FishJeep

I’m at my wits end on my 96 Cherokee classic. It has trouble maintaining an idle. After removing/cleaning the throttle body and replacing the idle air control valve the engine runs just fine and I can watch the RPM through its operating range and idle at 600 steady when I come to a stop. I can see the computer controlling the idle and everything’s fine. Then the next week something happens. It gets hard to start in the morning. Spark is good but it just doesn’t climb into its operating range and if I do manage to get the Cherokee rolling it just stalls out first stop sign. It will act this way until I take the throttle body off again. And so over and over. I have replaced three idle air controllers and most recently the throttle position sensor. So currently it just doesn’t want to start and idle. I disconnected the battery to reset the computer but still it doesn’t want to start and idle. Good spark so I know it’s not the crankshaft position sensor. Right now I’m thinking about the throttle body as the bore is not completely smooth throughout or maybe the computer is it receiving or sending signals correctly. Or maybe if I use a higher grade fuel. But like I said every time I removed the bottle body and clean it or replace parts everything runs fine for a week or two. It holds good fuel pressure,replaced injectors. any ideas? Scott

jordan96xj

When it struggling to idle, if you control the RPM with the gas pedal, does it respond and run properly? Asked another way, does it only have a problem if your foot is OFF the pedal?

If you have OBD2 (you probably do) you should get a BAFX OBD2 scanner (about $25 from amazon) so that you can see a lot better what your system is doing (from the computer's point of view), things like RPM, coolant temp, and fuel trims can help a lot to point in the right direction.

Have you used a meter to test system voltage when this is happening? I would also recommend testing your fuel pressure when this is happening.

FishJeep

When the problem is present and I'm rolling down the road if I take my foot off the pedal it will want to stall.
Where can I test the system voltage other than the battery?
Fuel pressure looks good in holding around 50 pounds.
I have a Bluetooth engine diagnostic using torque app. I can read temperature but I don't think I can get fuel trims. In my using the wrong app
Thanks

jordan96xj

Test voltage at the battery terminals with the system OFF and also while running, and especially while it is trying to die. Would be helpful. Note the values.

Good job testing fuel pressure (often skipped).

Torque will do fuel trims. Go to the realtime information section where you can look at dashboard style gauges. Press the screen and hold in a blank area to add a new gauge. When adding the new gauge it will present you with a list of data items available for attaching to the gauge. The items in green are the ones that actually have data coming from the computer. The 96, doesn't have a ton, but it does have the LTFT and STFT for bank 1. Those are what you need.

DCWinterXJ

This may be an incredible shot in the dark, especially since I am the only one who did this fix. But do you have a cap on the valve of your fuel rail? If not...try connecting the fuel pressure gauge and see if she starts every time....with good idle. My fix was tightening the schrader valve on the fuel rail itself. After I did that she always ran perfect. I am the only one who did this but I have a 96 motor in a 94 xj. I mean hey,,it is worth a shot. Your symptoms dont match entirely...so it may be the tps sensor or the map sensor.....but def try my trick....who knows. I am hoping it will help someone someday!

EZEARL

^^^^^^^^^^ that.

And here I thought I was the only one who had that issue. Found mine was loose(over one full turn)when going to release the pressure so I could replace the fuel filter. It had the cap on it. There were really no issues other than it did smooth my idle out.

FishJeep

Thanks, I do have a cap. I have used a fuel pressure gauge. It indicates 47lbs and still no start when its acting up.

DCWinterXJ

I had good pressure also, when the gauge was tightened onto the rail. That is how I noticed the weirdness. My engine would only start once and then would not start again after I turned it off until it sat for a while. But with the gauge connected
it restarted over and over and was seemingly perfect, I figured instead of stuffing the gauge in the engine cavity I would tighten the schrader valve. And that was it. Kept starting and running great. Like I said I was the only one who had this issue,
except for now EZEARL who just said he did. It really blew my mind. Except now I have a smoked rod bearing(unconfirmed). But she still starts up and runs great.

FishJeep

Wow, sorry to hear about your trouble. I found my shrader valve tight.

FishJeep

Now i disconnected the Idle control valve and it wants to start.. This is the same old story. Like i said before. Maybe I will remove the throttle body then disassemble the idle control valve housing and see what's inside. Still trying to get the fuel trims on my torque app.

DCWinterXJ

Thanks for understanding my pain but I feel your pain too. Before I found the schrader valve issue which did not show up right away I had a myriad of other problems I worked on. This forum got me through it. Have you checked your air filter? I know it's probably a stupid suggestion but a highly restricted filter can certainly cause problems. Since you are having many issues with the IAC, have you backed up for a second and checked that the wiring is sending the proper voltages or receiving the proper resistance levels? I don't recall the proper way to test the IAC but there is a ton of info on here and Cruiser prob posted how to test it at some point. Also how many TPS sensors have you tried? It is not unusual to get a bad one. When I replaced my TPS sensor my truck started shifting perfect...before that...disaster...although not your problem. I do know a weak battery can also cause problems. Even if it seems okay. Somebody once told me to unplug the coolant temp sensor as one test...it is at the front of the head near the first injector. The truck will start without it but will not get readings. Also an interesting test. I am making suggestions but I always yield to the top dawgs cause they have solved most of my problems. If I have anything else that comes to mind I will post it. Good luck of course.

fb97xj1

Dont take the IAC apart. You stand a good chance of killing it. Destroying it, even. You already replaced it three times anyway with no change, including a new TPS. It still acts up. What brands are you using? Have you checked the connectors and wiring? Engine grounds? Sometimes its somthing simple.

FishJeep

How do these fuel trims look? Not sure why the RPM fluctuated as I was only constantly raising RPM.

https://drive.google.com/file/d/1-EN...9zx8rVgcE/view

jordan96xj

Fuel trims were a little rich, but not enough to explain the symptoms. And the o2 sensors were swinging as they should.

The fact that your computer is reporting an RPM that doesn't match what you were doing in reality is probably the most concerning. You might have a crank or cam position sensor beginning to fail. This is that the computer depends on to know the current RPM. Which can of course mess up the computer's ability to sustain an even idle.

You should add throttle position to the gauges you are monitoring. At idle it should be around 16.5 - 17.0 % If the computer thinks the throttle is being manipulated it can also mess up the idle and allow stalls when at idle because the computer doesn't open the IAC to adjust idle when it thinks you are on the accelerator pedal.

FishJeep

A little bit of catch-up. As usual, hard starting. I stopped by a reputable mechanic who recommended using a noid light to determine if my 47lb fuel pressure was getting to my injector. Sure enough, when it was acting up the noid light wasn't pulsing and coincidentally no fire at the spark plug. I told the mechanic about it and he said it can only be three things. Crank position sensor or cam sensor or computer. Changed cam sensor first with no improvement. Bought computer off eBay programmed with my vin and mileage for $118. Fired up and ran like it should.I am one happy camper. Thanks for your help