Runs Rough 30s after start then OK

I have a '99 XJ Limited 4.0 Auto w/ approx. 235,000 miles on the OD. The engine has been very reliable but has developed a hesitation about 30 seconds after start-up. It does not matter if the engine is hot or cold, or if the outside temp is hot and cold. The engine starts great. After 30 seconds it runs incredibly rough for another 30 seconds, then it is just fine. If you accelerate while it is running rough, the engine will pop (like a backfire coming from the throttle body) and then runs fine.

I have researched many, many posts to no avail. I have new plugs, cap, rotor, and have cleaned the IAC. For different reasons, I have also replaced the water pump and thermostat. I also do not believe that it is an issue with the fuel pump. Even if I turn the key on for 10 seconds 2-3 times before starting I still get the same result.

Any guidance is appreciated. Thank you.

front 02 sensor most likely, mine did the exact same thing.
They get lazy and the PCM doesn't get the proper signal causing a severe lean condition.

Mine does the same thing & keeps setting DTC for pre-cat o2 sensor. Haven't got around to changing it yet cause it runs just fine after it warms up.

The problem is an O2 sensor, the fist one, before the Cat converter, BUT, it can be the heater relay or fuse to the heater in the O2 sensor, or burned or damaged wires to the O2 sensor, or a bad heater in the O2 sensor....

You need to check for 12 volts in one of the four wires to the O2 sensor, then two of the wires need to test as good grounds, and the fourth wire is the O2 sensor signal output to the PCM, all on the wiring harness side!!!! Two black wires are usually the grounds, white the signal wire, and fat orange wire (at least on renix, 87-90) was the 12 volt wire, maybe red on yours. The O2 sensor's internal heater should be about 8 ohms of resistance. If it is infinite, it is bad. If the 12 volts is missing check the fuse and relay!!!!! I think the newer ones are 12 volts to the heater, but it may be 5 volts?

The 12 volts and heater gets the O2 sensor hot enough to work faster, if it fails, you get the rough running like you are seeing until it goes open loop and gives up (which wastes fuel and damages the Cat converter), or until the O2 sensor gets hot enough on its own.

I appreciate the help. 02 sensor needs to be replaced after running the diagnostic test via Ecomike. Rockauto seems to have some good prices on the upstream 02. $27-$34. NTK appears to be the OEM manufacturer. I'm going to assume this is the best sensor available?? Thoughts?

That's what I bought for my junk; working fine so far.

My 99 was doing the EXACT same thing, I replaced the O2 sensors along with the header and I haven't had the issue since.

Just an update. It was definitely the front 02 sensor. It's running like a top. Thanks for the help.

I have a 98 TJ 4.0 with the EXCACT same symptoms. Im planning to buy this upstream o2 sensor, but first I have two questions, I cant find an o2 heater fuse or relay ANYWHERE. Second question.... would this o2 failure cause me to get HORRIBLE gas milage all the time? Or does it finally warm up and communicate with the computer? My gas milage is probably in the 11mpg range. thanks for any help.

Most likely a bad O2 sensor, but a bad T-stat and cold running engine can also drop MPGs.

There is an O2 sensor heater fuse and relay somewhere.

The loss of the heater power or the heater burned out in the O2 sensor usually casues a rough idle, not a low MPG.

thanks for the help ecomike.