RPMs dip / engine stalls at idle (intermittent)

Mercury

TJ Enthusiast
Joined
Jul 18, 2018
Messages
225
Location
Louisville, KY, USA
Hello!

So, strange thing that happens intermittently. I'll be at idle, sometimes it's right after I start the Jeep, sometimes I'll have been driving for hours and come to a light. The RPMs will start to lower, around 600 or so, then they'll drop to the 300s, the engine will rev a bit and the needle will bounce back up to the 800, then it'll dip again, and so on. Sometimes it'll just do that bouncing thing, sometimes it'll dip low enough to stall. This can continue for the rest of the drive, where it's driveable but whenever I stop I have to feather the accelerator, or it may just be that one stop. There may be weeks between incidents, or it may happen back to back.

I don't know if this is related to other problems my Jeep has, or if maybe this is exacerbating the others or vice versa.

Does anyone have any ideas?

99 TJ 4.0 Manual
 
You know, that occurred to me for another issue. The battery is an Interstate LA with 11 months on it, but I'm hard on batteries, so maybe I do need to upgrade.
 
When your foot is off the gas the engine depends on the IAC (idle air control) for all of its air. When it gets dirty from combustion byproducts it will cause idle problems. I'd clean it with an aerosol throttle body sprayed into the throttle body opening while the engine is idling. The IAC will suck the cleaner in through itself and its air passages and get the entire IAC system cleaner than if just the IAC was removed and cleaned. The IAC will flood with the cleaner and the engine will stall which is normal. Let it sit a few seconds to let the cleaner work then restart the engine and do it repeatedly until somewhere around 1/3 of the can of cleaner has passed through the IAC. Don't raise the engine rpms to stop the engine from dying, that'll cause the cleaner to bypass the IAC.

Aim the spray at the IAC passage inlet where the IAC draws its air in through...

TB & TPS Sensor Locations.jpg
 
I'm eager to hear what others have to say, but my first thought is that perhaps you should clean your idle air control. @Jerry Bransford once gave me instructions on how to do that, so I wonder if he agrees that this would help in your situation.

Edit: https://wranglertjforum.com/threads/how-to-clean-your-iac-idle-air-controller-valve.662/

I had the same thought and recently did this TB cleaning, but I didn't spray it directly into the IAC outlet like Jerry just described. I'll pick up a can of TB cleaner tomorrow and will try this.
 
I had the same thought and recently did this TB cleaning, but I didn't spray it directly into the IAC outlet like Jerry just described. I'll pick up a can of TB cleaner tomorrow and will try this.

When you do, remember this ...

Don't raise the engine rpms to stop the engine from dying, that'll cause the cleaner to bypass the IAC.
 
I haven't run the TB cleaner yet. Strangest thing today, I let the Jeep idle and warm for about 20 -30 minutes (it was covered in ice). It did fine, still heard the miss, but ran fine. The miss got louder towards the end. Once I started driving, the minute I'd let off the accelerator the RPMs would dip and it would try to stall again. I was able to pick up the TB cleaner, but every time I stopped I had to rest my foot on the skinny pedal.

Tomorrow I'll replace the IAC with the previous one (turns out it's an SMP, not Mopar) and run the TB cleaner through it all again.
 
I had an oddball problem with similar symptoms. I bought a 98 TJ 4.0 5 speed for my DD. Started having similar to your issues. Went through a lot of the suggestions here, standard stuff and parts replacement over a couple weeks to finally accidentally discover the solution. The spark wires had been changed not long before I purchased it and looked in good shape. The coil wire had a 90 off the rotor end to a straight end into the coil leaning against a bracket. I Just happened to see a spark jumping through the boot on the coil end to the bracket it was leaning against at the end of the boot. I put a 90 to 90 coil wire on, not touching anything and never have had a hickup since. It was hot summer, but suspect a little more humidity after first start up, don't think we had any rain during that time, also I suspect spark voltage is higher at low RPM to make the jump, but it started affecting it at driving speed later. Yep, I saved a few future replacement parts if needed. Good luck to you easily finding your solution. This is a great site with so many good people sharing anything they can. Thanks to all of you.
 
Last edited:
I had an oddball problem with similar symptoms. I bought a 98 TJ 4.0 5 speed for my DD. Started having similar to your issues. Went through a lot of the suggestions here, standard stuff and parts replacement over a couple weeks to finally accidentally discover the solution. The spark wires had been changed not long before I purchased it and looked in good shape. The coil wire had a 90 off the rotor end to a straight end into the coil leaning against a bracket. I Just happened to see a spark jumping through the boot on the coil end to the bracket it was leaning against at the end of the boot. I put a 90 to 90 coil wire on, not touching anything and never have had a hickup since. It was hot summer, but suspect a little more humidity after first start up, don't think we had any rain during that time, also I suspect spark voltage is higher at low RPM to make the jump, but it started affecting it at driving speed later. Yep, I saved a few future replacement parts if needed. Good luck to you easily finding your solution. This is a great site with so many good people sharing anything they can. Thanks to all of you.

