Idle issues

Lowcountryearl

Lowcountryearl

TJ Enthusiast
Joined
Jul 20, 2020
Messages
129
Location
Charleston SC
98 TJ, 173k 5 sp manual,,starts fine, idles fine about 825 rpm, when driving at the point of shifting and the clutch goes in the rpm's don't fall as much as I would think they should. Coming to a stop sign if you push in the clutch it starts off about 1500 and comes down to the normal idle after about 7-10 seconds. At that point if you are sitting at a redlight and you rev the engine, it immediately falls back to normal idle. Seems like when it's moving the RPMs start out at the higher end and fall to normal,,,if that makes since. Cleaned TB really good, changed IAC with a Mopar part, no change.
Any help would be appreciated
 
Lowcountryearl said:
98 TJ, 173k 5 sp manual,,starts fine, idles fine about 825 rpm, when driving at the point of shifting and the clutch goes in the rpm's don't fall as much as I would think they should. Coming to a stop sign if you push in the clutch it starts off about 1500 and comes down to the normal idle after about 7-10 seconds. At that point if you are sitting at a redlight and you rev the engine, it immediately falls back to normal idle. Seems like when it's moving the RPMs start out at the higher end and fall to normal,,,if that makes since. Cleaned TB really good, changed IAC with a Mopar part, no change.
Any help would be appreciated
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I'm pretty sure that an increased idle (~1,000RPM) at a cold start (completely cold engine) is to be expected. Is that what you're experiencing? Any OBDII codes stored?

If it seems higher than that, check the throttle cable for binding.
 
Negative, cold start does idle up for a minute or so, then drops to the normal 825 (in my case)
The scenario I am talking about is normal running temps, normal driving environment
 
Check to see if there is any binding in the throttle cables. Check the gas pedal cable, the cruise control cable, and any aftermarket throttle (such as hand throttle) cable for binding. You can spray lubricant into the cables to reduce friction and binding.

I had this happen when my hand throttle cable connector would bind with the cruise control cable connector at the throttle body. A quick blip of the gas pedal would resolve it. Ultimately the fix was to better clearance the cruise control and hand throttle clips.

Otherwise I would suspect a vacuum leak.
 
If it was a cable binding I would suspect it wouldn't only happen when the vehicle is moving. Once stopped like at a red light you can rev the engine up and it falls right down to normal idle, same with a vacuum leak,,maybe
I appreciate the feedback you're giving!
 
Do you have an OBDII reader and an app to read it? If so, watch the throttle position sensor data as it happens.

With the engine at normal idle, the throttle is closed, and the sensor will read a nominal amount. On my TJ fully closed is 13.3%. If your throttle doesn’t immediately return to that position when you let off the gas, then something is causing the throttle to stick.

If it does return all the way down immediately, then the problem is somewhere else. The two most likely culprits here would be the idle air controller (IAC) or a vacuum leak.
 
Clean the IAC, then check for vacuum leaks. Pay especial attention to the rear crankcase breather elbow - make sure its the correct one with the restrictor, not the wide open one.
 
I'm not very good at explaining,,if it was a sticking cable, vacuum leak then wouldn't it happen at a stop as well as while the jeep is rolling? I did replace the IAC with a Mopar part, no change.
So while the jeep is stopped like at a red light or my driveway, you can rev the engine and it immediately returns to idle but if you are traveling on a road and coming to an intersection or light and you push the clutch in to coast its starts out at 1500 and drops steady after about 7 seconds. Same if i am up shifting, in between gears the rpm's are about 1500 and if I keep the clutch in it will go to 1500 rpm's and go back to normal at around 7 seconds,,at that point still coasting and I rev the engine, it immediately returns to normal idle. Never seen anything like it myself.
 
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