The stfts are normal. The high stft value you are seeing is a sensor 2 reading (post cat) which is also normal.
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Weird Low Idle Issue
- Thread starter 4x4 AG
- Start date
Something for the OP to consider is does the low idle occur when shifting from park to drive while stationary or only when coming down from speed to a stop? There are several things happening when transitioning from speed to a stop both on the engine controls side as well as transmission side.
I notice it more when you are driving and come to a complete stop. where the vehicle rest back. California rolls it doesnt drop down. It also has to be warm, after driving for a while. if you just start it up and take off driving it will not do it initially.
I got a smoke machine to test for leaks this weekend. So will test the EVAP and intake spots. I have also done a soapy water test with back pressure on the exhaust. It showed some exhaust leaks at band clamps, plan to try and fix those, and ever so slightly at manifold to cats. So will address those as well.
At rolling speeds above 2mph the pcm is adding to your target idle speed which is why you don’t experience the problem during a slow roll. It also adds rpm for a period after cold starts and when the AC is on, again masking the problem.
Just for good measure, After you replaced your IAC, did you disconnect the battery? Do so clears the adaptive memory and resets the IAC stepper motor count. It may idle high after doing so, but it will come down shortly as it relearns.
Ah, I hadn't considered this forced stepper motor relearn. It definitely helps with pushing the PCM to relearn the fuel trims. Mine has been acting normal for the most part since a recent replacement but I have gotten an odd throttle hang on clutch in here and there. I understand why it's there but don't like it and have been working on dialing it out via my tune.
I'm planning to finally take care of installing a third brake light into my OR Fab swingout carrier this weekend, so a good time to disconnect the battery while I work on that.
As a tangent to the thread, you mentioned you have your idle at 824rpm and you sometimes have a hang. If you look at your timing table, there is a plateau in the idle areas from probably 500 up to roughly 800rpm where it then jumps up considerably. The pcm uses timing to control idle speed and sometimes setting your idle at the edge of this timing plateau can cause a hanging idle as it gets confused. Some oem tables have intentional ridges or troughs to help settle into the sweet spot. You can tune around this. It may help.
It most certainly will idle higher but after a drive it’ll calm down for sure.
That’s why I mentioned the increase idle associated with the A/C and Defrost (placing strain on the belt drive), I didn’t pay attention to the battery disconnect because with that much replacement of parts it didn’t cross my mind that he may not have disconnected it. I would disconnect my battery and reconnect when my TPS was bugging out and it would smooth out for a little. Eventually I bought a new Mopar and it ran like new. I did the same with my upstream O2 because it would just shut my jeep off whenever it felt like it so reseting or disconnecting the sensor upon startup worked until I got a new one
I'll take a look. The throttle hang tends to happen around 2200 or 1500 every once in a while. I've captured it once or twice while logging, but the logic driving it wasn't clear. I'm aware of it's 1216 rpm hold and see that in the logs even though and I've been working on these sections. I think I need to think about the relationship of the three thresholds on idle as well as the idle spark advance.
Yes at one point I did have the battery completely disconnected for an extended period.
Last IAC change it did idle up and settle back down.
I worked on sealing up exhaust leaks today. Doubt that is the cause, but was on the list.
Did not get to smoke test today. Hope it is a pesky vacuum leak and not transmission related. Feel like I can fix vacuum, transmission is above my YouTube certification
What is the best way to reset the PCM? Just unhook the battery for an extended period of time? Touch unhooked leads to each other?
Just leave it unhooked for at least 10 min. It doesn't completely reset but clears the memory for fuel trims and some other learned functions that don't require a specific learning mode like opda.
Ok. so had some time to tinker this weekend. seem to have exhaust leaks at band clamps sealed for now. Maybe still a small one at the manifold to cats, but small.
I did a smoke test on the intake and the Evap. Good thing, the smoke tester worked. bad thing, i didnt find anything. found a super small leak at on evap boot and got it to seal back up. couple small puffs from time to time around the intake tube, tightened it. seems to leak some around the IAC maybe, but nothing consistent. Nothing showed to leak on the evap either. so i dont know it is there.
I did unhook the batt for a few hours and rehooked it back up. hopefully that reset the PCM. idled high at first and then leveled out.
was asked to take fuel trims pic in Drive, with foot on brake, when it was idling low. This was at about 560rpm idle. not sure what this says. but i am in for any help.
I did a smoke test on the intake and the Evap. Good thing, the smoke tester worked. bad thing, i didnt find anything. found a super small leak at on evap boot and got it to seal back up. couple small puffs from time to time around the intake tube, tightened it. seems to leak some around the IAC maybe, but nothing consistent. Nothing showed to leak on the evap either. so i dont know it is there.
I did unhook the batt for a few hours and rehooked it back up. hopefully that reset the PCM. idled high at first and then leveled out.
was asked to take fuel trims pic in Drive, with foot on brake, when it was idling low. This was at about 560rpm idle. not sure what this says. but i am in for any help.
