I have a 1998 Cherokee 4.0 auto I just purchased, to start it you have to give it some throttle in park to keep it running and 2 foot the gas and brake In gear just barely but once you move 20 or so feet it will idle normal and never stall. Anyone have any ideas what it could be? No codes and the jeep has 300k miles but has a newer motor and trans. I bought it pretty cheap just don't know what the issue could be.

thanks in advance!

 

In order of concern (what I'd look into first):


1. IAC is sticky/tar and is not opening properly until some warmth hits the throttle body (normally IAC can be diagnosed by feathering the gas pedal a little, if that helps maintain idles, then the IAC is suspect).



2. Something on the engine belt bath is dragging down the motor until it gets moving a little and frees up (can be checked next time things are shutdown and cool, then remove the belt and test each spinning component by hand).



3. Something in the torque converter is dragging the motor down, as if the lock-up clutch is engaged and dragging on the motor, but then releases after a few feet for some reason.

 

I'll have to check the iac is it possible to clean or just replace? I don' think anything in the belt path is a issue but I'll pull the belt and check. Also o don' think it's the torque converter because it does it in neutral and park also. You have to manually hold the idle at 1k rpm even if you slowly let off the gas it will idle way down and stall. After a few min of holding it you can slowly let off and it will idle all day. Hot or cold it does this. It will be fine and idling after driving it stop shutit off and fire it right back up and it won' idle till you drive maybe 20ft and then it will idle fine.. I' wondering if it's a bad maf or iac. Also has a bad manifold exhaust leak not sure if that would cause it either

 

Exhaust leak won't help, but probably unrelated. Jordan's list is the best course of action.

IAC can be cleaned. Take the whole Throttle body off and clean it up really good, take the IAC out and clean it really well.
Good videos online for this.

 

It sounds like the IAC is the component to look at. But the "drive 20 feet part" is very unusual. There is very little brains in the Jeep that even knows that you did that. But there is a relationship between the IAC and the TPS (throttle position sensor) and the vehicle speed sensor (transmission) is also used by the computer for many decisions.


It kind of sounds like the IAC is not doing its jobs on initial start, but then is being commanded by the computer properly after the vehicle moves. If the TPS thinks that the throttle is open more than it actually is, the computer will command the IAC to stay closed (because rpms are being controlled by the driver via the pedal).


Are your sure it is a "move 20 feet" thing and not a "running for a few minutes" thing? This distinction could end up being important.

 

I was able to get it to idle after 5 minutes of holding the throttle at 1000 rpm and slowly removing my foot off the throttle and I mean super slow. It barely idled at 400 to 500 rpm and slowly idled up to around 800. I wonder if the throttle body and iacshould just be replaced since who knows how old it is.. I'll pull it apart tonight after work and see what I find. Thanks for the info guys

 

In general, if you can hold an idle with the pedal, but releasing the pedal causes the vehicle to bog below idle or die, that is a good indication that the IAC is failing or failing to be commanded correctly (the first is more common).


There shouldn't be a need to replace the throttle body. Probably just needs to be cleaned.

 

okay thanks new to jeeps haha I'll get the iac off and replace it along with cleaning the throttle body. I didn't have time yesterday but will try to get it done tonight. Thanks for the help will report back with findings!

 

Be gentle removing/installing the throttle body. The body and manifold are aluminum, so you should not try to muscle anything on/off. Have a spare throttle body gasket on hand (cheap from any local parts store). Just in case the existing one tears or otherwise gets damaged. Have an old toothbrush ready, and 2-3 cans of throttle body spray/cleaner (it will take more than 1). Having a new IAC on hand is best. I used a Standard Motor brand IAC about 1.5 years ago, and has not had any problems. You can clean the existing one, because they do get gummed up (causes them to stick and stop working), but sometimes the problem is electro-mechanical (inside the IAC) and no amount of cleaning will fix it.

 

I've had a SMP IAC on mine for ~5yrs with no issues at all. There's a way to test the IAC. When I tested my oem one after cleaning it was still in spec but didn't last but maybe two months.

 

Well finally had time to take the throttle body off and tear into it. The iac was completely plugged with gunk along with the butterfly valve. 30 min of scrubbing and cleaning everything it fires right up and idles perfect. Thank you guys for all the help 300k miles and climbing!

 

Nice - glad to hear you're up and running! Also thanks for posting the resolution.

 

Nice! Thanks for the follow-up.