Rushin

me figure out whats going on with my heep. 2001 Cherokee, 206k miles 4.0l ,4 speed auto. Engine is all stock aside from K&N intake that previous owner put on.

The issue is an intermittent low idle (400rpm) and sometimes it drops so low that engine stalls. if i blip the throttle slightly the rpm drops so low that engine totally dies.

Here is what i have done so far.

1. Replaced all common placed where there could be a vacuum leak and tested for leaks. The engine pulls good vacuum from multiple locations.

2. Scanned for codes and nothing came up.


I did bring the jeep to a shop that works on jeeps pretty often and after a couple hours of diag the guy told me he really does not have a clue whats going on but told me that the car goes into Open Loop for no reason at idle intermittently. His thought is i have a bad ECU. So his recommendation was to get a replacement ECU and see if that fixes the issue. Replacement ECU is only about $160 programmed. However i am not totally convinced that this is whats causing the issues.

One thing i just tried that made me a bit puzzled is i pulled the plug from the coolant temp sensor on the t-stat and the rpms went up to about 1000rpm, electric fan kicked in and CEL popped up. I guess this would make sense because the car would go into closed loop at this point, correct?

Has anybody seen this sort of issue before? IS it worth getting a replacement ECU?

Thank you and happy 4th!

jordan96xj

That is a normal defensive setting the computer takes when it senses the loss of the coolant temperature sensor. It assumes, since it can't be sure, that the engine is very hot and needs to have the maximum amount of mechanical fan and electrical fan cooling. It manipulated the idle to 1000rpm to achieve this.

Going to closed loop is normally associated with loosing data from a sensor that is involved with air/fuel management. Particularly the upstream O2 sensors. It can happen for a variety of reasons though, and it is always closed loop when the vehicle first starts, and for several minutes thereafter. It also probably goes to closed loop at WOT (wide open throttle), which the mechanic might have instigating while messing around with the idle condition. Or it could also be a sign that the TPS (throttle position sensor) is occasionally telling the computer that it is all the way open (falsely).

In general, but not definitively, these things are showing that the ECU is responding as designed.

So no, it is not reasonable to replace your ECU at this point. However, it is reasonable to replace your jeep mechanic at this point. If he works on older Jeeps, he should know better than to throw up his hands at a low idle condition and suggest swapping the ECU.

What he should have done was test some sensors and your IAC (Idle Air Control) and fix the problem.

Given that the rpm kicked up to 1000 when the coolant sensor was disconnected, but has trouble keeping 700-800rpm at idle. I would start by pulling off the throttle body, giving it a good clean, and remove, clean, and inspect the IAC. (which is not an expensive part). It is very likely that the IAC is not doing its job at the lower range of its movement, but was able to pull open a bit (to let more air in) to increase the idle to 1000rpm.

wyrick

Jordan is giving extremely sound advice. I consider myself knowledgeable on fuel injection but he is giving info that I feel would rival the engineers who designed our XJs. The only thing I have to add also backs up the statement of pulling the TB and giving it a thorough cleaning. The IAC sees a lot of carbon build up and is a solenoid valve commonly mistaken for a sensor. If it gets sticky or has excessive carbon build up it may not function properly. A good cleaning should almost be part of normal maintenance. If it doesn't fix the problem it at least rules out the piece mostly responsible for air in the idle air/fuel mixture. Like he said, not expensive so replacing it won't hurt.

Rushin

Thank you for the input. I totally forgot to mention that when first had this issue a few months ago i did pull the TB, cleaned it really well, replaced the IAC valve and when i accidentally broke the map sensor nipple i replaced that as well.

My original thought before i brought it to the "mechanic" was to replace both upstream o2 sensors. The mechanic told me that they tested the sensors and everything seemed to be fine. They uses an OEM scantool apparently. Because this is an intermittent issue he thought it was ecu related vs something else.

