98 rough idle with no cel

[grasshoppa]

hello everyone. Ive searched on here for a week and can not find anybody with the same problem.
98 xj 4.0 4x4
1. When the motor is cold the motor rolls over 4 or 5 times before it starts, but if you run it down the road turn it off and start it back it fires right up.

2. the jeep idles like crap. It feels like it is stumbling. It drives just fine with no considerable loss of power but at a red light I can look at my hand trembling pretty good from the steering wheel. There is no CEL light on.

I was hoping it might be the temp sensor not functioning properly . I pulled the plug wires one at a time while it was running and every plug wire i pulled it ran a little bit rougher but not much (i could still tell a diff between plug wire on and not). I sprayed starting fluid all over trying to find a vac leak but got nothing. I changed the plugs and was going to start buying diff parts but i dont have the money to chase this problem around (damn $200 bucks for fuel pump). i figured maybe someone would have a solution. thanks

 

[birchlakeXJ]

Finish your tuneup. Can't look to more complex causes until that is done. Champion copper plugs are a good choice for your 98, (ensure gap is .035) new plug wires, distributor cap and rotor. The importance of a fresh secondary ignition cannot be overstated.

The extended crank time is often tied to a faulty check valve on the fuel pump assembly. Did you replace just the pump on the assembly or the entire assembly? What brand?

 

[Flyfisher]

How long since a general tune-up? Plugs, wires, dist. cap, etc? Why are you thinking fuel pump? $$ better spend with a fuel pressure tester.

 

[kdailey4315]

I'm having the same exact problem as you are having right now. Mine is a 98 Limited. Here's what I've changed that has made zero difference:
Full tune up. Plugs, wires, cap, rotor.
TSP
MAP
IAC
Coolant Sensor
I'm guessing that leaves me with an injector problem (which i think would throw a code) or a vacuum leak somewhere.

 

[Reverend Jim]

My 98 Sport was doing the same thing a month ago. Ran 2 bottles of Techron fuel injector cleaner in the next 2 tank fill-ups and it idles fine now. Maybe I got lucky:dunno:

 

[dcampbell98XJ]

you may not think that the O2 sensor has anything to do with it but it is possible. My 98 classic has had a rough idle on start up since i bought it 2 years ago. couldnt figure it out. did a tune up, ran injector cleaner and everything then i started to replace parts. i bought a TPS, CPS, cap and rotor, plugs wires and everything. still nothing. then i decided to check the connections on my upstream O2 sensor and when i started doing that it looked like the original with 160,000 miles i decided i would replace it. It was seized on there and stripped out. after i touched it the CEL started coming on. finally replaced it last week, and NO more rough idle. so it might be worth taking a look at rather then changing a fuel pump or injectors.

 

[Matthew Currie]

I'm having the same exact problem as you are having right now. Mine is a 98 Limited. Here's what I've changed that has made zero difference:
Full tune up. Plugs, wires, cap, rotor.
TSP
MAP
IAC
Coolant Sensor
I'm guessing that leaves me with an injector problem (which i think would throw a code) or a vacuum leak somewhere.

A mechanical injector problem - clogged, leaking, etc. will not throw a code. It might be time to test for that, but first I'd second the suggestion of a bottle or two of Techron or the like, to see if it helps.

 

[kdailey4315]

A mechanical injector problem - clogged, leaking, etc. will not throw a code. It might be time to test for that, but first I'd second the suggestion of a bottle or two of Techron or the like, to see if it helps.

That's what I think my problem is. I'm going to run a can of BK44 (I think that's what it's called) through when I hit 1/4 tank.

 

[RenegadeStang]

That's what I think my problem is. I'm going to run a can of BK44 (I think that's what it's called) through when I hit 1/4 tank.

You've got burgers on the mind. :confused1 BG's 44K is pretty good stuff.

On a seperate note, I was having rough idle issues on my '98 a few years back. Replaced all six of my power and ground cables with upgraded cables I made myself, got rid of the problems. I figure there was corrosion inside some of the cables.

 

[kdailey4315]

You've got burgers on the mind. :confused1 BG's 44K is pretty good stuff.

On a seperate note, I was having rough idle issues on my '98 a few years back. Replaced all six of my power and ground cables with upgraded cables I made myself, got rid of the problems. I figure there was corrosion inside some of the cables.

HAHA. If BK still had the Italian Chicken Sandwich I would weigh 500lbs.

