My reason was a plugged cat. Do you hear a rattle when running underneath? My fuel mileage is much higher now and good god the power she has now.

 

No rattles, everything's tight and she's got lots of power, at least I think. This spring I'll put in a high flow though.

 

Mine would still get up and go, now she'll throw you back in the seat.

 

Since its OBD2, put it on a scan tool and view live data to make sure coolant temp goes high enough to achieve closed loop. Then look at your fuel trims to determine if fuel trim is extremely high or low. Don't just use a code reader

 

I've got one of those powerful snap-on readers at work ill use tomorrow, thanks

 

Did you do the down stream o2 sensor?I get over 2o in my Renix also sea foam kills o2 sensors..

 

From what people have told me the downstream is only there to set off a CEL. others have said that's not the case for OBD2 jeeps so Im going to replace it anyway

 

I've read that a cracked exhaust manifold will drop your MPG's

 

New exhaust manifold.

 

Let us know how this goes

 

Subscribed. I get 8 mpg so I'm with ya lol. When you get gas, try pouring it in on the slowest setting. The faster the fuel goes into the tank the more air it brings. Less air in theta I will help with mpg. At least that's what someone told me.

 

Downstream O2 is catalyst efficiency monitoring. It checks that the cat is working, does not affect fuel trim

 

I might've missed this, but have you verified the temperature reported by the coolant temperature sensor with an infrared thermometer?

Any chance you've tested the MAP? I believe there's a way to see if it's working properly by checking its impedance while running.

What on earth are you talking about? Miles driven / gallons used = Miles Per Gallon...

Only on OBD-I. His Jeep is an OBD-II model, so all O2s are used for fuel trim.

 

OBD1 does not do catalyst monitoring, there's no post cat O2. If you read attached article, you'll see OBD2 compares pre-cat and post-cat to determine if the converter is doing its job. Only under certain conditions does the PCM look at the after cat O2 readings.

http://www.aa1car.com/library/obd_monitor_not_ready.htm

 

My 1999 FSM was too ambiguous and only said "O2 Sensors". While it did not say explicitly Upstream and Downstream, it also did not say Upstream Only. The 2000 XJ FSM has a much better explanation of what happens:

OXYGEN SENSOR—PCM INPUT
DESCRIPTION
The Oxygen Sensors (O2S) are attached to, and protrude into the vehicle exhaust system. Depending on the emission package, the vehicle may contain either 2 or 4 sensors. On non-California emissions packages, 2 sensors are used: upstream (referred to as 1/1) and downstream (referred to as 1/2). On California emissions packages, 4 sensors are used: 2
upstream (referred to as 1/1 and 2/1) and 2 downstream (referred to as 1/2 and 2/2).

OPERATION
An O2 sensor is a galvanic battery that provides the PCM with a voltage signal (0-1 volt) inversely proportional to the amount of oxygen in the exhaust. In other words, if the oxygen content is low, the voltage output is high; if the oxygen content is high the output voltage is low. The PCM uses this information to adjust injector pulse-width to achieve the 14.7–to–1 air/fuel ratio necessary for proper engine operation and to control emissions.

An O2 sensor must have a source of oxygen from outside of the exhaust stream for comparison. Current O2 sensors receive their fresh oxygen (outside air) supply through the wire harness. This is why it is important to never solder an O2 sensor connector, or pack the connector with grease.

Four wires (circuits) are used on each O2 sensor: a 12–volt feed circuit for the sensor heating element; a ground circuit for the heater element; a low-noise sensor return circuit to the PCM, and an input circuit from the sensor back to the PCM to detect sensor operation.

Oxygen Sensor Heaters/Heater Relays: On a certain non-California emission package, the heaters on both sensors are fed battery voltage from the ASD relay which is controlled by the PCM. Refer to ASD relay for more information. On another non-California emission package, the heaters on both sensors are fed battery voltage from the two O2S heater relays. The O2S relays are also controlled by the PCM. On the California emission package, the heaters on all 4 sensors are fed battery voltage from the two O2S Heater Relays.

The O2 sensor uses a Positive Thermal Co-efficient (PTC) heater element. As temperature increases, resistance increases. At ambient temperatures around 70°F, the resistance of the heating element is approximately 6 ohms. As the sensor’s temperature increases, resistance in the heater element increases. This allows the heater to maintain the optimum operating temperature of approximately 930°-1100°F (500°-600° C). Although the sensors operate the same, there are physical differences, due to the environment that they operate in, that keep them from being interchangeable.

Maintaining correct sensor temperature at all times allows the system to enter into closed loop operation sooner. Also, it allows the system to remain in closed loop operation during periods of extended idle. In Closed Loop operation, the PCM monitors certain O2 sensor input(s) along with other inputs, and adjusts the injector pulse width accordingly. During Open Loop operation, the PCM ignores the O2 sensor input. The PCM adjusts injector pulse width based on preprogrammed (fixed) values and inputs from other sensors.

Upstream Sensor (Non-California Emissions): The upstream O2S sensor (1/1 sensor) is located in the exhaust downpipe before the catalytic convertor. 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 fuel delivery to maintain the correct oxygen content at the downstream oxygen sensor. The PCM will change the air/fuel ratio until the upstream sensor inputs a voltage that the PCM has determined will make the downstream sensor output (oxygen content) correct. The upstream oxygen sensor also provides an input to determine catalyst efficiency.

Downstream Sensor (Non-California Emissions): The downstream heated oxygen sensor (1/2 sensor) is located near the outlet end of the catalytic convertor. The downstream sensor is also used to determine the correct air fuel ratio. As the oxygen content changes at the downstream the PCM calculates how much air fuel ratio change is required. The PCM then looks at the upstream oxygen sensor voltage and changes fuel delivery until the upstream sensor voltage changes enough to correct the downstream sensor voltage (oxygen content). The downstream oxygen sensor also provides an input to determine catalyst efficiency.

Upstream Sensors (California Emissions): Two upstream sensors are used (1/1 and 2/1). The 1/1 sensor is the first sensor to receive exhaust gases from the #1 cylinder. Both of the upstream O2S sensors are located in the exhaust manifold just before the mini-catalytic convertors. They provide 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 fuel delivery to maintain the correct oxygen content at the downstream oxygen sensors. The PCM will change the air/fuel ratio until the upstream sensors input a voltage that the PCM has determined will make the downstream sensors output (oxygen content) correct. The upstream oxygen sensors also provide an input to determine mini-catalyst efficiency.

Downstream Sensors (California Emissions): Two downstream sensors are used (1/2 and 2/2). The downstream sensors are located in the exhaust downpipes just after the mini-catalytic convertors. The downstream is also used to determine the correct air fuel ratio. As the oxygen content changes at the downstream the PCM calculates how much air fuel ratio change is required. The PCM then looks at the upstream oxygen sensor voltage and changes fuel delivery until the upstream sensor voltage changes enough to correct the downstream sensor voltage
(oxygen content). The downstream oxygen sensors also provide an input to determine mini-catalyst efficiency.