What should be my next step here. Considering i replaced the IAC valve already.

wyrick

lol Jordan?

wyrick

If I remember right a vac leak would increase the idle, if I'm right you can rule that out exponentially. Although the TPS could cause that I don't believe that is common, and IMO if all your connectors and vac lines are as they should be the ECU is not that far fetched. What makes this so hard is it's intermittent, difficult to nail down or find a good pattern. I hope more people join the conversation but if I was facing the same problem I would triple check everything and then consider the ECU.

Rushin

I am just not wanting to throw parts at it. I did get a spoke machine not too long ago but i really dont think its that. I really am having a hard time figuring this out.

I might try another shop but thats like $100 every time somebody "guesses" what could be bad.

wyrick

What's a spoke machine?


No need to throw parts at it, triple check everything first. If you don't already have one, use some of that next $100 and get a decent multimeter and download the Chrysler shop manual and go to town (I'd send you a copy of mine but I'm OBD1). Somewhere within the relationship between the injection system and the PCM an electrical signal is not what it should be, and this is my suggestion on how to find it


Hopefully someone else on the forum has run into this, found a fix, and reads this post. Otherwise it's a tedious path ahead my friend.

Rushin

Honestly I am so sick of working on cars now that I would rather pay somebody to figure it out. But the problem is that most mechanics I have to deal with are idiots and know less than I do about cars in general.

So I am hoping somebody had a similar issue and can help me figure this out without me having to test every ****ing wire in this thing.

wyrick

[QUOTE=Rushin;3274412]Honestly I am so sick of working on cars now that I would rather pay somebody to figure it out. But the problem is that most mechanics I have to deal with are idiots and know less than I do about cars in general.

I understand completely, parts store are even worse!

fb97xj1

I am sure he meant smoke machine, not spoke. It connects to a vacuum line or hose and smoke is sent thru them. Anywhere you see smoke escaping is a vacuum leak. Some use a theatrical fog machines from Walmart or wherever, or just build thier own. Look up' fog machine vacuum leak' on Youtube. There a couple variations making one. Some smoke machines use an air compressor, some do not. You dont need one really. Just blow thru the hose.

wyrick

Ahhh, that makes a lot more sense. Thanks!

jordan96xj

Rushin, do you have a android phone or tablet? (or family member or friend that has one?)

For $30, you can get a very good OBD2 scan setup using the following items:

BAFX Products 34t5 Bluetooth OBDII Scan Tool for Android Devices
by BAFX Products
Link: https://amzn.com/B005NLQAHS

Torque Pro App for Android
https://play.google.com/store/apps/d...l.torque&hl=en

Primarily, what I would want to look at would be long and short term fuel trims, o2 sensor voltages, throttle position value, and vacuum psi values.

It will show you all of this. My guess is that we'll see something in that data that isn't quite right.

Also, let's rule out basic voltage and ground issues by doing the stuff in this video:


Wyrick, Thanks for the kind words.

Rushin

Alright! Thank you for the input guys. Update: I went ahead and got a new Idle Control Valve. The one i had there before was a cheap ebay one. I pulled it out and noticed that the seal ring was smashed and broken and the valve itself did not look right for some reason. So far with a new valve the idle is much better in general and i have not had the issue for a few days now. So fingers crossed this fixed the issue.

I do have a pretty good scan tool. So if the issue comes back i will try and look at the live data and report the numbers i see.

97grand4.0

Whoa. PCM replacement is the LAST thing you want to do and only after good troubleshooting leads you there.
Closed loop is good, it's when your up to temperature and the pcm is taking data from the 02 sensor to trim the fuel. Getting there can be a problem. First thing you need is a scan tool to monitor the TPS signal, the CTS signal and others.
Absent alot more data, my GUESS based on what you said so far, is you haven't any idea what the Tstat is opening at. 195 is the recommended stat, that's the temp it starts to open at. You need a scan tool to see what temp the CTS is actually reporting. $40 on ebay. If the engine does not maintain the obd2 temp it will run rich from preprogrammed tables in the pcm. This can foul your plugs after only a few weeks depending and it will run rough, start funky and maybe stall out.
TPS, a scan tool can graph it check smooth operation
Of course the IAC is another famous one that will cause idle problems, should have said that first. Junk yards and ebay are your friend here. Even just having another to throw in to rule it out.