 

[O-Gauge Steamer]

Nearly 2 years ago I picked up the 98 I am chipping away at. It had issues which caused the price to drop like a rock. Ran rough, was throwing the dreaded P0700/P0705 codes, was missing the front right fender flare and the brush guard was crooked. Not to mention the dent in the roof.

My then 19 year old could not believe I bought it. On the way home... Buy a set of plugs/wires/cap/rotor and a can of injector cleaner. Had the boy change out the ignition parts. From the look of the cap, it still had the factory parts on it at 125,000 miles. Cleaned the NSS. Ran the can of cleaner, added 5 gallons of fuel and another can of cleaner... Straightened out the brush guard mounting bracket (it is good to own an oxy-acetylene torch) and replaced the flare.

I also replaced all of the power delivery cables with custom ones made by my favourite battery shop here.

Started running perfectly. So good, in fact, that I added a Supercharger to it. As for the dent on the roof, you can't see it due to the lift so it does not matter...

 

[Ecomike]

Pull the spark plugs and examine them. You can tell a lot by examining color, and spark gap, fouling if any and type of fouling on each plug. Inspect the rotor and cap for erosion, corrosion, dust film (that causes missfire / crossfiring), cracks or other damage. Check the Dizzy shaft for side to side slop.

Remove and clean the IAC and clean the throttle body port of the IAC. Many good how too threads on that, google it. IAC =I dle Air Control valve.

Test the sensors with a cheap ohm meter, especially the CTS. Make sure the vacuum line to the MAP sensor is tight, no vac leaks.

TEST and verify the fuel pressure. many parts stores loan test tools for free (just need a security deposit). Use a code scanner to check for pending codes (they do not set of CEL lights!!!).

 

[NW99XJ]

Cant believe no one has asked whether or not you have an exhaust leak?
I'd get all up close and personal with your headers, look them over with a fine tooth comb. Often times a crack in the exhaust manifold cant be seen but could be heard. Listen closely after a cold start and after it's ran a while to see if you can hear any leakage.
I'd second the motion for a fuel pressure test. This will let you know what the regulator and the pump are doing as well as let you know if you have any leaky injectors.
You should be right around 49psi ±5....if you have that at idle your good, but if it drops after you shut off the engine, then you have a leaky injector. This could be fixed by a new set of O-rings. It of course wont hurt to clean the fuel system, you're on the right track there. All good advice too with the o2 sensors and doing a tune up. DEFINITELY clean up the throttle body and the IAC. (the iac is easy, its two T25 screws on the side - get a soft parts brush and a can of throttle body cleaner and go to town on it.) The advice on the battery cables is actually pretty good too. The Jeep engines are very finicky when it comes to voltage. A shotty ground will mess up more than you can imagine. Cables can be made pretty easy, or found thru various shops around town.

 

[praetorian]

Something else u might want to check is the fuel pump relay. Before mine went out I was have starting issues mostly but some rough idling when it would first start up. I switched the relay for the electric fan relay and the problems went away.

 

[shaman1204]

I have (am?) chasing this exact same problem. The difference is my 2000 fires right up. It has a rough idle that disappears once the RPMs go up. I have:

replaced all sensors on the throttle body
checked compression (in spec)
checked fuel pressure (in spec)
replaced fuel injectors with Mustang 4 pintle injectors
ran various cans of fuel system cleaner through tanks of gas.
replaced all four O2 sensors (i have the Cali exhaust)
hooked it up to a scanner, closed loop happens as it should, my fuel trim levels are all in spec, so it's not running rich/lean

Once it warms up I am going to check the transmission mount. I have read that if they are bad it will transfer vibrations all through the Jeep.

If that doesn't do it I am going to ignore it. It doesn't seem to be hurting anything. Yet.

 

[Ecomike]

Did you check for the 12 volts to the front 2- O2 sensor's electric heaters? If they don't get that 12 volts at idle, the O2 sensors get too cold and it causes a rough idle. There is a relay, O2 sensor power relay, and sometimes the wires get damaged, too close to front drive shaft or E manifold.

Unless yours is newer, where the Precats replaced the electric heaters? I am not sure when that happened? I do not know, yet, what happens if the Pre cats go bad, but it may be the same as loss of the 12 volts to the heater on the older models. The precats are there to get the O2 sensor hot early, and keep them hot at idle.

One way to check is to back probe the front O2 sensor(s) one at a time, and monitor them for 5-10 minutes at idle. If it switches to rich, after working properly at 2000 rpm for a minute, that may be the issue.

Most of my rough idle issues have bad plugs, wires, cap rotor, or injector related, or a bad head gasket, or bad grounds, battery connections.