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Oxygen Sensor Replacement
 
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Pelican Technical Article:

Oxygen Sensor Replacement

Steve Vernon

Time:

1 hour1 hr

Tab:

$70 to $150

Talent:

**

Tools:

22mm or 22mm crows-foot wrench, floor jack, jack stands, wheel chocks, safety glasses, torque wrench

Applicable Models:

Porsche 944 Turbo (1986-89)

Parts Required:

O2 Sensor

Hot Tip:

Let the car cool before working on it

Performance Gain:

Proper running motor

Complementary Modification:

New air filter

The oxygen sensors (also called O2 sensors) are one of the most important elements of modern fuel injection systems. A finely tuned fuel injection system with an oxygen sensor can maintain an air/fuel ratio within a close tolerance of .02 percent. Keeping the engine at the stoichiometric ratio (14.7:1 air/fuel ratio) helps the engine generate the most power with the least amount of emissions.

There are a few signs that your oxygen sensor may be failing. In general, it is difficult to diagnose problems with the sensor, unless all of the other components in the fuel injection system have been checked and determined to be operating correctly. Some of the symptoms of a failed oxygen sensor system are: Irregular idle during warm-up, irregular idle with warm engine, engine will not accelerate and backfires, poor engine performance, fuel consumption is high, and driving performance is weak, CO concentration at idle is too high or too low.

O2 sensors must be heated up to normal exhaust system temperature to work properly. There are two types of O2 sensors used on 944s. Early 944s used a single-wire O2 sensor, which is simply heated by the engine's exhaust up to normal operating temperature. The single wire provides the output signal to the DME. Since it takes some time for the O2 sensor to get heated up, the DME operates in an Open Loop until it reaches normal temperature.

Later 944s use a three-wire O2 sensor where there is one signal wire and two wires to provide power to the sensor to heat it up more rapidly. This reduces the amount of time that the DME operates in Open Loop before the sensor provides a good output signal. Make sure to check the type of sensor your vehicle has before ordering.

Always purchase the correct sensor for the car. The wire resistance and proper connection to the harness are vital for the performance of the sensor. Snipping and soldering wires together can affect the resistance in the wire and cause the sensor to give false readings. While the factory sensors are expensive, in the long run you will probably not end up saving yourself any money and end up doing the same job twice if you install cheap generic sensors.

Begin by safely jacking up and supporting your car. Please see our article on safely raising and supporting your vehicle for further assistance. ALWAYS work on a cool car and exhaust system.

While access to the oxygen sensor is from below the vehicle the connection is located in the wiring loom behind the intake manifold (red arrow) at the rear of the engine bay.
Figure 1

While access to the oxygen sensor is from below the vehicle the connection is located in the wiring loom behind the intake manifold (red arrow) at the rear of the engine bay.

Begin by locating the wiring connection for the O2 sensor.
Figure 2

Begin by locating the wiring connection for the O2 sensor. It will be a circular plug (red arrow) unless someone has used a universal connection when replacing a sensor in the past. Unplug the connection and move to underneath the vehicle. If you cannot find the oxygen sensor connection from above, you can remove the sensor and then trace the wires up.

With the vehicle safely raised and supported and the exhaust cool, locate the O2 sensor from underneath the car.
Figure 3

With the vehicle safely raised and supported and the exhaust cool, locate the O2 sensor from underneath the car. Follow the turbo crossover pipe from the exhaust header to where it runs up by the lower control arms. The sensor is located below the turbo on the exhaust input side of the turbo (red arrow).

Use a 22mm crows-foot socket (red arrow) or wrench and remove the sensor from the pipe.
Figure 4

Use a 22mm crows-foot socket (red arrow) or wrench and remove the sensor from the pipe. Use care as these can get corroded in place and there is not much room to work.

You cannot tell if an oxygen sensor is working correctly just by looking at it (red arrow), but you can check for problems with the motor by looking for oil on the sensor.
Figure 5

You cannot tell if an oxygen sensor is working correctly just by looking at it (red arrow), but you can check for problems with the motor by looking for oil on the sensor. The new sensor should come with a small amount of anti-seize on the threads (yellow arrow). Do not get any of the anti-seize on the end of the sensor when installing. Always install the sensor by hand until it is seated correctly and then finish tightening by wrench. You do not want to cross thread the sensor or you will have a heck of a job in front of you to fix it. Installation is the reverse of removal.


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Comments and Suggestions:
Alabama Sam Comments: Regarding the 22 MM crow's foot socket to remove the sensor, the sensor's location on top of the exhaust pipe makes it difficult to get much leverage. On my 1989 NA I disconnected the sensor wire, pulled it down to where it was hanging below the car and threaded the wire through a six point box wrench. You get at it from forward of the sensor, not rearward. This allows the application of a lot more torque. PS That still wasn't enough for my car. I had to drop the exhaust pipe from the header back to the cat and take the pipe to a bench and apply much heat to get it out. The threads were galled but I managed to dress them up with a tap sufficiently to be able to screw the new sensor in.
August 26, 2015
  Followup from the Pelican Staff: Thanks for sharing your installation process and experience. These type of comments add so much to the Pelican tech community.
- Nick at Pelican Parts
 
Nick Comments: Just as an FYI. You recommend a torque wrench for this job, but don't give a torque spec.

Thanks for the how to!
- Nick
June 16, 2015
  Followup from the Pelican Staff: Here ya go: 36 - 43 ft.lbs (50 - 60 Nm)
- Nick at Pelican Parts
 

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