JeepMN

1999 xj 4.0 AW4 np242

I'm basically just wondering if there is a known way to test our ECU's/PCM's? I've looked through every thread on this website about PCM's and couldn't find a single one about testing it so that leads me to believe I'm SOL..

Reason I'm asking is cause I flopped my Jeep on it's side and my PCM was definitely underwater for around 2 hours and basically the only thing wrong other than body damage from the flop is that my fuel mileage has been even worse(not good to start with) since the misfortune. Basically around 8 mpg. So, could I have a faulty PCM on my hands, even though I don't have any other symptoms of one like misfires or cutting off? All my electronics work as intended.

Btw i've replaced spark plugs, wires, cap, rotor, air filter, front 02 sensor, catback exhaust, cleaned TB/IAC, regular oil changes, coolant flush, 2 new calipers up front(none of them are sticking now).

Thoughts anyone? Thanks in advance.

JeepMN

And if it matters

4.5" lift
33'' duratracs
4.56 gears

I went from around 12 mpg(still think it was running rich before the flop but didnt care enough to look too much into it), to around 8 mpg and me looking at my fuel gauge and seeing it drop while driving around town..

CCKen

The PCM is pretty much a sealed device. If anything, the connectors to the PCM need to be flushed out with CRC QD Electronic Cleaner. Same with the pins on the PCM.

If the PCM suffered water intrusion your Jeep probably wouldn't run at all.

The only way the PCM could be tested, internally inspected, is to send it to a shop that does that sort of thing.

Edit: Did the downstream (after Cat) O2S, and its connector) get submerged? The downstream O2S does contribute to gas mileage.

JeepMN

Thanks for the clarification Ken. I'll go ahead and spray out all the connections someday this weekend just for gigs.

And as far as I know, the downstream sensor was never submerged but I guess crazier things have happened. From reading on the forums though I thought the back O2 sensor had no relation to fuel readings? Obviously I'm wrong by you saying that but what function does it serve?

CCKen

Get a six pack and enjoy this long drawn out piece on Oxygen Sensors...

FSM stuff.

Oxygen Sensors

OPERATION (FSM Section 14)

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 (vent holes in the body of the O2S for NGK Sensors). 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:

The heaters on both sensors are fed battery voltage from the ASD relay which is controlled by the PCM.

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:

The upstream O2S 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:

The downstream heated oxygen 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.

OXYGEN SENSOR (O2S) MONITOR (FSM Section 25)

Effective control of exhaust emissions is achieved by an oxygen feedback system. The most important element of the feedback system is the O2S. The O2S is located in the exhaust path. Once it reaches operating temperature 300° to 350°C (572° to 662°F), the sensor generates a voltage that is inversely proportional to the amount of oxygen in the exhaust. The information obtained by the sensor is used to calculate the fuel injector pulse width. This maintains a
14.7 to 1 Air Fuel (A/F) ratio. At this mixture ratio, the catalyst works best to remove hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxide (NOx) from the exhaust.

The O2S is also the main sensing element for the Catalyst and Fuel Monitors. The O2S can fail in any or all of the following manners:

• slow response rate
• reduced output voltage
• dynamic shift
• shorted or open circuits

Response rate is the time required for the sensor to switch from lean to rich once it is exposed to a richer than optimum A/F mixture or vice versa. As the sensor starts malfunctioning, it could take longer to detect the changes in the oxygen content of the exhaust gas.

The output voltage of the O2S ranges from 0 to 1 volt. A good sensor can easily generate any output voltage in this range as it is exposed to different concentrations of oxygen. To detect a shift in the A/F mixture (lean or rich), the output voltage has to change beyond a threshold value. A malfunctioning sensor could have difficulty changing beyond the threshold value.

OXYGEN SENSOR HEATER MONITOR

If there is an oxygen sensor (O2S) shorted to voltage DTC, as well as an O2S heater DTC, the O2S fault MUST be repaired first. Before checking the O2S fault, verify that the heater circuit is operating correctly.

Effective control of exhaust emissions is achieved by an oxygen feedback system. The most important element of the feedback system is the O2S. The O2S is located in the exhaust path. Once it reaches operating temperature 300°C to 350°C (572°F to 662°F), the sensor generates a voltage that is inversely proportional to the amount of oxygen in the exhaust. The information obtained by the sensor is used to calculate the fuel injector pulse width. This maintains a 14.7 to 1 Air Fuel (A/F) ratio. At this mixture ratio, the catalyst works best to remove hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxide (NOx) from the exhaust.

The voltage readings taken from the O2S sensor are very temperature sensitive. The readings are not accurate below 300°C. Heating of the O2S sensor is done to allow the engine controller to shift to closed loop control as soon as possible. The heating element used to heat the O2S sensor must be tested to ensure that it is heating the sensor properly.

The O2S sensor circuit is monitored for a drop in voltage. The sensor output is used to test the heater by isolating the effect of the heater element on the O2S sensor output voltage from the other effects.


CATALYST MONITOR

To comply with clean air regulations, vehicles are equipped with catalytic converters. These converters reduce the emission of hydrocarbons, oxides of nitrogen and carbon monoxide.
Normal vehicle miles or engine misfire can cause a catalyst to decay. A meltdown of the ceramic core can cause a reduction of the exhaust passage. This can increase vehicle emissions and deteriorate engine performance, driveability and fuel economy.

The catalyst monitor uses dual oxygen sensors (O2S’s) to monitor the efficiency of the converter. The dual O2S’s sensor strategy is based on the fact that as a catalyst deteriorates, its oxygen storage capacity and its efficiency are both reduced. By monitoring the oxygen storage capacity of a catalyst, its efficiency can be indirectly calculated. The upstream O2S is used to detect the amount of oxygen in the exhaust gas before the gas enters the catalytic converter.
The PCM calculates the A/F mixture from the output of the O2S. A low voltage indicates high oxygen content (lean mixture). A high voltage indicates a low content of oxygen (rich mixture).
When the upstream O2S detects a lean condition, there is an abundance of oxygen in the exhaust gas.

A functioning converter would store this oxygen so it can use it for the oxidation of HC and CO. As the converter absorbs the oxygen, there will be a lack of oxygen downstream of the converter. The output of the downstream O2S will indicate limited activity in this condition.
As the converter loses the ability to store oxygen, the condition can be detected from the behavior of the downstream O2S. When the efficiency drops, no chemical reaction takes place. This means the concentration of oxygen will be the same downstream as upstream. The output voltage of the downstream O2S copies the voltage of the upstream sensor. The only difference is a time lag (seen by the PCM) between the switching of the O2S’s.

To monitor the system, the number of lean-to-rich switches of upstream and downstream O2S’s is counted. The ratio of downstream switches to upstream switches is used to determine whether the catalyst is operating properly. An effective catalyst will have fewer downstream switches than it has upstream switches i.e., a ratio closer to zero. For a totally ineffective catalyst, this ratio will be one-to one, indicating that no oxidation occurs in the device.

The system must be monitored so that when catalyst efficiency deteriorates and exhaust emissions increase to over the legal limit, the MIL will be illuminate

firehawk618

There's no way for you to test an ecu.

The places that sell refurbished ones I always question.....

How exactly do they refurbish the ecu? Clean it up and send it back out?

There's no way that these places have some kind of a bench test other than quick simple tests.

Those won't expose intermittent or issues that crop up after time.....

But back to your issue. About the only thing you can do is make damned sure the connectors have no water / corrosion in them and you could take apart the ecu and visually inspect it for water ingress.