Low idle, miss at idle, no codes, no problems under load

[DUB]

This in a 2002 4.0. Not exactly sure when this started but this low idle is annoying, as you need to bump the throttle at a stop to get the R's up to start letting off the clutch (more than normal). I have read a ton of threads on this and tried just about everything with exception to a couple of more major things.. Idle is around 450, with a sputter or miss during idle only. Rig runs just fine under load.

I have replaced: Plugs, TPS, IAC & cleaned TB, waiting for a new MAP sensor, all O2's, coil rail, checked for vac leaks, etc... Nothing has changed. Should I consider valve springs?

 

[DUB]

Is valve spring replacement even a consideration? Wondering if the miss/low idle exists in the valve train itself.

 

[DUB]

No takers.... Could use some advise here, my OCD is going crazy!

These are the last things I can think of to do:
Change CPS (ordered)
Check injectors for leaks
Replace injector O-rings
I did spray carb clean around the top of the intake and injectors last night, nothing...

I have read that a vacuum leak will cause high idle, so I assume this is not a symptom?

 

[JuanTJ]

Just my thoughts.... Have you checked the fuel pressure? Also, have done a compression test on the valves? These tests should help you narrow down what the problem is.

 

[DUB]

Just my thoughts.... Have you checked the fuel pressure? Also, have done a compression test on the valves? These tests should help you narrow down what the problem is.

I hadn't considered a compression test because the plugs look so good and the thing runs perfect under a load. I could do that though. Also have not checked fuel pressure. Is that just done at the rail while running?

 

[JuanTJ]

Yes, pressure gauge on the fuel rail.

 

[AndyG]

This in a 2002 4.0. Not exactly sure when this started but this low idle is annoying, as you need to bump the throttle at a stop to get the R's up to start letting off the clutch (more than normal). I have read a ton of threads on this and tried just about everything with exception to a couple of more major things.. Idle is around 450, with a sputter or miss during idle only. Rig runs just fine under load.

I have replaced: Plugs, TPS, IAC & cleaned TB, waiting for a new MAP sensor, all O2's, coil rail, checked for vac leaks, etc... Nothing has changed. Should I consider valve springs?

Have you checked the catalytic converters

 

[basedbb69]

Have you checked the catalytic converters

How do you check those, my jeep is currently having the same issue. Runs fine if i'm driving but when I'm at a complete stop it starts idling rough. No code. I have plugs on the way because I haven't changed those since I got it, so hopefully that it. But who knows.

 

[DUB]

How do you check those, my jeep is currently having the same issue. Runs fine if i'm driving but when I'm at a complete stop it starts idling rough. No code. I have plugs on the way because I haven't changed those since I got it, so hopefully that it. But who knows.

Have you checked the catalytic converters

I have the last one removed and the front two are in great shape, had them off last week for a RMS job. This is a 2002, so no O2 in the rear cat..

 

[DUB]

How do you check those, my jeep is currently having the same issue. Runs fine if i'm driving but when I'm at a complete stop it starts idling rough. No code. I have plugs on the way because I haven't changed those since I got it, so hopefully that it. But who knows.

This seems to be a common issue that IAC cleaning usually fixes but others chase it. My Jeep does this exact same thing, even at a roll it idles perfectly, until stopped.

 

[AndyG]

How do you check those, my jeep is currently having the same issue. Runs fine if i'm driving but when I'm at a complete stop it starts idling rough. No code. I have plugs on the way because I haven't changed those since I got it, so hopefully that it. But who knows.

The first thing is you can put your hand at the exhaust and see if you feel any real pressure- That’s not an acid test but at least a good start.

Then put an infrared thermometer on them and see if the temperature Delta is dramatic- If the lower pipes are really cool below the couch then heat maybe not transferring transferring.

You can actually even pull the sensors and stick a camera in too.

 

[Moglocker]

I have the last one removed and the front two are in great shape, had them off last week for a RMS job. This is a 2002, so no O2 in the rear cat..

Other then the iac,you should try another throttle body?

 

[DUB]

Other then the iac,you should try another throttle body?

What goes wrong with them? I cleaned the hell out of mine.

 

[DUB]

The first thing is you can put your hand at the exhaust and see if you feel any real pressure- That’s not an acid test but at least a good start.

Then put an infrared thermometer on them and see if the temperature Delta is dramatic- If the lower pipes are really cool below the couch then heat maybe not transferring transferring.

You can actually even pull the sensors and stick a camera in too.

I know mine has pressure, it blows condensation 10' out of the tail pipe when warming up!

 

[Moglocker]

What goes wrong with them? I cleaned the hell out of mine.

My shaft must be wore out,if I spray carb cleaner where the shaft meets the main body ,it kills the engine,my dads wj won’t do it,and my idle is below 400 rpm’s

 

[DUB]

My shaft must be wore out,if I spray carb cleaner where the shaft meets the main body ,it kills the engine,my dads wj won’t do it,and my idle is below 400 rpm’s

Good deal. I'm going to order a cheapo off of Scamazon to try before spending $$. I'm throwing the kitchen sink at this thing!

 

[Moglocker]

Good deal. I'm going to order a cheapo off of Scamazon to try before spending $$. I'm throwing the kitchen sink at this thing!

Let me know how it works,I have the same thing

 

[DUB]

Let me know how it works,I have the same thing

You haven't tried it yet? I ordered a 62mm for $113 just now. I'll probably buy a better one if it fixes my problem.

 

[Moglocker]

You haven't tried it yet? I ordered a 62mm for $113 just now. I'll probably buy a better one if it fixes my problem.

I would have checked for play in the shaft,mine is lose as a goose(lol)did you spray around it?

 

[WestCoastDan]

Which brand of new O2 sensor did you use?

Aside from all of the other troubleshooting and replacement parts you've added, your description of the problem (low, rough idle that clears up when RPMs increase) is exactly what our '03 was doing before I replaced the front (upper) O2 sensor.
That resolved my issue instantly.