RPM dips/engine stalls at idle (intermittent), 99 4.0

[Mercury]

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

 

[TerryMason]

Maybe a vacuum leak, dirty idle air controller, or dirty throttle body.

Does it throw any codes?

 

[Mercury]

Maybe a vacuum leak, dirty idle air controller, or dirty throttle body.

Does it throw any codes?

Would a vacuum leak cause this? I'm pretty sure I'm chasing another vacuum leak. I found one in the throttle body where the throttle shaft rotates at the bearing, so I pulled the old o-rings, added new ones with fresh grease, and shifted the bearing a bit so it has fresh metal on the shaft. It resolved a lot of this missfire issue I'm having, but it's not completely gone. Tomorrow I'm going to run a can of TB cleaner through it while it runs.

I replaced the IAC within the past few months trying to chase the misfire issue, so I'll try switching it back to the original one as well.

 

[JPNinPA]

IAC controls the idle. TPS controls skinny pedal / fuel ratio. If the TPS is not reading zero pedal this rev issue may happen. Or if the TPS had bad connection.
With engine off, by hand open and close the TB butterfly valve 20-50 times. Blow off the TPS with air and unplug and replug it back in.

The only other thing is the fuel pressure. If its not 49 +/-5 you will have similar issues


Never look down on anyone unless you are helping them up - Jesse Jackson

 

[Mercury]

Iac controls the idle. TPS controls skinny pedal / fuel ratio. If the TPS is not reading no pedal this rev may happen. Or if the TPS had bad connection.
With engine off by hand open and close the TB 20-50 times. Blow off the TpS with air and unplug snd replug back in.

Thi only other thing is the fuel pressure. If its not 49 +/-5 you will have similar issues


Never look down on anyone unless you are helping them up - Jesse Jackson

So IAC or TPS could be the issue. The connection should be fairly solid, I had the TB out to replace the o rings a few nights ago and everything was recently buttoned back together. My last TPS went out a few months ago, but the replacement, while it has decent reviews, isn't Mopar, so I may need to replace it again. IAC is new as well, but, like the TPS, isn't Mopar.

Fuel pressure is steady at 48 at idle, dips to 47 for a hair when I press the gas but then goes back to 48.

 

[JPNinPA]

Did you reset your PCM after replacing parts? If you haven’t the system is tring to use old data to run. It will take app500 mi and 50 cold starts to relearn. Before thst it uses factory presets.

When the TPS is not connected the vehicle will have issues at idle. Esp when slowing to a stop. Like a stop sign. Idel will remain high until uou stop then nearly stall. Other signs of abad TPS are blank spots when depressing the skinny pedal. You press down snd rpm remain the same. Press more past the dead zone and it all of a sudden catches up.

And do not wind or unwind the TPS when installing. It should just go on the blade and bolts line up.

Never look down on anyone unless you are helping them up - Jesse Jackson

 

[Mercury]

Did you reset your PCM after replacing parts? If you haven’t the system is tring to use old data to run. It will take app500 mi and 50 cold starts to relearn. Before thst it uses factory presets.

When the TPS is not connected the vehicle will have issues at idle. Esp when slowing to a stop. Like a stop sign. Idel will remain high until uou stop then nearly stall. Other signs of abad TPS are blank spots when depressing the skinny pedal. You press down snd rpm remain the same. Press more past the dead zone and it all of a sudden catches up.

And do not wind or unwind the TPS when installing. It should just go on the blade and bolts line up.

Never look down on anyone unless you are helping them up - Jesse Jackson

So THAT'S what's going on. I've noticed a lot lately that even when it's running normal-ish whenever I would pull up to a stop sign/light it would hover at 1100 - 1200 RPMs, then as soon as the Jeep came to a total stop it would start to stall. Still don't understand how just slightly rolling still keeps the RPMs high, and that when it's fully stopped something kills it. How does the Jeep know it's fully stopped?

The vehicle has had 500 miles put on since the sensor swap, but nowhere near 50 cold starts. What's the method to force a relearn? Is it disconnect the battery cables from the battery and touch them together?

When I installed the TPS I had to rotate the shaft slightly for the blade to slide in and seat, but that was it.

 

[Mercury]

I'll add that I replaced the TPS recently. My last one went out and I had to drive back from Chicago in the wee hours with a very unhappy Jeep. The new one is a cheapie, so maybe I just need to order a Mopar one and call it a day.

 

[JPNinPA]

How does it know?? PCM uses the TPS to know how far the skinny pedal is depressed. It also uses O2 sensors to keep the gas ratio in-the zone. When you’re coasting down the highway it is likely 1500-1800rpm maybe less. When you apply the break the wheels slor. Then the trans stalls the engine. Disengages and waits for rpm to pick up to engage.


