jeepiah98

Hey Guys,
I’ve been having some idling/hesitation problems at startup with my 99 XJ. When I first start it up it’s fine, and just after about a minute (if that) it will idle down really low (200-300) for a brief moment and then rebound up to 1100-1200, and then back down to normal. If I start it up and start driving it will sometimes hesitate at acceleration unless I give it full throttle, which brings me right out of it.

It only happens when I first start the engine (not necessarily a cold start though) and goes away after a few moments. Also, I can hear a “pop” coming from the front driver’s side of the engine… and I hear the pop right when it bogs down. It’s so strange because the engine runs perfectly fine otherwise, and no CEL… thoughts on what this could be? O2 sensor?

Thanks guys!
Scotty

CoffeeCommando

That's an 02 sensor. I had that exact same problem with my 02 sensor and never got a code for it.


Note: DO NOT BUY A BOSCH 02 SENSOR. Go to NAPA and pick up an NTK, or don't buy one at all.


Also, I recommend replacing the rear as well. Code or no code as they get older they start responding slower (relaying alternating signals to the ECU). I replaced a sensor in the front once and it worked fine for a day (it was the 3rd bosch I had bought) and then I could smell richness from the driver's seat the next. I had no code.


Then I decided to replace the rear. After I replaced the rear I got a code telling me the front 02 sensor was responding too slowly. These things relay signals within fractions of a second to the ECM. If it's slow it's constantly out of phase and alternating lean/rich. It seems as though the ECU references one from the other. When one is responding faster it is in better condition, but the ECU doesn't know that unless one IS operating faster (IE a new one). If they're both slow then they're both working together and doing the job right... they're just doing it slower so the ECU can't make as many quick corrections to correct air-fuel-ratio. Might as well not even have it plugged in in that scenario. I actually left it unplugged and got better gas mileage without having to sniff gas fumes. THEN I found out the real deal about Bosch 02 sensors after searching around online and went to NAPA to pick up 2 shiney brand new 02 sensors.


Have had execellent gas mileage and performance since then. The idle is smoother too.


I posted this a few months ago: https://www.cherokeeforum.com/f2/02-...ion-fyi-94823/

VTJeep

EXACT SAME ISSUE. replaced both o2's w/ the high quality 4-wire pre-heated ones. the issue cleared up for a while, but came back w/ in a month or so.
now when i start it up i have to floor it when taking off to get it to clear up...
here is a list of replaced parts:

-new battery leads
-new cap/rotor
-new plugs/wires
-new (both from autopartswarehouse.com i think..) TPS and IAC valve
-blew out the gas lines and checked/cleaned the gas filter.
-pulled the TB and cleaned it
-new o2's twice now...

-what else... oh yeah, plenty of sea foam. even did it directly into TB... no white smoke, no improvements...

right after having the o2's done the first time, pulling out of the mechanics drive it kinda hesitated and run funny for a second, then cleared up and i went from 11mpg (all highway, easy on throttle) to 19mpg (same driving conditions)...
so idk. im at a total loss here. it SEEMS like a sensor issue (at least electrical) cuase when it gets WOT its gone till i start it again... if it was mechanical then going WOT shouldnt make a dif??? like i said: at a loss...

x2 on doing voth the o2's at same time.

CoffeeCommando

How do the connections look? The wiring? Has any damage been done to them anywhere so that the shielding is slighly missing and the metal is making contact somewhere?

The front is down there by the exhaust so that's something to consider.

I myself have recently starting getting the gas fumes to return. I also have a P0171 code though. I've had it since March and don't know how to get rid of it. I'm wondering if switching the ground with the signal sensor when I put on a new 02 barrel plug damaged the PCM 02 circuit.

That's the only thing I can think of... but the gas mileage isn't all that bad. I'm wondering if maybe it's just degraded a little and is on the line between just fine and relaying a "lean" signal when it's actually not lean at all.

CoffeeCommando

All 02's for the XJ are 4 wire. What brand did you use?

VTJeep

not all o2's a 4-wire, some are 2... idk if they fit my jeep, but im pretty sure they exist... i think
but, idk what brand, they cost like 110/ea (not that price has bearing on compatibility w/ vehicle...) the exhaust is in good shape, idk about the actual connection points for the o2's but im thinking they are good. just got some exhaust work done so everything under there should be good to go.
my CEL will pop on randomly. sometimes it stays on for a while (day or 2) others times it will go on and off in a couple hours...

im gonna go look at o2's just to make sure about the amt. of wires...

CoffeeCommando

I know this. Different vehicles have different sensors. OURS have 4 wires. My XJ, and everyone I have seen, has 4 wires on the other side of the connector.

Heater, Heater Ground, 02 Signal, System ground. If you somehow manage to connect something with anything less than what's is supposed to have you are gauranteed problems.

VTJeep

ok, i was wondering about the dif. amt. of wires on the o2's...
as far as i know nothing else but the o2's has been changed. i will have to check on the other parts you mentioned... thanks!

jeepiah98

Thanks, Commando!

I just got back from Napa and got the NTK as suggested.... Also a blow torch in case I have trouble removing the old one... I am going to try this myself now (right after lunch) and see how it goes. I picked up a 22mm wrench from auto zone figuring an adjustable would not be a good idea... Wish me luck!!,

Scotty

jeepiah98

Oh, and by the by, I disconnected the sensor this morning before running my errands... Had no problems at all, the idle purred like a kitten, and I swear the acceleration was better or at least smoother. Also, no fumes... I guess it defaults to factory operating specs without the sensor.

It ran so well I almost don't want to in tall the new one, haha... Though I know that's no good either.

Scotty

CoffeeCommando

Get an 02 socket. You don't want to damage it. Follow the directions. Torqueing it too much could destroy the internal sensor on it.. and don't drop it.