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New O2 sensor, now it's running rich!

TRSCobra

TRSCobra

NAXJA Forum User
Location
Gig Harbor, WA
I replaced the upstream O2 sensor because it was having the same 2000 rpm stumble that the last O2 sensor fixed some 5 years ago. Now the jeep is running rich and idles really rough. No CEL, either. I'm aware of a small crack in the exhaust manifold (have a replacement waiting to go on) but the crack was there long before I changed the sensor out.

Is it possible that the new sensor is more sensitive to the leak than the old one?
 
Did you disconnect the battery and reset the PCM? This may be part of the problem if you did not do it.
 
I got it off ebay, but it's a legitimate aftermarket brand. Been a good month since I bought it, i just forget what brand it was.

Yes, I've disconnectec the battery several times. And a cel just showed up tonight! yay!
 
You profile say it's a 98. Once you reset the PCM, it takes a bit to relearn the optimum running parameters so it can run a bit rough at first.

What code are you getting?
 
P0171 System too lean. Must be that crack in the manifold making the sensor read lean, causing the computer to richen the mixture... Looks like I'll get to that manifold swap this week!
 
A bad ground can cause this code. All the problems I had on the 97 led back to the grounds.
 
Replaced the manifold and she's running great so far. No CEL, either. :)
 
Just curious--I've never actually encountered a leaking exhaust manifold that I can say caused a "false lean" reading on the O2 sensor--could you here the leak? That obvious?
 
^^^ I could hear it if i had the windows down in mid-high load situations. It was only a hairline crack halfway around one of the pipes, too!
 
xjbubba said:
Just curious--I've never actually encountered a leaking exhaust manifold that I can say caused a "false lean" reading on the O2 sensor--could you here the leak? That obvious?
Click to expand...

Seriously? Besides the crankshaft position sensor, this is the most typical issue of the XJ:thumbup:
 
NW-ZJ-SCOTT said:
Seriously? Besides the crankshaft position sensor, this is the most typical issue of the XJ:thumbup:
Click to expand...

You're saying "false lean" is the "most typical issue of the XJ"?
My question actually was intended to be general, but using the OP's experience here as an example (for my edification). It would seem obvious that any exhaust leak that result in a false lean (O2 sensor tells ECM to add fuel, when the engine is really not running lean), could be heard; but maybe not. That's why I asked.:wierd:
 
I spoke too soon. Once the ecu relearned everything (about 30 miles later), the rich condition returned. No code yet, though.

It must be the O2 sensor. It's a "Standard OExtra" unit that I got for a steal on ebay...
 
I haven't seen anywhere in this thread where you've changed the Coolant Temp Sensor. There are resistance values that should correspond to specific coolant temps as the engine gets up to running temp. The chart I followed to check mine (it was shot) is here somewhere on NAXJA. Price is in the low $20 range, and it's not hard to do. A faulty CTS will keep the engine in 'warm-up', possibly contributing to a rich condition.
 
rocklandxjer said:
Don't write it off as the O2 sensor yet though. It could be something else, that's causing the problem. What, I don't know. Recet the computer and try again?
Click to expand...

I've reset the computer several times since I installed the new sensor. It runs great until about 30 miles or so, then it starts running like crap and the exhaust stinks. The problem started immediately after putting the new sensor in.
 
Here's an update:

I bought a new Bosch sensor from a local parts store, and put that on. Also noticed that my cat was rattling (probably from cooking itself on the rich mixture) and my muffler was rusted through.

Put in the new o2 sensor and had the cat/muffler/tailpipe replaced. 50+ miles on it and she's running great.
 
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