Rough Idle and "Jerky" Acceleration

I have a 1998 Cherokee Sport, 4L 6 Cylinder engine. 146,000 miles.

For about a year and a half I have an issue with jerky acceleration at lower speeds. Around the same time I started experiencing some problems with a rough idle. When the engine is cold it idles very loudly and the RPMs oscillate in an inconsistent pattern. During this, the exhaust system is very loud and is shaking underneath the vehicle. On several rare occasions the engine has shut off because the RPMs have dropped so low.

I’m not sure if the two problems (idle and acceleration) are related. It feels like the same thing because my RPMs also fluctuate when driving at lower speeds. When this happens and the RPMs drop, it feels similar to running out of gas. Also when the jerky acceleration happens, I hear a clunk/click and I can feel that clunk/click in the gas pedal. Spark plugs were replaced just a few months before this all started.

This happens very inconsistently. I had a mechanic look at it, but it wouldn’t happen when he was in the car. Lately, it has become less inconsistent and happens almost every time I drive. Outside temperature seems to have no effect. It almost always stops after a few minutes and the engine is warm. But even this is inconsistent as sometimes I will drive for a long time, stop the car to gas up, and the problem is back even though the engine is plenty warm. The only consistency is that it’s always at the beginning of my drive. This never happens after accelerating away from a Red light for example.

Things I’ve tried:

  • Tested fuel injectors (all fine)
  • Manually cleaning fuel injectors
  • Manually cleaning idle air control
  • Using a fuel conditioner/additive somewhat regularly for the past 6 months.
  • Tested Fuel Pressure (was fine the one time I checked it)

Any ideas? Thank you! First time poster, long time reader.

You can clean a faulty Idle Air Control valve.

But it still remains a faulty Idle Air Control valve.


Would the idle air control still cause problems when not idling?


There’s a transition from idling to accelerating.

And if the IAC valve is lazy to move to the proper position from idling to accelerating and allows too much air into the engine, the engine misfires.


Sounds like a pretty inexpensive fix. Can’t hurt to try. I’ll report back.

Wouldn’t the engine check light come on if the valve is faulty?


It’s an engine control device.

Not an emission control device.



Replace the upstream oxygen sensor.

These engines go into closed loop very quickly, if the oxygen sensor is slow reacting the engine will run lean. This problem goes away after the oxygen sensor becomes hot enough to perform properly. You can observe the oxygen sensor input and problem with a scan tool.

Other possibilities could be bad fuel filter or bad plug wires. Don’t know about newer cars, but when older cars start to get tired and start leaking around the valve covers, when the oil gets on the plug wires they don’t conduct current as well and you can get a rough idle or miss.

This might be another helpful tid bit. During the rough idle while the
engine is cold, the exhaust is very white. I don’t know if it’s smoke or
steam. My gut says steam.

When the rough idling is over, the exhaust goes back to normal.

I’m not a Jeep expert, but I seem to remember hearing the 4.0 straight six was known for cracked heads, for certain model years . . . ?

YES and coolant will seep into the engine oil if head cracks.
I was entertaining an idea about Cherokee of 1998-2001 years at some point, so I recall some info :slight_smile:
Easy to google for images how it looks with the oil-filling cap removed.
Some later head casts were made to have no such an issue: once again, one can look with a flashlight through the same hole and find cast number to see it head is from faulty series or not.
Google to the help :slight_smile:

The symptoms made me think of the cracked head

A block test would quickly determine if combustion gases are getting into the coolant

1 Like

This is a tough one, as there’s many things that can cause it. It sounds like you’ve done the common sense diy’er things already. It does sort of sound like the engine is running lean and/or the idle air control gadget is malfunctioning, so the above ideas are good ones. I’ve experienced a jerky acceleration problem before w/ my Corolla during neighborhood driving, but that happened all the time, not just when the engine was cold. If anything the symptom was reduced when the engine was cold. So you probably got a different reason. That one for me turned out to be caused by a combination of things, but the most important was the fuel injectors were a little clogged. One of the other culprits was the throttle position sensor was intermittently malfunctioning, so you might want to give that a go as a possible cause for your problem.

Just speculating now, but if the throttle body is gunked up w/egr and pcv residue, that could cause it. As well a faulty/dirty MAF. So giving all that a cleaning might be worthwhile. Other ideas

  • check for pending diagnostic codes
  • make sure the engine is running at the correct temperature
  • a thorough test for vacuum leaks
  • if you have a conventional distributor w/electronic ignition, check for carbon tracking, replace the dizzy cap, rotor, all the spark plug wires, and the spark plugs. if you can figure out a way to get a pro to evaluate w/an o’scope test of the ignition system, so much the better. look for sparks jumping round in the engine compartment when it is dark is the diy’er alternative.
  • check compression and (if applicable) valve clearances

White smoke out the exhaust when the engine is cold and the ambient temp is cool or cold is normal. As long as it goes away after 5 minutes or so. It’s just water vapor, a byproduct of the combustion of gasoline. If you got white smoke coming out the tailpipe during acceleration when the engine is warm, that’s another story. One you probably don’t want to hear.

Thanks you guys I appreciate all the suggestions.

When I was digging around under the hood I found a sensor that was just
sitting there, unattached and not plugged in. After a little bit of
searching I have discovered that it’s the crankshaft position sensor. I
don’t know why this would be unplugged and unbolted as I’ve heard this is a
difficult piece to unattach and reattach.

Anyway, could this missing senor be a problem? When I looked up what it
does, I saw that it does help regulate fuel injection. I’m a bit of a car
newbie but I am determined to learn!

Thanks guys.

Are you sure it’s the crankshaft position sensor? And not the camshaft position sensor?

Because without the crankshaft position sensor connected, the engine won’t start/run.

Where it will with the camshaft position sensor disconnected.

The crank sensor is located on the bell housing for the transmission


This is what I found

Could be that the previous owner replaced it and left the old one just hanging in there. I’ll have to check more thoroughly next time I’m out there.

I’ve replaced many of these sensors.

And it’s possible the old one was left when the it was replaced.

You usually plug the sensor in at the connector, and then feed it down the back of the engine to the bell housing.


On many vehicles of that era the crankshaft position sensor is the only way the engine computer knows when and in which order to fire the spark plugs. So the above comment is correct, the engine is unlikely to even start without that sensor being installed & connected. So you the one in the photo must be an old sensor that was removed at some point, and replaced with a new one. The shop just forgot to remove it from the engine compartment.

It’s possible the thing that the sensor senses is faulty, and that could cause all sorts of engine naughtiness, including your symptom. Those sensors are known to fail to throw diagnostic codes (& to not turn on the check engine light) sometimes, even when they are faulty. The computer has a difficult time to know they have failed if they fail intermittently.

There’s one thing that is inconsistent with the sensor being the cause of your symptom tho. CK Sensor malfunctions usually occur when the engine is hot, especially likely to happen when the ambient temperature is hot, the engine is fully warmed up, you pop into 7-11 for a big gulp, only to find the car won’t start when you return to the parking lot. That doesn’t sound like your symptom. A shop w/a Jeep scan tool can probably test it for you to rule it out as a possible cause. If not, start with the ideas posted above.