Topic: hanging idle and weird throttle revs

[roadkill]

I have already replaced the TPS and AIC as well as cleaning the TB and IAT sensor.  I have checked for vacuum leaks and can't find any.  also no codes are being tripped.  I have a Banks header and cat back and a 4.0 TB and TB spacer. on 2001 TJ

here recently the jeep is taking a long time for the revs to fall as I'm shifting from 1 to 2 and sometimes from 2-3.  all other shifts the rpms fall quickly.  I have noticed that the rpms hang and fall slowly when I'm doing less than @25mph.  if I floor it, then shift the rpms fall quick.  if I drive it easy and shift they hang.  I shift at 3000.  if I'm behind slow traffic and want to shift early (@ 2500 or less for example) the rpms hang and the motor has to pull them back down after i let out the clutch.  in other words if I accelerate fast it shifts fine.  accelerate like a grandpa and they hang.  this is fine on the hwy till I'm in 4 low then it becomes a nuisance.  

also sometimes the engine revs when I am coasting to a stop and stay up 1200-1500 till I come to a complete stop.  other times it stays at 1000 till I stop then drops to normal.  and when sitting still if i rev it up the rpms drop slowly (comparetively speaking) till it pauses breifly at @ 1500 then it falls quickly down to normal idle.

any ideas or theories or is this just a normal Jeep thing for my setup.  I don't think it has always done this but it may have and I'm just now noticing it since I'm driving more city and less hwy than in the past.

[Chowmaster]

well im no expert but it doesnt sound very normal to me. i always like to start with simple things, like maybe something to do with your throttle is sticking? also not sure what you mean by no codes being tripped, did you hook up a scan tool or is there just no mil on?

[roadkill]

had a scan tool hooked up and also did the off/on key thing.  checked all throttle linkages, nothing is binding or hanging anywhere.  its got to be either a sensor/computer thing.  whatever it is I'm sure its a small simple thing.  the jeep still drives great and runs great, just a problem with a hanging idle while shifting from 1st and accelerating slowly

[roadkill]

I've been thinking, since I changed the TPS and IAC motor, the only other sensors that have input for the idle and throttle would be the MAP sensor and O2 sensors.  the jeep is not using an excessive amount of fuel, still getting 13-15 mpg (lifted on 33s, 4.88 gears and usually lots of wind where I live).  I think the O2 sensors are probably fine (jeep currently has 55,000 on it)  what about the MAP sensor?  what does it do in relation to the way idle speed is affected and how the jeep runs?

[Jhcujo]

I have been watching this post for a few days because I was having the same problem with my jeep. Today I went out and got my exhaust fixed from the manifold back. It was in pretty sad shape. Since I picked it up it runs a lot better. The idle drops right off and is very smooth. I see that you have the header and cat. Is it possible that you might be leaking somewhere? I do not know if this helps you or not but one more thing to look for.

[roadkill]

its possible, sometimes I hear what sounds like an exhaust leak when its cold but then it goes away.  could be just the lifters ticking, who knows.  its not bad enoughfor me to start worrying about yet.  I was fighting doughnut leaks but replaced the bolts and the leak has stopped thus far. maybe I'm hearing a slight leak from it again.  if it was an exhaust leak doing this I would think it would happen at any speed and not the first 2 gears and under @ 30 mph.  I think its a sensor problem as its somewhat intermitent.  as long as I accelerate hard its not there, drive reasonable and short shift and it rears its ugly head.  

I tried the MAP sensor today, no change in driveability.  I think I'm about to give up on the problem.  the only thing I seem to be doing is making my wallet lighter and my anger go higher.  I just hate driving slow and always having the motor rev up at will and stay revved till I come to a complete stop.  sometimes with no throttle input while taking off the engine will rev up and stay revved till I come to a complete stop or punch it andshift gears.  other times it drives like it should.

[roadkill]

I have been watching this post for a few days because I was having the same problem with my jeep. Today I went out and got my exhaust fixed from the manifold back. It was in pretty sad shape. Since I picked it up it runs a lot better. The idle drops right off and is very smooth. I see that you have the header and cat. Is it possible that you might be leaking somewhere? I do not know if this helps you or not but one more thing to look for.

did they replace your O2 sensors also.  or just the pipe?

[Jhcujo]

They replaced from the converter back and fixed a leak at the manifold. It is all basicly stock but she runs a lot better. Another thing I did at the sametime as the exhaust is I replaced the PCV conection on the vale cover that goes to the intake. Mine was busted and kept coming lose and spitting oil everywhere.  It has to be some thing between those two for me. Cause now the idle drops right off and stays at just under 1000.

[roadkill]

well I found a leak on my header today while stabbing the tranny back up. ( I had broke the 3-4 gear fork and had to replace it)  before the tranny broke I had noticed an exhaust leak that got worse.  the header is cracked where the 4 tubes join into one.  according to my maintenance records the header had @ 10,000 miles and was 10 days away from being installed for one full year.  

A new Banks is on its way (warranty, but I am still out the freight to and from plus labor, although I supply the labor; its costs my time)  I'll see if a non cracked header will help my hanging idle in about a week.

[yellowta]

Did you swap the sensors in the throttle body with your stock 4 cylinder pieces when you swapped in the 4 litter unit? I had the same problem when I did my old YJ, then I read the instructions on the web and found that the idle hardware is different and was causing my hanging idle and high idle problems.  Just a thought

[roadkill]

yes they were all swapped when the TB swap was performed several years ago.