What about on L-Jet cars? The idle mixture is set via a bypass screw which is part of the air flow meter. What are the implications of leaving the O2 sensor disconnected?

I currently have the O2 sensor disconnected, and it seems to run better that way. With the O2 sensor connected the mixture is constantly changing, even at idle. The effect is a noticeable "throbbing" of the engine at idle as the mixture goes rich and then lean, with a period of about 2 seconds.
During this "throbbing" the engine speed varies by about 100 R.P.M.. With the sensor disconnected the idle is pretty stable. When off idle, but not at WOT, the car feels weak with the sensor connected, but pretty good with it disconnected. Is there any real reason why I should try to get it to behave better with the sensor connected, or should it be O.K. to just leave it off?

The car is a US 83S (M19 engine), but it has the camshafts from a Euro 83S (M11 engine). While I'm asking hard questions maybe I'll throw these ones out. What ignition timing might be correct for this setup? With the timing set as the manual suggests for the original cam (~8 degrees at 650rpm idle) the manifold vacuum at idle is rather low (~11" Hg), if I advance it quite a bit (~20 degrees at 750rpm idle) by turning the distributor I can get about 17" Hg which seems more 'normal'. Any thoughts? Would it seem O.K./safe to try running with this 'advanced' setting? What are the dangers of running
with the timing "too far" advanced?

I know that the camshaft I now have was originally used with a higher compression engine (10:1 vs. 9.3:1), had K-Jet vs. L-Jet, had larger throttle body and intake and exhaust valves, and is longer duration (Intake Opens 9 degrees ATDC vs. 11, Intake closes 52 degrees ATDC vs. 46, Exhaust opens 37 degrees BBDC vs. 25, Exhaust closes 2 degrees BTDC vs. 2 degrees ATDC). Unfortunately my expertise as to what this means in terms of engine setup is essentially nonexistent ;).

Any advice/speculation/guesses welcomed.

Eric Buckley


>with 57psi being the optimal psi stock with stock injectors, is there a calculation out there that helps you determine where is a good psi to start with larger than stock injectors

There's a fuel injection flow calculator at
Fuel injectors are rated at 43.5 psi of fuel pressure. A 19 lb. injector operated at 57 psi would flow 21.75 lbs./hr. Plugging different fuel pressures into the calculator for 24 lb. injectors, it shows that a 24 lb. injector would have to be operated at about 36 psi of fuel pressure to flow the same as the 19 lb. injector did at 57 psi. When on the dyno, make sure you have a safe full throttle air/fuel ratio through the whole RPM range, and tune by the power produced, and not by trying to obtain some specific air/fuel ratio. The idea air/fuel ratio can be different for different cars, and also for different RPM points on the same car.

>Also it appears the Repair manual has details about coding a plug. Is this the same plug, as it is not very specific.

The coding plug is not the same as the variable resistor (potentiometer) mentioned. There are several different coding plugs available, depending on whether the car is equipped with or without cats, with an automatic or manual transmission, and on fuel octane. The coding plugs are all eight position connectors, with different coding plugs having different pins connected to each other internally. The configuration of which pins are connected inside the particular coding plug tells the computer what settings to use. All cars will have one of the coding plugs, and it does not plug in place of the O2 sensor.

The potentiometer is used independently of whatever coding plug is in the car. It's used to adjust the mixture. On the early 32 valve cars, the mixture adjustment was done by removing a tamper resistant plug on the MAF sensor housing, and turning the adjusting potentiometer that was in there.
On those cars, the O2 sensor is unplugged, the adjustment made, and the O2 sensor is reconnected. The potentiometer is the coarse adjustment, and the O2 sensor signal is used by the computer to continually do the fine tuning of the mixture. On the later cars, a potentiometer was no longer put into in the MAF sensor. Cars without cats use those same MAF sensors without potentiometers, but a potentiometer to allow for mixture adjustment was located near the LH computer instead.

>Or put it another way - is it worth me spending a lot of time getting the idle mixture on my non-cat car spot on for better full throttle performance?

The full throttle mixture would depend on airflow sensor signal and the chip mapping when the full throttle switch is activated. Any idle mixture settings probably wouldn't have any real effect then.

>Amazing that an electronic fuel injection system designed to be interactive with a mixture monitoring device could run adequately without the interaction. Did you do any custom mapping?

Without the O2 sensor, the computer uses the airflow sensor signal information and the mapping on the chips to control fuel flow into the engine. It's not as accurate as if the O2 sensor information was also used, since the computer doesn't actually know what the air/fuel ratio really is.
Instead it kind of assumes what the mixture should be based on what the airflow sensor is telling it the mass of air entering the engine is. That's the same way it works at full throttle, when the system goes open loop and the O2 sensor signal is ignored.

Mike Schmidt
'88 928S4 Auto Black/Black "PORSCHE" cloth