Interesting! Just spark jumping from through the wires. Shoot, I'll check it tomorrow. I'm open to just about any suggestion at this point 😆
 
  • Like
Reactions: 801 Driver
Left my lights on once waiting on my kid .. had to jump of the jeep ... 3 times and keep it running with the pedal for a bit before it would stay running .. after it was fine .. guessing the ecm registers battery voltage and has issues below a certain volt
 
  • Like
Reactions: 801 Driver
Left my lights on once waiting on my kid .. had to jump of the jeep ... 3 times and keep it running with the pedal for a bit before it would stay running .. after it was fine .. guessing the ecm registers battery voltage and has issues below a certain volt

Learning the 4.0 is fairly sensitive to voltage issues. When my alternator was going out I had all sorts of O2 sensor codes, both voltage and heater circuit, and they would switch around which sensor had which code. Pulled my hair for nearly a year, switched the alternator for a slightly larger model, suddenly no O2 codes.
 
  • Like
Reactions: 801 Driver
One of my statements in life. "Technology is wonderful, but only when it is working." —- to me includes our computers, ECMs, and about everything else that is the "new improved electronic model." 1955 Chevrolet owners manual included instructions on how to adjust the valve lifter gap every 35k miles. 2020 Chevrolet owners manual has a warning "Don't drink the battery acid."

Glancing to make sure your wires are not near a ground like mine were would be an easy visual sweep. TB cleaner would be my next first thing to look at. Yep you might have done recently, but, ya know? Switch sensor next. Good luck to you. Iffen neither works, come back for more ideas of what to ck. JMO Hell, most of us are here to help you spend $ doing something to your Jeep, ya know.



+
 
Last edited:
  • Like
Reactions: Jerry Bransford
One of my statements in life. "Technology is wonderful, but only when it is working." —- to me includes our computers, ECMs, and about everything else that is the "new improved electronic model." 1955 Chevrolet owners manual included instructions on how to adjust the valve lifter gap every 35k miles. 2020 Chevrolet owners manual has a warning "Don't drink the battery acid."

Glancing to make sure your wires are not near a ground like mine were would be an easy visual sweep. TB cleaner would be my next first thing to look at. Yep you might have done recently, but, ya know? Switch sensor next. Good luck to you. Iffen neither works, come back for more ideas of what to ck. JMO Hell, most of us are here to help you spend $ doing something to your Jeep, ya know.



+

We were a bit more self reliant back in the day 😆

It doesn't look like any of mine were near a ground, but it was worth checking.

Replaced the IAC with the old, SMP one, and ran TB cleaner through the IAC port, about 1/3 a can. When I had the IAC out I made sure to scrub it out with a toothbrush and TB cleaner. Now it's all buttoned back together. It was idling fine after I replaced it, idling better after the cleaning. Miss is still there, and I haven't had a chance to test drive it yet and see if the issue persists, but working on it.

Ran smoke through the intake again, didn't notice any leaks, but I could see underneath the intake, so it's always possible there's a leak there.

I ordered a timing light (couldn't find a rental anywhere) and it'll be in tomorrow. We'll see if there's an erratic anything.
 
If you have a miss the IAC will not work properly. It has preset values that it aims for. Misfires will require more airflow than normal and cause the idle to be low. It's shocking to me how many times folks will fail the IAC, TPS, MAP etc. when there are clearly other issues. I can count on one hand in my 20+ years of experience as a tech how many of each of those I have actually diagnosed as bad.
 
If you have a miss the IAC will not work properly. It has preset values that it aims for. Misfires will require more airflow than normal and cause the idle to be low. It's shocking to me how many times folks will fail the IAC, TPS, MAP etc. when there are clearly other issues. I can count on one hand in my 20+ years of experience as a tech how many of each of those I have actually diagnosed as bad.

It's just strange that plenty of times I wouldn't have this issue for hundreds of miles of driving, then start it the next day and this happens the whole day. Then the following day, it's fine. Just haven't found any sort of environmental or driving factors that preface the stalling.

Do you have any ideas for diagnostics?
 
have you checked or changed your spark plugs ? that's normally the 1st thing I check , seen some huge gaps on 2 different TJ's it seems people don't like to change them !
 
It's just strange that plenty of times I wouldn't have this issue for hundreds of miles of driving, then start it the next day and this happens the whole day. Then the following day, it's fine. Just haven't found any sort of environmental or driving factors that preface the stalling.

Do you have any ideas for diagnostics?

Intermittent misfires can be ignition like coil, cap, rotor, wires or plugs. It can also be fuel like low pressure, bad injectors, or bad PCM. It can also be mechanical like a leaking head gasket, bad lifters or a sticking valve. Really too much for the internet to diagnose. I recommend if you are unable to self check those items to take it to a shop.