s346k

certainly not. I have a 180 t stay in my jeep. 100% stock otherwise. I get 22+ mpg. I got the same mileage before the swap.

Lead Foot

Since it looked like mainly 1,2, and 6 were running rich I checked the injectors. Spray pattern looks good.

B11A5157-B596-4C1A-8A9B-82F56A1CA4D9-2842-000001091CA73245_zps481c2fb4.mp4

Radi

I think you have no real options other than guessing until you can see the data feed from the sensors. Might be a bad O2, might be PCM trouble, could be 10 other things. Hard to say. I assume there aren't any codes set? Or they haven't been read?

salad

lol rage much? My point is that you SIGNIFICANTLY modified your XJ. And then something broke. Guessing the problem from here is nearly impossible and it's likely more than one thing. Go back and triple-check everything you've worked on. An inaccurate MAP reading is the prime suspect for a mileage drop that significant, whether the sensor is bad, dirty, the vacuum fitting is clogged, you have a vacuum leak, or there's a wiring problem is hard to tell.

In the later model OBD-II XJs the downstream O2 sensor is absolutely used by the PCM for setting fuel mix. It needs to be there for it to run properly. OEM is NTK and performs better than Bosch on Chrysler years (Bosch is only OE on Renix) and comes with the correct connector... no wiring hacking.

Of course you could have an issue with a number of other sensors too, IAT, CTS, TPS, they're all important and should be tested. Increasing the parasitic loss on the engine by adding that molasses (Lucas Oil Stabilizer I presume you're pouring in) isn't helping either.

What codes is the PCM throwing?

SoCalxJ4.0

If your downstream 02 is getting the same reading as the upstream, you are going to have issues.

Lead Foot

Thought I would check the intake/exhaust manifold bolts since it looked like the center cylinders were running more lean than the front and back cylinders. Was able to tighten them up about 1/2 to 1 full turn. So they weren't really loose. Took it down the road and didn't notice a difference. I also hooked the PCV valve on the rear of the valve cover to the vacuum port by the throttle body like it is supposed to be.

I slammed on the gas from a stop once and let it rev all the way to 4500 and go through all the gears. Then I got a check engine light. Drove down to the auto parts store to run codes and did a compression check. Figured might as well get new plugs while I was at it. I had 2 codes. First was downstream o2 sensor voltage high (which makes sense as that o2 sensor isn't hooked up). Second code was bank 1 running lean. Figured that was the code that set off the CEL when I went WOT.

Shot of the spark plugs when I took them out. No change from tightening the manifold bolts. #6 on the left, #1 on the right.



My compression test showed as follows. #6 135, #5 135, #4 145, #3 145, #2 120, #1 90. I retested 1 and 2 and got the same results twice.

I put in new Autolite platinum plugs, noticed a little better throttle response but that's it. Also changed the oil and filter. After reading through the articles on bobistheoilguy.com I went with Mobile1 0-30w. Oil pressure is still good at about 30psi, I did notice a little more valve clatter though.

Lead Foot

Again for anyone who missed the only codes that have been thrown were downstream O2 sensor voltage (no O2 sensor there) and bank 1 running lean (only appeared after a full WOT through the gears from a stop.

I did do some trial by error testing and started unplugging some sensors. Map sensor, drop in idle/stumble and almost died. Throttle Position Sensor, drop in idle/stumble and almost died. Intake Idle Air Controller, no noticeable change. Intake Air Temperature Sensor, no noticeable change. Coolant temp sensor, electric fan kicked on and gauge temp dropped, no noticeable engine change. O2 sensor fuse, no noticeable change. (Can anyone verify unplugging the fuse is enough?)

Lead Foot

I had the idea of grounding the O2 signal to see if there was a difference in idle. I didn't notice any change. I should have been able to hear a difference in the idle since the ECU will see 0 as a voltage correct?

If memory serves me correctly, the difference between closed loop and open loop is just the O2 and IAT sensors correct? If there wasn't a noticeable difference in idle when they were unplugged then doesn't that mean the jeep isn't running in Closed loop?

SoCalxJ4.0

I'm still pretty certain that its not going to complete the "loop", cycle properly with that rear 02 out. 98 motor right? These things are ran by computers. Everything in the system is there for a reason.

Lead Foot

Mine is a 98 but this is strait from the 99 factor service manual.

UPSTREAM HEATED OXYGEN SENSOR
The upstream O2S sensor is located in the exhaust
downpipe (Fig. 6). It provides an input voltage to the
PCM. The input tells the PCM the oxygen content of
the exhaust gas. The PCM uses this information to
fine tune the air/fuel ratio by adjusting injector pulse
width.


DOWNSTREAM HEATED OXYGEN SENSOR
The downstream heated oxygen sensor is located
near the outlet end of the catalytic convertor (Fig. 6).
The downstream heated oxygen sensor input is used
to detect catalytic convertor deterioration. As the convertor
deteriorates, the input from the downstream
sensor begins to match the upstream sensor input
except for a slight time delay. By comparing the
downstream heated oxygen sensor input to the input
from the upstream sensor, the PCM calculates catalytic
convertor efficiency.
When the catalytic converter efficiency drops below
emission standards, the PCM stores a diagnostic
trouble code and illuminates the Malfunction Indicator
Lamp (MIL).

SoCalxJ4.0

Word. My 96 downstream went out and it started running like it was on 5 cyl. I dunno, that's all I got bud.

DougyFresh

Instead of arguing about the rear o2's purpose, why don't you just take the 15 minutes that it would take to hook it up correctly and do so?

854x4

You killed the o2 sensor by driving that long with it unhooked. When they are unhooked the engines wont adjust the fuel mixture to compensate. Also when computers are reset they need to be driven to relearn. You need to put a OE type sensor not a universal one. Remember unhooked or non functioning o2 sensor affect fuel mileage. Last thing running a sock as an air filter is letting more air in the intake, more air = more fuel to burn the air.

Lead Foot

Its not an argument when the FSM clearly states that it only reads cat condition and doesnt effect A/F ratio.

I dont have an O2 sensor for the rear nor do I have a cat, or even a bung to put an O2 sensor in downstream. That and it looks like I need to order an OE Mopar sensor online so I need to wait for it. Does that still sound like 15 minutes to hook it up? If it was that simple then I would.

Lead Foot

I've never heard of an O2 sensor going bad because it was simply unhooked... Can you elaborate on this more please? I know what the purpose is of an O2 sensor, of course it adjusts A/F Ratio.

More air does equal more fuel, but your application of the theory seems off to me. The engine sucks as much air as it needs, I am making it easier by not running a filter (currently, dont even have that sock on). Its the same concept as running a cold air intake or K&N filter ect. To say that it will kill fuel mileage is completely off. Turbos force even more air into an intake, yet NA engines with added on turbos and adjusted ECUs to add MORE FUEL dont see worse fuel mileage... See what I mean?