Never look down on anyone unless you are helping them up - Jesse Jackson

 

[JPNinPA]

1. Disconnect the POSITIVE battery Terminal and touch it to ground for 30 seconds. Or try to start with positive disconnected. This is to discharge the PCM capacitors, which maintain the Adaptive Memory.
2. Reconnect the Battery Cable
3. Turn Ignition Switch to the "ON" position but DO NOT start the engine
4. Turn Headlight "ON"
5. Turn Headlights "OFF"
6. Turn Ignition Key "OFF"

The PCM Adaptive memory has now been flashed, or erased from the PCM.


Never look down on anyone unless you are helping them up - Jesse Jackson

 

[JPNinPA]

When I installed the TPS I had to rotate the shaft slightly for the blade to slide in and seat, but that was it.

You should only need to turn less than 1/8 turn


Never look down on anyone unless you are helping them up - Jesse Jackson

 

[Mercury]

How does it know?? PCM uses the TPS to know how far the skinny pedal is depressed. It also uses O2 sensors to keep the gas ratio in-the zone. When you’re coasting down the highway it is likely 1500-1800rpm maybe less. When you apply the break the wheels slor. Then the trans stalls the engine. Disengages and waits for rpm to pick up to engage.


Never look down on anyone unless you are helping them up - Jesse Jackson

Oh, sorry, it's a manual. That's where I'm confused, I can't think of any way the Jeep can tell it's in motion, but plenty of times when I roll up to a stop it'll idle high right until the second I stop moving, then drop back to normal. I can't figure out the sense in it.

1. Disconnect the POSITIVE battery Terminal and touch it to ground for 30 seconds. Or try to start with positive disconnected. This is to discharge the PCM capacitors, which maintain the Adaptive Memory.
2. Reconnect the Battery Cable
3. Turn Ignition Switch to the "ON" position but DO NOT start the engine
4. Turn Headlight "ON"
5. Turn Headlights "OFF"
6. Turn Ignition Key "OFF"

The PCM Adaptive memory has now been flashed, or erased from the PCM.


Never look down on anyone unless you are helping them up - Jesse Jackson

So am I leaving the negative cable attached to battery ground and touching the positive cable to the battery ground w/ negative attached, or am I disconnecting both cables and touching the cable lugs together?

You should only need to turn less than 1/8 turn


Never look down on anyone unless you are helping them up - Jesse Jackson

Perfect, it was hardly any turn at all.

 

[Mercury]

I switched the IAC back to the SMP one I had in before. Took off the air box hose and ran TB cleaner through the IAC port with the engine running. Put about 1/3 a can through it, stalled it a few times. Buttoned all back up now. It was idling better afterwards, though the miss is still present. I haven't test driven it yet, but it certainly sounds better at idle.

I've been listening, and, while I'm not 100% sure, it seems like it takes about 10 or 20 seconds of running before the miss starts.

I'm going to attempt the relearn after I get that clarification.

 

[JPNinPA]

So am I leaving the negative cable attached to battery ground and touching the positive cable to the battery ground w/ negative attached, or am I disconnecting both cables and touching the cable lugs together?



Perfect, it was hardly any turn at all.

Disconnect both.



Never look down on anyone unless you are helping them up - Jesse Jackson

 

[JPNinPA]

Oh, sorry, it's a manual. That's where I'm confused, I can't think of any way the Jeep can tell it's in motion, but plenty of times when I roll up to a stop it'll idle high right until the second I stop moving, then drop back to normal. I can't figure out the sense in it.

Speedo gear.

Never look down on anyone unless you are helping them up - Jesse Jackson

 

[Mercury]

Speedo gear.

View attachment 31674

Never look down on anyone unless you are helping them up - Jesse Jackson

That makes so much sense. Thank you, that's been driving me nuts for over a year. Is it normal for that to happen, though?

 

[JPNinPA]

What to happen?? The near stall when you slow below 5 mph?

Happened on mine when I cleaned the TB and didn’t plug it in correctly. Happened on the patriot we own and the liberty when it failed.


Never look down on anyone unless you are helping them up - Jesse Jackson

 

[Mercury]

What to happen?? The near stall when you slow below 5 mph?

Happened on mine ehen i cleaned the TB and didn’t plug it in correctly. Happened on the patriot we own and the liberty when it failed.


Never look down on anyone unless you are helping them up - Jesse Jackson

Not just the stall, the fact that it idles at 1100 until the wheels totally stop, then it'll drop to 750 - 800 again. I had no idea what was causing that. Sometimes it'll start dropping to stall, but most of the time it just goes to the normal 750 - 800. But once it gets in one of those "stall moods" it'll keep trying to stall at idle for hours.

 

[JPNinPA]

Just make sure the gaskets or orings are not worn,torn and are in place gor yhe TPS, IAC and TB.


Never look down on anyone unless you are helping them up - Jesse Jackson

 

[Mercury]

Just make sure the gaskets or orings are not worn,torn and are in place gor yhe TPS, IAC and TB.


Never look down on anyone unless you are helping them up - Jesse Jackson

I'm with you. New IAC o ring, new TB gasket, and the TPS came with a new gasket a few months ago.