Miss when cold and at idle

I have a '95 XJ with a straight-6 that has developed an issue. I am not very familiar with fuel injection so I am in largely uncharted waters here. It has always had a slight miss at idle since I bought it (nothing worth writing home about so I thought nothing of it). Recently however, it started to idle poorly and miss fairly significantly at speed. I put in some fuel injector cleaner and it seemed to help, it still idles like crap and misses when it is cold, but once it warms up, it is back to only a slight miss at idle (not as slight as it was but still not significantly noticeable).

Has anyone encountered this before? Things I have thought of are vacuum leak and timing but I cannot find any sort of vacuum leak and I could not find a spec to check the timing against. Plugs, wires, cap, and rotor are all new as of about 2k miles ago.

Sounds like the O2 sensor or circuit. The O2 sensors uses an electric internal heater that needs 14 volts at the sensor to keep it hot enough at idle for the sensor to work.

The sensor could be bad, the power relay or fuse to the sensor that supplies the heater power could be bad (the wires have been known to get rerouted and or eaten by drive shafts, exhaust manifolds, road hazards, and so on)

Here is a thread

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

I created 7 years ago on O2 sensor testing and diagnostics. Read it, Test the O2 circuit and sensor and see what if anything is wrong, do not run and buy a sensor, check the fuse, the relay, use a multi meter to test for power and for a ground at the wiring harness connector to the sensor....

Focus on the Pre-cat sensor (not the post cat sensor if there is one that year).

That is where I would start.

Forgot to mention, the O2 sensor itself is new but I will take a look at the heater circuit, thanks!

the_mechanic said:

Forgot to mention, the O2 sensor itself is new but I will take a look at the heater circuit, thanks!

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I read regularly here that there is brand issue with Jeep O2 sensors. What year is yours?

87-90 is OK to use Bosch.

But I hear 91-2001 do not like them. I think NGK is the recommended one for 91-01 but check some threads here first. Never use a universal!!!!

Mine is a '95, put a Bosch O2 sensor in it (not universal) when the issue first began and it didn't seem to make much of a difference.

the_mechanic said:

Mine is a '95, put a Bosch O2 sensor in it (not universal) when the issue first began and it didn't seem to make much of a difference.

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I am pretty sure I have read many, many, posts here saying that Bosch O2 sensors just do not work right on 91-01 Jeeps. But that may history, and that may no longer be the case. Maybe some recent buyers of O2 sensors for HO rigs will post up the latest news on that Bosch or not Bosch issue.

But the right thing to do is get an analog multi meter (20,000 ohm minimum impedance) or a $10-$20 Bluetooth OBD-II rig off Ebay from China and install Torque pro for $5 on your cell phone and monitor the actual idle voltage of the O2 sensor feed back to the ECU computer and see if it is switching back and forth rapidly, or going rich and staying there cause the idle miss as the O2 sensor drives the ECU crazy in a borderline area.

Easiest thing to do first is use $5 digital meter to check for 12-14 volts on the heater wire at the O2 sensor wiring harness connector while the engine is running. Also disconnecting the O2 sensor in a test that stops the miss, would be a giveaway that the problem is there in the O2 system