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Idle drops after engine warms up.

A

AnthonyZ28

NAXJA Forum User
Location
Hallettsville, Texas
Whenever I start it up, it idles just fine at 1K. But after the engine (4.0) warms up and I park/sit in neutral, (AX15) the idle gets unstable and starts fluctuating anywhere from 600-800. Causing a fairly shaky/rough idle. This only happens once the engine is warmed up.
 
I had a shaky rough idle once.

Turned out to be one bad spark plug.

Ran fairly smooth at higher revs, but ran like crap at normal idle.
 
AnthonyZ28 said:
Whenever I start it up, it idles just fine at 1K. But after the engine (4.0) warms up and I park/sit in neutral, (AX15) the idle gets unstable and starts fluctuating anywhere from 600-800. Causing a fairly shaky/rough idle. This only happens once the engine is warmed up.
Click to expand...

(1) The O2 sensor internal electric heater is either bad, or the power to it is missing, 12-14V.

It is switching from open loop to closed loop and having problems in closed loop. Most probably the O2 sensor system. The power to the heater goes through a fuse (on later years, HO) and a relay......

What year????
 
Dirty Idle Air Controller? Sounds like maybe it's struggling to maintain an idle that the computer is happy with. In addition to Ecomike's suggestions.
 
heyhar said:
Dirty Idle Air Controller? Sounds like maybe it's struggling to maintain an idle that the computer is happy with. In addition to Ecomike's suggestions.
Click to expand...

I think the IAC gets a bad rap. It must be working for the idle to drop, unless the ECU cuts back a rich fuel rate.
 
I got a rude introduction to IACs with the 3.8 Taurus we had years ago. The pintle was made on nylon, and over time would deteriorate, causing erratic idle, as the computer was struggling to maintain an idle. Until the whole thing got sucked into the engine, and it would idle at close to 3k. I know the JEEPS use a metal plunger, but I've seen one of them-my '96 when I first got it- very dirty to the point that it wouldn't seat, and the idle would float, with the engine computer trying to compensate with the injectors. Certainly now would be a good time to check or replace the O2 and Coolant Temp Sensors, the two main components that allow Closed Loop Operation.

That being said, I don't know that I've ever noticed a higher idle when cold on any of our XJs. Even with an old, barely functioning O2 sensor. I often thought they should, like a vehicle with a carb and a choke. Our Subie, though, will idle higher when cold, and if you tap the gas, it will settle down, as if a choke were kicked down. Go figure...
 
Typically, the O2 sensor doesn't cause the problems you've described. Honestly, a thorough tune-up is the best place to start. Other posters have told of a funky plug causing similar symptoms. I agree, as I've seen stuff like that too.
 
Did either one of you read my post, post #5?

(1) The O2 sensor internal electric heater is either bad, or the power to it is missing, 12-14V.

It is switching from open loop to closed loop and having problems in closed loop. Most probably the O2 sensor system. The power to the heater goes through a fuse (on later years, HO) and a relay......
Click to expand...

See:

http://www.naxja.org/forum/showthread.php?t=1012701&highlight=sensor+test

Test everything before you buy a damn thing!!!!
 
I was just looking for that test. I used it a few years ago, and successfully diagnosed the lazy O2 sensor in my '96. There's also a test to check the function of the Coolant Temp Sensor, somewhere in the depths of NAXJA. Probably the two most important sensors on the vehicle.

Maybe start with a can or two of BG44k in the gas tank. Dirty injectors?
 
My last idle wander from 500 to 700 turned out to be a loose, poor contact fuel injector connector.

I was pulling the injector connectors one at time to see if the engine noticed the loss. The one that does not change the engine running to a rougher idle is the cylinder that has a miss.

You can do the same thing pulling the spark plug wire one at a time.

The CTS and IAT-MAT are tested for ohms resistance cold and hot, and compared to the factory spec data table.
 
heyhar said:
I was just looking for that test. I used it a few years ago, and successfully diagnosed the lazy O2 sensor in my '96. There's also a test to check the function of the Coolant Temp Sensor, somewhere in the depths of NAXJA. Probably the two most important sensors on the vehicle.

Maybe start with a can or two of BG44k in the gas tank. Dirty injectors?
Click to expand...

If the heater element in the O2 sensor is bad (open, infinite resistance on an ohm meter) or the 12-14 volts is missing on the heater power wire (bad wire, fuse or relay) it will miss and wander at idle as the O2 sensor struggles to stay warm enough to work right. They get too cold at idle to work, thus the electric heater!!!
 
Ecomike said:
If the heater element in the O2 sensor is bad (open, infinite resistance on an ohm meter) or the 12-14 volts is missing on the heater power wire (bad wire, fuse or relay) it will miss and wander at idle as the O2 sensor struggles to stay warm enough to work right. They get too cold at idle to work, thus the electric heater!!!
Click to expand...

A lot of this is above my head. I understand what the problem could be, but as far as where any of this is located, I have no clue. I've looked and cant find anything. I've read somethings on how there's 2 locations? Is one of them located somewhere along the exhaust?
 
Number of O2 sensors varies by year but the '93 has only one and it is very easy to find and replace.

I just replaced the one on my '93 with an NTK unit. Get the drivers side front up high enough to get under (ramps will do) and you can't miss the sensor screwed into the exhaust pipe. It has a cable leading to the left to a connection supported by a metal bracket. Unplug and remove the old sensor and follow directions that come with the new one as to proper installation torque.

I would just replace the sensor unless you are sure the current one is relatively new. I had a Bosch unit before but the consensus seems to be that the NTK units are the best.

Not that I necessarily stick with the consensus as I also went back to Bosch platinum plugs after giving the Champion coppers a more than fair tryout. Mileage is back up to ~21 and all is well, at least with this aspect of XJ ownership.
 
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