So in the end...what is the proper air-fuel ratio for a 928?

-I just finished scraping the dirt from under my fingernails from my latest escapade, the installation of an air-fuel meter. That happened at about 0100 last night, which means my neighbors' sleep was disturbed shortly thereafter...

Well, in the short drive last night I confirmed what I'd suspected all along: My '85 S2 is indeed running rich...

[Not that all that black smoke wasn't telling me something already!]

Well, this meter is a PLX M-300, and includes a Bosch O2 sensor that I mounted in the Ott X-pipe. The indicator is in digits, not LED's, so it is pretty precise. It is so precise that it is a bit hard to read- the digits change very rapidly. When the car was shut down, it read 14.67, which is normal, I guess. After startup, my car went to a fairly steady 11.00 to 1, with the last two digits constantly changing. Under acceleration for the first few miles, it read between 10.00 to 1 and "RICH", which indicates off the scale. After the car warmed up, it seemed to settle into approximately a 12.50 to 1 situation, but still went off scale RICH whenever I put the throttle to the floor. Upon coast-down, I'd get the typical loud popping and backfires, and the meter indicated between 16.00 to 1 and "LEAN", which is to be expected.

I plan to spend the day running errands and burning my left arm by threading it under the spider with a 3 mm allen wrench with which to adjust the mixture at the MAF. What approximate air-fuel ratio should I be striving for under 1. Normal driving [lean please; gas is expensive!], and 2. Hard acceleration?

Thanx in advance!

N!

 

1. At normal/cruise conditions (..non-WOT), your air/fuel should be 14.7:1, which is stoichiometric.

2. Under hard acceleration (..WOT), best power is typically made at 12.5:1, so shoot for that.

At idle, you should be at stoich, or slightly richer.

On deceleration (..throttle closed completely), you should be full lean since the injectors are turned off until RPM's reach 1300 RPM where they are turned on again to avoid stalling (..S4's do that for sure).

 

Hi Normy,
The mixture pot will affect idle mixture the most, it does also have some effect higher up the airflow scale, but small.

Check that temp sensor 2, as measured at the ECU is reading OK. If all else fails you could fool the ECU to make the mixture all round weaker by adding a resistor is parallel with the sensor.

What you describe is not normal for a S2 ! Also check that there isn't a permanent WOT switch signal at the ECU.

Regards

 

The mixture pot is only for idle and part throttle conditions.
No way is the mixture gauge capable of really measuring correct mixture. it is just an early warning device. Its plus or minus a full point, minimum. Ive tested them on the dyno with sensitive equip. 13.5:1 to 12.2 :1 all read the same on the guage.
as was stated, cruise should be an average , after hunting back and forth, lean to rich based on the o2 sensor feeding the computer. disconnected, you would be on the lean side and the lights on the guage would be stable. upon full throttle , the fuel enrichment swith, would richen up the mixture.
the o2 sensor that you are using is pretty crude to be testing for specific ranges.

the part throttle mixture, can be adjusted with the pot, and you should go to a smog shop or garage and put a sniffer in the tail pipe and adjust.
however, if you dont care about smog and polution, or your car is a racer, forget about that, as those settings have little to do with performance.

mk

 

Does your car ever bog @ WOT?

 

Do you have a 5 wire O2 sensor? Or just a 1,3, or 4 wire unit?

If you don't have a 5 wire, the O2 sensor tells you one of 3 things. Rich of 14.7:1, lean of 14.7:1, or at 14.7:1. Thats IT. Anything else is just pretty lights, or flashing numbers created by the makers marketing department.

The problem is that other than the flip in voltage from rich to lean, the O2 sensors are rather unpredictable. The same voltage out of 2 diffrent sensors doesn't result in the same mixture. Heck, the same voltage, at two temps doesn't mean the same mixture outof the same sensor!

So take the narrow band O2 sensor's readings with a grain of salt.

Now, if you've got a wide band unit, your good to go. That is a 5 wire sensor.

 

Thanx Lag for answering my question! Straight and to-the-point as always.

VU- It is indeed a 5 wire Bosch O2 sensor. It has a VW part number etched in the side, not sure from which car. The PLX unit is a wide-band, with a narrow-band output so that a second O2 sensor for the engine computer is not needed. Pretty clever....

The S2 series of cars never had O2 sensors from the factory, since they weren't catalyzed. Indeed, when I installed mine last night, I discovered that the "knock-out" in the sound insulation for the O2 wire had never been removed.

-After some screwing around with the mixture pot, I finally managed to get to a point where it isn't too rich, yet still has decent power. However, this leaning out has definitely worsened my minor hesitation. In the end, I wound up turning the pot exactly 4 turns counter-clockwise.

John Speake: I've tested the continuity and the resistance of the wire from the T2 sensor [which is new], and it meets the specifications in the book. Same with the wires from the MAF. The switches on the side of the throttle body worked as per the manual as well.

[scratches head]

Geez, with all this extra fuel maybe I really should add a blower, just to even things out!

I suspect that I'm going to need to take this car to the chassis dyno here in town. I've heard that they can generally give you a good idea why a car exhibits certain symptoms- a point in the right direction, per se. You'll probably see my air-fuel gauge on Ebay in a few months. It's fun to look at in the meantime, though it takes up a lot of room in the glove compartment.

Well, I drove around tonight with a clipboard and recorded some readings [all numbers approximate]:


IDLE:

Cold engine: 14.6
Warm engine: 15.0

Steady:

20 MPH, 2nd gear, 2000 RPM: 13.8

35 MPH, 2nd gear, 3400 RPM: 12.7


40 MPH/ 70 KMH:

Steady, 4th gear, 2000 RPM: 13.5

Steady, 5th gear, 1400 RPM: 14.0


80 MPH/ 130 KMH:

Steady, 4th gear, 4000 RPM: 14.3

Steady, 5th gear, 2900 RPM: 13.8


Wide Open Throttle:

80 MPH- 100 MPH, 5th gear: 10.7 [some hesitation]

50 MPH- 70 MPH, 5th gear: "RICH" for two seconds then variable between mid 10's and mid 11's. Some minor hesitation.

Another interesting point that I noticed today: As the day warmed, the mixture became progressively leaner. I started the engine from cold in the morning [75 F/ 24 C], and in the afternoon [90 F/ 32 C], and the trend continued. Porsche has programmed the chip to lean the mixture as density altitude increases due to increased temperature. I have wondered about this for some time- whether they would lean it to match the "thinner" air or richen it in order to help cool the engine.

Of course, these numbers are indeed relative terms. As pointed out, I guess they really don't tell the true story....

N!

 

Those are some strange numbers. Your idle is a little bit lean, but most of the rest of the numbers boarder on acceptable to rich. I'm thinking something is dumping fuel into the system at a non concistant rate.

What have you check so far?

The list of things I ended up checking when I was undergoing a similer rich mixture problem.

Temp I
Temp II
Gross AFM failure
Fine AFM failure
fuel pressure regulator and dampaner leaks
fuel rail pressure
WOT enrichment switch
leaking injectors

Thats pretty much it. Your car has comperable sensors and units, and there is really no way to know exactly which would be causeing you problems. So start at the top, and go to the bottem. Enjoy...

Mine turned out to be a way out of spec. AFM. Good luck.