Sounds like a bad sensor.

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After a dead battery or changing parts the PCM has to relearn the IAC stepper settings

Typically crappy idle that on a manual tranny stalls at stop lights unless you add some skinny pedal until driven for a hour or two then relearns and works normally again


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Agreed with Digger84. The PCM either has to relearn the settings, or you can clear the PCM and let it start over. I'd suggest you disconnect your negative battery clamp for the night, and reconnect it in the morning. That will clear your PCM memory and allow it to start over tomorrow morning. That should fix the immediate problem, and it will take about 100 miles or so of driving to completely reset your PCM.

Thank you for the replies, will give this a try today!

 

Agreed with Digger84. The PCM either has to relearn the settings, or you can clear the PCM and let it start over. I'd suggest you disconnect your negative battery clamp for the night, and reconnect it in the morning. That will clear your PCM memory and allow it to start over tomorrow morning. That should fix the immediate problem, and it will take about 100 miles or so of driving to completely reset your PCM.

 

First, you only need to disconnect the battery for 3-4 minutes to reset the computer. It may run poorly after resetting it, give the computer time to relearn all its adaptive memory parameters by driving it around. I'd reinstall the new TPS and drive it for a couple hours and see if that helps.

 

I can barely get the jeep started by flooring it, and as soon as I let off, it literally sounds like it is idling at 100rpm and shaking considerably. I don't think it would be wise to drive it in this condition, won't i damage something? I don't know why unplugging the battery throws off the TPS so much. Makes no sense to me. Currently I have the new TPS plugged in, with the negative terminal unplugged, I will leave it like this for 3-4 hours, and then try to fire it up.

 

Have you been having to jump start your Jeep? Is the battery fully charged? A dead or nearly dead battery will produce that same symptom.

And again, there is NO need to leave the battery disconnected more than 3-4 minutes to reset the computer.

 

The battery is about 5 months old. The only other symptom I have had is a check engine light a few days ago for a vacuum leak, which resolved itself after tightening the gas cap (and it was already tight, I thought this was pretty random)

 

You may have a leaking injector
When you push skinny pedal down it adds air to help correct over rich

Be careful in a closed space gasoline fumes plus spark turn your vehicle and your home into one big fire

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Did you also clean the crud out in the orifice in the throttle body the IAC's plunger fits into? If you did, perhaps the IAC is indeed bad though that is a rarity. Normally cleaning the IAC and the orifice restores it back to good operation.

 

I cleaned the orifice with many small q tips soaked with carb cleaner. I also cleaned the top of the throttle body. Everything was extremely black, lol.

 

That q-tip may not have gotten back into the orifice far enough. I dunno if pipe cleaners are even sold any more but I'd sure try something like that, baby bottle nipple brush, etc. soaked in TB/carb cleaner to make sure that orifice is clean enough so it can easily pass enough air for the engine to run on. And of course maybe the IAC isn't working if the orifice is clean and wide open.

 

Ill go ahead and clean that area a little better, and then report back in a bit. Not looking forward to removing that bottom screw again, but oh well, haha.

 

The cable connectors just unsnap, stick a larger size flat-blade screwdriver in there and twist it to pop them off.

Have a fresh TB gasket ready, the one that's in there now will not likely survive. And make darned sure not to overtighten the four screws that hold the TB on when replacing it. Doing so will strip the threads out in the aluminum intake manifold.

 

For what its worth... once the computer is cleared and you re-connect the battery....

Learning the TPS:
Turn the ignition on but don't start the engine. Slowly floor the throttle 2-3 times. The engine control unit has now learned the throttle position sensor full range.

Learning idle speeds:
After resetting the engine control unit.. start engine and let it get into closed loop mode. (usually engine temp needs to be at least 180 or more)
Let it idle 60 seconds in each of these four situations.
1. AC off in park or neutral
2. AC on in park of Neutral
3. AC off in drive
4. AC on in drive
(only 1 & 2 if you have a manual trans)

1995sahara4.0, based on what I read above I don't think this will help your TPS / idle situation. But at least now y'all know how to quickly get the TPS and idle learned.

Most people don't know these quick learn tips. Of coarse a guy can just hook up the battery and drive.. but the computer has to learn on the fly which only compounds trouble shooting.

Just my 2 cents...

Awesome info! Thanks you both for all the help you provided. Here is what happened. I took apart the throttle body, and it was pretty gunky. Spent about 1 hour cleaning every nook and cranny, then I put the new TPS on it, secured the recleaned IAC on it, and installed it back on using the original gasket (it looked to be in great condition). put the battery on, and fired it up. It took about 10 cranks, but finally turned on and idled really poorly but didn't stall. I then revved it a bit, and quickly reversed out of garage and took it for a 30 mile freeway drive. It now purrs like a kitten, I've never felt such a smooth idle out of a 4.0. The RPM needle stays at about 400 at idle, no hiccups, no vibrations. Pretty awesome stuff! Thanks again!

 

For what its worth... once the computer is cleared and you re-connect the battery....

Learning the TPS:
Turn the ignition on but don't start the engine. Slowly floor the throttle 2-3 times. The engine control unit has now learned the throttle position sensor full range.

Learning idle speeds:
After resetting the engine control unit.. start engine and let it get into closed loop mode. (usually engine temp needs to be at least 180 or more)
Let it idle 60 seconds in each of these four situations.
1. AC off in park or neutral
2. AC on in park of Neutral
3. AC off in drive
4. AC on in drive
(only 1 & 2 if you have a manual trans)

1995sahara4.0, based on what I read above I don't think this will help your TPS / idle situation. But at least now y'all know how to quickly get the TPS and idle learned.

Most people don't know these quick learn tips. Of coarse a guy can just hook up the battery and drive.. but the computer has to learn on the fly which only compounds trouble shooting.

Just my 2 cents...

 

First a cold engine goes closed loop way sooner in one minute or less on my 1998 TJ

And even with the temp sensor harness plug connected to an extra sensor sitting free in the air it still goes closed loop in under a minute

I doubted it before I ran it with an obd2 monitor and watched how quickly it went closed loop

Next learning idle with the iac stepper takes awhile been there done that

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If you haven't already, you may want to check for vacuum leaks. This too can affect engine idle, but Your Jeep should idle around 750-800 RPMs for both, auto or man trans.

 

A vacuum leak would cause a higher not lower idle. I always suspect a problem with the IAC or O2 sensor when the idle is a little too low.

 

Vacuum leaks makes idle too fast and will not cause a slow idle

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If you haven't already, you may want to check for vacuum leaks. This too can affect engine idle, but Your Jeep should idle around 750-800 RPMs for both, auto or man trans.

A vacuum leak would cause a higher not lower idle. I always suspect a problem with the IAC or O2 sensor when the idle is a little too low.

It's been running around 4-500rpm perfectly for most of it's 240k mile life. Hmm..

 

looks like you are on the verge of a functional idle here my man! I wouldnt be comfortable with that low though. But it is working well, you say. Anything else to check? O2 sensor?

You must be feeling so relieved now its running good, you are not feeling to need to check that low rev.. I guess haha Its a common sensation

 

Haha thanks, it has been driving PERFECTLY ever since (knocks on wood!) To be honest, I don't want to work up anything further. It idles so smoothly, you can't even tell the engine is on. It seems to be very happy and peppy, so I will just leave it here. If it gives me problems in the future, I will go ahead and do a further work-up. For now, this forum has saved me once again!