I paid close attention to this question when i drove tonight. once you come to a complete stop it will sometimes idle at the 650prm range. never much up in the 670's, maybe bounces around it some. but alot in 650-660 range.
Sometimes it will drop down into the 560-570rpm range. if i shift to Nuetral, the idle doesnt change, but you can feel the transmission pump disengage, so there is less vibration at that low rpm. if you shift back into Drive, the idle stays the same, but you can feel pump kick back in.
If you let off the brake and let it roll some, the idle sometimes bumps back up. then if i apply the brake again, it will stay at that 650 rpm range.
It just seems weird and erratic. Wondering if the IAC i have isnt a dud?
Good news @4x4 AG you do not have trans drag. You do not have any transmission issue as a matter of fact. It does appear that you do have a small vacuum leak. You can tell by the LTFT +6-8% at idle which means the PCM is adding fuel (indication the engine is running lean and needs fuel to neutral out) the trims move down to +2% at 1,500 rpm meaning you don’t have a fuel delivery issue so your fuel system is essentially ruled out of the equation.
My next thoughts are this:
Vacuum Leaks: check around vacuum hoses, throttle body gasket, intake manifold gasket, PCV hose
IAC / idle air control positioning / idle learn issue: Disconnect the battery for a couple minutes and then reconnect and you should be good to go to relearn
PCM
Fuel trims don't lie, if there was any kind of engine/transmission issue or sensor issue the trims would show it. 2 to 4 positive is pretty much as great as it gets. Anything negative is a problem. And over 8% positive is usually where you start getting concerned. Both trims are about equal so we know the engine compresion, injectors, plugs etc are all good or else one bank would require more trimming then the other one to achieve stochiometry.
The trims go down at higher RPM that verifies that you do have a very small vacuum leak, but nothing the computer can't compensate for very easily.
Those computers all go bad, you can't find a replacement for them anywhere. They have all kinds of issues, not controlling the IAC properly at idle being one of them. But I’m not all too knowledgeable with computers, maybe @Wranglerfix may be able to help. Too many other possibilities aside from the computer though.
My next thoughts are this:
Vacuum Leaks: check around vacuum hoses, throttle body gasket, intake manifold gasket, PCV hose
IAC / idle air control positioning / idle learn issue: Disconnect the battery for a couple minutes and then reconnect and you should be good to go to relearn
PCM
Fuel trims don't lie, if there was any kind of engine/transmission issue or sensor issue the trims would show it. 2 to 4 positive is pretty much as great as it gets. Anything negative is a problem. And over 8% positive is usually where you start getting concerned. Both trims are about equal so we know the engine compresion, injectors, plugs etc are all good or else one bank would require more trimming then the other one to achieve stochiometry.
The trims go down at higher RPM that verifies that you do have a very small vacuum leak, but nothing the computer can't compensate for very easily.
Those computers all go bad, you can't find a replacement for them anywhere. They have all kinds of issues, not controlling the IAC properly at idle being one of them. But I’m not all too knowledgeable with computers, maybe @Wranglerfix may be able to help. Too many other possibilities aside from the computer though.
i have tried several. it is back on the factory Mopar one now. I ordered some off Amazon, but all came in really beat up packaging, and never made a difference.
Didn't know if i should try to order straight from Mopar for better results.
I appreciate the write up. and glad nothing seems wrong with the trans. I have heard about the @Wranglerfix but thought 04's werent as prone. I know there was some leakage around the throttle body, so maybe i RTV that up. Didnt seem excessive, so i didnt think it was an issue. Maybe that is all it is.
I will continue to look, and appreciate this forum and your help. great resource to keep these old things running. This jeep is my 15yo boys HS vehicle. I drove an 81 CJ7 Laredo in HS.
Mopar is really the only thing I’d recommend, I’ve tried 10 different TPS and warranties and they all didn’t fix my issue till I tried Mopar. Now I don’t second guess myself on spending the extra money.
This is my first vehicle in highschool. It held up to the beating a teenager could give it, wheeling on not. Only downside is hanging out with friends and cramming them in the backseat
This is my first vehicle in highschool. It held up to the beating a teenager could give it, wheeling on not. Only downside is hanging out with friends and cramming them in the backseat
Ok. Worked on the jeep a little this weekend.
Smoked it really good. Found one bad evap boot, replaced.
Seems to leak a lot at the throttle body I tried RTV where I could, but no help.
Is the throttle body supposed to leak some smoke? Coming from around the butterfly spring area and out of where the sensors are.
Smoked it really good. Found one bad evap boot, replaced.
Seems to leak a lot at the throttle body I tried RTV where I could, but no help.
Is the throttle body supposed to leak some smoke? Coming from around the butterfly spring area and out of where the sensors are.
Here's my smoke test. No issues with mine...no idle problems. Replaced the throttle body gasket JIC and probably need to o-ring the throttle body but haven't.
-Mac
-Mac
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