Fuel Injection

This section contains special information on the fuel injection system that is not found in the Service or the Bentley manuals.

All information contained in this FAQ is provided by BMW enthusiasts who are not typically fully trained in the art of BMW maintenance. As such, all information in this FAQ is provided "as-is". Any use of this information is strictly the responsibility of the using party. The supplier of the information and the Webmeister assume no liability for incorrect information or use of this information.

Oxygen Sensor - Testing & Replacement

Air Flow Meter Pointer Repositioning and Repair

Universal Oxygen Sensors - "Steve D'Gerolamo" <steved3@village.ios.com>

There is a universal 1-wire oxygen sensor available from Bosch (# 11027) which should sell for under $25.00. Many of the WD's are selling a 3-wire sensor (#13913) as a universal oxygen sensor which should sell for under $40.00. This latter sensor does have a plug on it which may have to be adapted (ie, cut off the connector and crimp on solderless connectors-heat shrink tubing will finish the connections nicely) to mate with the connection running to the engine. The Bosch oxygen sensors are all basically the same with different part numbers for different plug connectors and lengths of wire. Use only Bosch oxygen sensors for your BMW applications.

Steve D'Gerolamo c/o The Ultimate Garage, Emerson, NJ (201-262-0412)


In last month's column I wrote a little about the theory and operation of your engine's Lambda sensor This month I will address the inspection and testing of this critical component.

As I stated in part one of this article, the O2 sensor usually performs unnoticed for at least 50,000 to 60,000 miles. If you suspect your sensor has failed, or is not operating up to specifications, there are several inspections and tests that can be performed before laying out the money for a new sensor.

The first point of inspection should be the connector for the sensor. Because an O2 sensor's output is a very small voltage (1 volt or less), any problem at its connection can cause the output to be erroneous. Make sure this connection is free from corrosion, grease. or any other foreign substance. Also check that the sensor's wires are not damaged, especially from heat from the exhaust manifold.

Next jack the car and place stands under it to support the car's weight. NEVER WORK UNDER A CAR WITHOUT JACK STANDS. The O2 sensor, which looks like an overweight spark plug can be found between the exhaust manifold and the catalytic converter Once again carry out an inspection, this time of the area surrounding the sensor An oxygen sensor must have a free flow of ambient air to com pare with the exhaust gas in order to function properly. If a sensor is blocked by mud or other trash and can not `breathe" it will not function.

With the inspections completed, test the output of the sensor with your Digital Multi-Meter. Don't make the mistake of using an older generation analog meter to test the sensor, as possible damage to the electronics of the DME computer and the O2 sensor can result.

Depending on the age of your Bimmer and the Motronic system installed, there will either be a one, three, or four wire O2 sensor. With only one wire, there's no problem determining which wire is the signal wire. If it is a three or four wire system and you don't have an Electronic Trouble Shooting manual to look up the details of the wires, there is a simple method to determine which wire is the signal wire. Place your DMM in the DC volt position and attach the black common test lead to a good chassis ground. Then find the O2 sensor's connector and disconnect it from the car's computer With the engine running, use the red test lead (positive) to probe the wires of the O2 sensor. Only one wire should have an output and it should be under one volt.

Once you find the correct wire, stop the engine and reconnect the sensor You will then need to break out the signal from the sensor with it still connected to the computer. I have spliced in a short test wire which enables me to test the sensor at any time. You could also use a test lead end that has a small alligator clip with a piercing tooth that can pene- trate the wire's insulation. I prefer the first method as it allows me to install a long test set of wires which I can then hook up to my meter and place in the front seat of my car. This setup allows me to test the sensor under real world circumstances as I drive.

With the sensor hooked up to the meter, restart the engine and let it warm up to norma) operating temperature. Set the meter to DC volts and then set it to use the four volt range. If your meter has the ability to record minimum, maximum, and average voltage readings. use this feature as it allows you to record and save the O2 sensor's signal the whole time it's under test. With the engine at temperature, run the throttle up to 2000 RPM and hold it for 30 seconds and then release it. Goose the throttle once and then press the meters hold button. This locks in all the voltage readings captured during the test. If your sensor is working properly, the minium voltage should be under 0.200 DC volts, the maximum at least 0.700 DC volts and the average close to 0.450 DC volts. A rich running engine would show a much higher average. Use the chart as a simple guide for diagnosing oxygen sensors.


