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rough idle for few seconds after startup

M

muddeprived

NAXJA Forum User
Location
PA
Need some assistance here. I'm experiencing this rough idle that occurs right after you start the jeep after it has sat overnight. It would start up just fine but then idle rough and start rocking back and forth for just a few seconds, i would say 3-5 seconds. After that it idles like normal. I've noticed the past couple months that it would shake a little when I come to a stop. The idle is just a little off and you get a slight rumble from the motor every couple seconds. I have no codes.

Can you tell me where I should start looking? My haynes manual isn't much help and i've searched through so many threads trying to find something similar but most of them talk about rough idle that occurs when accelerating or all the time. I don't have those problems.

i have a 2001 sport.
 
Here's my IAC:

PIC-0079.jpg


PIC-0080.jpg


What's a good way to clean that tip? Do you think it's bad enough to cause my idle issue?
 
I managed to scrape it all off with a q-tip and water. It came out lookin like new so I stuck it back on and reassembled. I'm gonna wait an hour or so just in case a little bit of water may have gotten under the sensor cap. The throttle body isn't raelly gunked up. It's blackened with carbon buildup but not thick. Just a thin layer of it from the throttle plate down.
 
Don't use regular "carb cleaner", make sure you get cleaner specified for TB.

Toothbrush, q-tips, TB cleaner, elbow grease. Be gentle with the tip of the IAC, it can break.

As for a rough idle after sitting overnight that clears up after a bit--pooling fuel in intake/cylinder from an injector leaking down overnight.
 
joe_peters said:
Don't use regular "carb cleaner", make sure you get cleaner specified for TB.

Toothbrush, q-tips, TB cleaner, elbow grease. Be gentle with the tip of the IAC, it can break.

As for a rough idle after sitting overnight that clears up after a bit--pooling fuel in intake/cylinder from an injector leaking down overnight.
Click to expand...
x2 Also, be careful wiyh water around any of your electrical sensors. It corrodes them after a short time.
 
joe_peters said:
Don't use regular "carb cleaner", make sure you get cleaner specified for TB.

Toothbrush, q-tips, TB cleaner, elbow grease. Be gentle with the tip of the IAC, it can break.

As for a rough idle after sitting overnight that clears up after a bit--pooling fuel in intake/cylinder from an injector leaking down overnight.
Click to expand...

Thanks for the tip on the leaking injectors. Is there a way to test to see if they are leaking? I don't want to just randomly replace them cuz they are like $40 each...and I have 6.
 
muddeprived said:
Thanks for the tip on the leaking injectors. Is there a way to test to see if they are leaking? I don't want to just randomly replace them cuz they are like $40 each...and I have 6.
Click to expand...

Just noticed that you have 2001, and that would put you in the "heat soak" problem range.

Do a search for heat soak. Dealership fix was an insulation kit for the #3 injector, pretty mickey mouse.

I would look for the problem with #3.

Best bet would be to lower the engine compartment temperature after shutdown, you could try a timer on the efan so it would run 2-3 minutes after shutdown.
 
I think this "rough idle that occurs right after you start the jeep after it has sat overnight" elliminates the heat soak option.

Could be a bad B+ timer relay (IIRC) not properly resetting the IAC after shut down, but I suspect the only happens over night issue kills that idea too.

What about the battery cables?

Being a 2001 with no error codes make it tough to isolate. I am still leaning towards the prior suggestion of a leaky injector. A hot engine would flash a small slow leak quickly, while an overnight cool down, with a slow tinny leak would be another story.

One way to test this might be to bleed the fuel rail into a cup right after shut down then try it again the next morning. If the problem is gone it is a leaky injector(s).

Seafoam in the fuel and intake might help if it is a leaky injector. But be warned a leaky injector can wipe out cylinder compression quickly if the leak is bad enough. And it might never throw a code before the damage is done, though a miss fire code usually appears well before it gets too bad with OBD-II systems.
 
not to hijack, but my 98 has been having this problem lately with rough idle for a few seconds after sitting overnight. It sounds like it is a leaky injector from reading on here.

Is this a serious issue that should be taken care of or just a nuisance? My computer does through a misfire code. And how do I know which injector it is, or do you have to replace all of them?
 
jeepster_259 said:
not to hijack, but my 98 has been having this problem lately with rough idle for a few seconds after sitting overnight. It sounds like it is a leaky injector from reading on here.

Is this a serious issue that should be taken care of or just a nuisance? My computer does through a misfire code. And how do I know which injector it is, or do you have to replace all of them?
Click to expand...

If they are old enough I would replace them. I got my new ones pretty cheap at fiveomotorsports.com

A set of new injectors can save a lot of gas over time, and is la ot cheaper than replacing or rebuilding an engine.

I think you can pull the fuel rail with injectors attached, and turn the ignition switch to on (do not start it) which cycles the fuel pump a few seconds. Then watch for leakers. Make sure the metal retainer clips are still on all the injector inlet side to hold them to fuel rail!!!!!
 
You can get them on ebay (cleaned and tested) WAY cheaper than that. I had to laugh this weekend on the Trucks (IIRC) series on Spike TV. THey spent $777 on new injectors for their Cheep Cherokee stroker buildup.
 
jeepster_259 said:
yikes, $250 for all 6.

What can happen if you don't replace them?
Click to expand...

Which ones are you looking at? I think I paid about $165 for the set, but I forget the exact price. One of the best investments I made. Mine were the old OEMs, leaking externally (fire hazard), and not running that great internally.

I got the Ford/Bosch Yellow tops, $215.60 for a set of 6 now. Still half of what others charge.

Pull the fuel rail and run the leak test I described first.
 
I'm gonna check the injectors. It's got 92k miles on the oem injectors. I'll letcha know what I find out.

I REALLY hate these 4.0's, although they are reliable, just a pain in the ASS to diagnose. I abused the piss out of my tj's 2.5 for years, covering that s.o.b. in mud all the time and never had a single motor-related issue. I get this xj a year ago and i've had 7 cel's, hot running motor, and a rough idle that's impossible to figure out...and I didn't get a chance to cover it in mud. :rof: Grrrr....
 
Ecomike said:
Which ones are you looking at? I think I paid about $165 for the set, but I forget the exact price. One of the best investments I made. Mine were the old OEMs, leaking externally (fire hazard), and not running that great internally.

I got the Ford/Bosch Yellow tops, $215.60 for a set of 6 now. Still half of what others charge.

Pull the fuel rail and run the leak test I described first.
Click to expand...

One question, do I have to pull the fuel rail while it's cold or can I do it once it's already been run?
 
muddeprived said:
One question, do I have to pull the fuel rail while it's cold or can I do it once it's already been run?
Click to expand...

No problem, just don't get burned. Let it cool off some, but no, it does not need to be cold.
 
Talyn said:
Cool is better... and depressurize the fuel rail first.
Click to expand...

Why?

If he makes sure the upper injector metal push on keepers are still there and in place it will not leak. He is just pulling the rail, so he can switch the power on (but not start) to check the injectors ends for leaks at the tips!

Be sure to buy a set of injector o'rings, to replace the o'rings before you re assemble it. Also a little motor oil on the o'rings is a must when you reinstall.
 
napa store in my area can clean and test fuel injectors for flow
 
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