Running Lean

[Kirby2]

Probably a well known issue, but my wife's 944 N/A won't pass smog because of NO2. The air fuel ratio is too lean...great CO and CO2 numbers but too high NO2.
Engine starts first pull, runs cool, no missing, no cutting out, good power.
(new: plugs, wires,cap and rotor, recond & balanced injectors, fuel reg, fuel filter, 5 day old cat, no air leaks)

[nynor]

altitude sensor is bad? oxygen sensor? hell, i might try just unplugging the O2 sensor, forcing the computer to rely on the fuel maps...

you can turn the dme switch so that it is a bit richer... maybe retard the timing a bit as well, just to pass the emissions.

not an expert, just hacking my way through life.

[Zero10]

I'd check O2 sensor, and try some injector cleaner.
Check for vacuum leaks as well. What is the manifold vacuum at idle?

[rigitrite]

It doesn't have to be very much lean to shoot the NO2 through the roof. If you have access to an exhaust gas analyzer, you should try and set the lamda value to .99 for the optimum emissions performance. Changing this value .05 either way will kill your emissions, especially the NO2.

[Porsche944]

I beg to differ.

I have experience with early cars, late cars, and turbos, and I always set the A/F ratio so the O2 sensor reads just over 0.6 volts.

I recently had my '87 NA (with 250K+ miles on the odometer) inspected and it had 0 (Zero) NoX.

[Kekec944/84]

I have a 84 944, how do you set the air fuel ratio?

[Porsche944]

After looking at my handy little graph (Motronic Engine Management, p 5), I'd say you have a serious vacuum leak.

What is happening is there is too much oxygen in the air charge. The CO and HC numbers are low because all of the HCs and COs have been turned into lots of CO2s and H2Os.

However, there is still some "fuel" to burn because the Earth's atmosphere is just a tad over 70% Nitrogen. Add that to the high temperatures we get from a complete fuel burn, and we get NoX, or NO2.

The only way this situation can happen is if there is "unmetered" air entering the intake. Unmetered refers to the fact that air has entered the intake tract upstream of the Air Flow Meter. Its just a fancy way to say vacuum or intake leak.

Anyway, I'd start with checking vacuum at the Fuel Pressure Regulator and the brake booster. Typically the brake vacuum will be a tad higher. This is because brake vacuum comes directly off of the intake manifold, while vacuum from the FPR comes from the Throttle Body.

While you're checking for leaks, put some small nylon cable ties on all of your vacuum fittings. I've seen engines gain as much as 3" Hg just by tie wrapping a few fittings.

Hope this helps.

[rigitrite]

Cliff,
I'm sure you're correct on the O2 sensor voltage. I'm refering to the stoichiometric value though; 1.0 lamda being perfect 14.7:1 air to fuel. I don't know how that translates to the O2 value, but I bet 0.6 volts is 1.0 lamda.

[Porsche944]

Actually 0.50 volts is 1.0 lamda.

I use 0.6 volts because of how the DME is set up to compensate for no input from the O2 sensor.

If you look at the O2 sensor input on the DME schematic you will see that the sensor input goes to a voltage divider network that has a "bias" of 0.45 volts.

With no O2 sensor input, the DME "sees" the 0.45 volts as a lean condition and richens the fuel/air mixture. This richer mixture insures that the engine does not run lean or produce detonation (knock or ping).

In this situation the DME is in "Open Loop Control." In this mode the DME "ignors" the O2 sensor and uses a default set of fuel and ignition data to compute injector duty cycle.

Once the O2 sensor heats up, and the output voltage is greater than the 0.45 volt bias, the DME shifts to "Closed Loop Control." In this mode the DME uses the air load signal from the AFM, the input from the O2 sensor, inputs from the speed and reference sensors, and a temperature input to compute ignition advance angle and injector duty cycle to produce an O2 sensor output of 0.5 volts.

By disconnecting the O2 sensor and setting the AFM so that the output from the O2 sensor is >0.50 volts I am insuring that the A/F ratio will be on the rich side should the O2 sensor fail during normal operation.

To set the AFM you need a dwell/tach, 1/4" drive 7mm socket, 6mm Allen wrench, a 6" jumper wire, and a digital multimeter. An analog meter will not work. The input impedence is way to low and tends to load the circuit under test.

As an added bonus I have an LED A/F ratio "meter" that I use in conjuntion with my DVM.

Anyway, you disconnect the O2 sensor and connect your meter to the sensor out put (black wire) and ground. Set the meter to read 2 volts DC. NOTE: On later cars you can 'back probe' the O2 sensor connector. The connector is a pliable rubber and a test probe can be inserted through the back of the connector. This way the connector will hold the test probe and give you a good electrical contact. This also makes it very easy to do both open and closed loop testing by simply connecting or disconnecting the sensor.

Connect the Dwell/Tach to the ignition coil (Green is (+), black is (-) ).

The 7mm socket is for adjusting the idle by-pass on the throttle body. The 6mm Allen is for the AFM adjustment. The jumper is used to disable the idle stabilizer.

To disable the idle stabilizer, put the jumper between pins B and C of the diagnostic connecter that is located on the driver side fender just out board and below the emissions purge valves.

With your meters and such connected (no jumper installed yet), start the engine and let it come up to temperature. Note the engine RPM and DVM reading.

Install the jumper to disable the Idle Stabilizer. RPM may drop significantly, so be ready to open the idle by-pass on the throttle body.

Turn the idle bypass as necessary to set the idle RPM to 840 +-50 RPM. Clock wise= slower, Counter clock wise= faster.

With the idle set, adjust the AFM to get a DVM reading >0.50 volts DC. Make your adjustments a little at a time. Turn the AFM bypass about 1/4 turn and WAIT a few seconds for the DVM reading to change. Turn the AFM bypass clock wise for a richer mixture (DVM reading goes up), or counter clock wise for a leaner mixture (DVM reading goes down).

What you are doing here is changing the amount of air that is measured by the AFM. By opening the bypass, less air impinges on the flapper, which causes the flapper to close slightly. The flapper moves a wiper across a resistor track that is part of a voltage divider. The wiper "picks off" a voltage that proportional to incoming air flow rate and sends it to the DME.

As you "tweak" the AFM you may see engine RPM either rise of fall. Use the idle bypass to set the idle RPM so you maintain 840 RPM. Basically what you are trying to do is "balance" engine RPM with the A/F ratio, so you may need to make several adjustments.

Once you have the O2 sensor and RPM readings stable, remove the ISV jumper. Note engine RPM and DVM readings.

At this point you can reconnect the O2 sensor. Engine RPM may change briefly, and the DMV reading should start to vary (or dither) across 0.50 volts.

What is happening is the DME is altering the injector duty cycle to produce an O2 sensor output that averages 0.50 volts.

If you "blip" the throttle you should see the DMV reading drop and then start to dither again. If you hold the throttle open so the engine is running at about 2K RPM, the DMV should dither more rapidly.

At this point you can shut the engine down and remove the meters.

Take the car for a drive. Start out in regular traffic. Note how well (or how bad) the engine runs. Keep and eye on oil pressure and coolant temperature. Don't do any jack rabbit starts, just normal street driving.

Once you have this base line, take the car for an extended drive on a highway. You can be a little more "spirited" here, but don't get busted! What you want to do is run the car hard enough and long enough to "flush" the CAT.

After you have completed the test drives, recheck your O2 sensor and RPM readings. They really shouldn't change, but its better to be safe than sorry.

At this point you should be able to get the car inspected, assuming of course that you have found and corrected your intake leak.

Hope this helps.

[Kekec944/84]

Thanks Porsche944 I will tinker with that when I get a chance.