Min Max Average TestResults ------- ------- ------- ----------------------------------

<200 >700 400-500 Oxygen Sensor is OK

>200 NA 400-500 Replace Sensor

NA <700 400-500 Replace Sensor

<200 >700 <400 Running lean

<200 <700 <400 System is lean. Enrichen mixture to see if the sensor reacts, if not, replace sensor.

<200 >700 >500 Running rich

>200 >700 >500 System is rich. Lean mixture to see if the sensor reacts, if not, replace sensor.

all numbers in millivolts (mV).

Another important test to consider is how fast a sensor reacts to a mixture change. Check this by forcing the engine lean (remove the oil filler cap during engine operation), and then rich (inject propane into the air inlet, or restrict the flow of intake air). A properly functioning sensor should change voltages instantly. Faulty sensors generally lock onto one voltage and do not cycle up and down. Shorted sensors will read 0 volts.

Of course, it goes without saying, you should never condemn the O2 sensor unless you're sure the rest of the engine is running properly. There are many engine problems could point out a leaky fuel injector would cause a higher then average O2 sensor reading, a torn intake hose would cause a lean mixture, etc.

One final caution, some people are under the impression that disconnecting the O2 sensor will benefit them and give them more power This is not the case. The sensor is a very important component in the engine management system and is there to fine tune the fuel injection system and give you an optimum running engine.


Air Flow Meter Pointer Repositioning and Repair - Barry Jay Burr <justbarrys@snip.net>

Correcting Idle Fallout due to air flow meter carbon band wear on Ljetronic and Motronic models:

The problem of intermittent stutter at mid throttle, failure to hold idle, and the engine reponding when the throttle is depresses as if it were out of gas very often is due to wear on the carbon band within the Air Flow Meter.

The dealers will say replace the AFM. Thatcan cost you a few hundred dollars or more. Instead of that, try thefollowing procedure. My old 84 318 still had the original AFM at228,000 miles because of doing this procedure several times, and I'm pretty sure that AFM is still in action, now somewhere out in Minnesota:

Remove the black plastic lid.Notice the carbon dust on the pointer tracks on the carbon band.VERY GENTLY, ever so lightly lift up the pointer from the band using a small bladed screwdriver or thin hook tipped nose pliers.Blow the dust away, compressed air is preferred. If you don't have a compressor, see below.

To verify band wear or loss of pointer tension is the problem, holdthe pointer at about the place it would be at idle. If you are notfamiliar with this, try about a centimeter from the resting end. Thentry to start the car. If just by pressing down on the pointer at the right spot gets the car to hold idle, then you have found the cause of your problem.

Now to regain tension at the pointer:

1. Notice the end of the copper pointer arm where it joins thepivot.

2. Notice where the arm is a double V shape, with an open slot between the inner and outer sections of the V

3. Press down on the arm, only on one side of the V, just away from the pivot until you just feel a little chance of it bending a bit. Nowdo the other side of the V, again just away from the pivot.

4. Pressing down GENTLY on the tip of the pointer with a fingertip, move the pointer forward and back across the range of movement of thec arbon band.


You may not get it exactly right at first, but once you get the ideaand can see exactly what you need to do, a little more or less pressure exerted on either side of the pointer arm will restore full contact over the full range of throttle motion.

You may then need to adjust the pointer position a tooth leaner or richer because of the slightly different resistance at any given point. This is done by moving that big black gear a tooth forward or backward.The gear retainer is easily lefted by a screwdriver. Be sure to put a dab of white-out or something to mark the original position so you can go back to initial setting if you need to.

The goal of this bending and repositioning of the arm and pointer is to both:

A. restore tension to the arm so it presses down sufficiently on the carbon band again and B. reposition the pointer ever so little further inward or outward so that it is now rubbing across an unworn part of the carbon band. You may then need to adjust the pointer position a tooth leaner because of the slightly different resistance at any given point. [If you don't have an compressor, then try this: Take a bicycle tire and tube, remove the valve; hose clamp a length of fuel line hose onto the valve stem; cut the stem and valve out of another bicycle tube, preferable the presta valve style as used on road bikes; hose clamp that second valve and stem to the free end of the fuel line hose. Now use a bicycle pump on the valve at the free end of the hose. Simply by pressing lightly on the tip of the presta valve, you can get precise controlled applied bursts of however much you pump up the tire to. That can easily be 120psi or more for two to three seconds on a road race tire, or 75psi or more for five seconds or so with a balloon mountain bike tire. ]