Topic: 2.5 stumble/misfire... O2 issue?

[mikepotts]

im still working the bugs out of my YJ (traded for it about 3 wks ago) and there is a persistent stumble upon accel and decel right about 35-45% throttle (maybe lower, hard to tell), during my diagnosis, i noticed the cat is gone along with the O2 sensor!  yes, completely gone... all that is left is the melted connector that used to be connected to it.  i picked up a good connector and length of wire from pull a part yesterday and will be getting a bung welded in so i can put a new sensor in place.  my question is, could the lack of an O2 sensor cause this problem?  thanks guys!

[Tr00b]

I'm  usually wrong on this stuff but here it goes:

The downstream O2 sensor past the cat is usually only to sense whether the cat is working or not. if this is broken or disconnected, it usually causes a CEL. My cat was hollow, and the sensor was in bad shape, but it never threw a CEL.

The upstream O2 sensor near the manifold does the dirty work on air to fuel ratio and is the one you should be truly worried about as far as that pesky hesitation.

IMO I would look more at your throttle position sensor. My Jeep had hesitation and the cause was the O2 sensor. But the hesitation went away when the Jeep warmed up. If you have a consistent hesistation at a specific throttle position, it could be bad. Test it... I replaced mine before I tested it and it ended up being the O2 sensor instead.

Next, I have consistent problems with the intake air temperature sensor getting fouled by valvecover vent oil. It will cause stumbling and other weird anomalies in the way it runs. Its on the rearmost intake runner, you take it out with a wrench and it has a 2 pin connector. Spray her off with some MAFS cleaner or comparable solvent...

[sharpxmen]

YJs only have 1 O2 sensor at the exit of the exhaust manifold on the downpipe.

to me it sounds like it's the TPS (throttle position sensor), if the O2 is gone you should have a CEL, i would read the codes (check the FAQ on how to do that and what the codes are) and along with the O2 you might have some other one that would tell you what the problem is. Fix the O2 anyway and check the codes.

[mikepotts]

thanks guys, i tried to check the codes when i first got the jeep and either the light is blown or there is some other issue, but the CEL doesnt work.  i too had suspected the TPS, im gonna do as mentioned... put the O2 in it and go from there.  ive delt with primarially ford efi, so im a little weak on the jeep stuff, when the O2 is unplugged does the ECU go into "limp mode" like some do or does it go into "open loop" like others? 

[sharpxmen]

it goes into open loop and drives relatively fine, that's why i'm thinking TPS. Of course it could be the more obvious which is an ignition problem, might want to have a look at the plugs, wires, cap & rotor - i mean if you didn't replace them you should anyway, they're dirt cheap and not worth fiddling around with cleaning and other activities :)

does the CEL come on when you put the ignition on (before starting the engine)? if not your bulb might be the problem, try and replace that first.

If the O2 is unplugged you will have a CEL so my money is that the bulb was removed by the person that had the Jeep before you.

[grumpygy]

it goes into open loop and drives relatively fine, that's why i'm thinking TPS. Of course it could be the more obvious which is an ignition problem, might want to have a look at the plugs, wires, cap & rotor - i mean if you didn't replace them you should anyway, they're dirt cheap and not worth fiddling around with cleaning and other activities :)

does the CEL come on when you put the ignition on (before starting the engine)? if not your bulb might be the problem, try and replace that first.

If the O2 is unplugged you will have a CEL so my money is that the bulb was removed by the person that had the Jeep before you.

Just a quick thought they could have even unplugged the CEL light cause they got tired of seeing it.

[mikepotts]

i bet you guys are right about the light!  either unplugged or removed entirely, i will check that and yes a full tuneup is on the list.  so glad i joined here!   

[Jeffy]

There are 6 screws holding the bezel in place.  4 for the main and 2 for the pop-out piece.  There are 4 for the white gauge cluster.  You can slip it past the headlight switch tab without removing it if you're careful.  The plug is on the left side.  Simple to disconnect them swing the cluster out.  It could be that the light was removed, burnt out or has corrosion on the connection.  Pretty simple to check though.

[mikepotts]

thanks jeffy!  i will check that out tomorrow.

[mikepotts]

hey guys, i figured i would update this.  i got a bung welded in and O2 sensor installed, it seemed to run a little better for a day or two then back to the same ole thing, so next up i swapped the t-bodys (with TPS) from the spare motor and no change.  ive been covered up with work and hadnt touched it again.  BUT, i have been gathering parts SYE, XJ front shaft (to cut down for the rear), new (to me) center gauge cluster, taurus fan, 70 amp relay, ect... i will get back to work when it settles down.

[sharpxmen]

did you sort out the CEL? that could help with the troubleshooting.

[mikepotts]

did you sort out the CEL? that could help with the troubleshooting.

i installed the gauge cluster donated by a friend and that fixed the clock, temp gauge and im thinking once i get a sending unit the oil pressure gauge.  then i moved on and pulled the main bezel to find someone had sure enough, removed the buld from the "maint req'd" spot... funny thing is, there is no socket in the CEL location... i thought maybe it used the maint req light for code retreval, but it never started flashing when i put it in retreval mode???