and ECU were the only things left.
 
grapehead said:
Wow, that is the worst cap and rotor I've ever seen!
Click to expand...
I couldnt believe it was even running at all. The crazy part is that new parts didnt make it run significantly better.
 
paradisePWoffrd said:
The crazy part is that new parts didnt make it run significantly better.
Click to expand...
That was my takeaway reading through the thread, haha. Apparently everything had adapted over time to have just the right conditions, until the crank sensor failed.
 
Back
Top