My car is a '74 2.0. I've replaced all the vacuum tubing, cleaned the PCV system, rebuilt the TPS, set fuel pressure, verified no air leaks, and replaced the MPS and CHT sensors, and it's now starting and idling fairly well. I need to "dial in" the idle mix and I'm assuming that the throttle body air bleed screw and the mix adjuster knob on the ECU are not where they should be. My question: is there a starting position for the bleed screw and ECU knob, after which minor adjustments can be made once the engine is fully warmed up?

Supposedly, you set the idle to 950ish with the bleed screw and then tune the mix to a particular CO level with the dial. My car came with the ecu marked where the factory set it (almost in the middle of the possible range). After rebuilding the mps etc, it's still there, or pretty close.

What emerygt350 said.

Assuming timing is good and all FI stuff is working/calibrated properly and no vacuum leaks...
Once you get the TB air bleed screw set, turn the ECU knob counter clockwise one click at a time, waiting about 20 seconds or so between each click. As soon as the idle starts hunting, turn it back (clockwise) until the hunting stops. If you're more than a few clicks from center (I think there are 10 positions?) then you've probably got something else in the system that needs to be addressed.

OK, that's very helpful, thanks. I'll check to see if there's a factory mark on the ECU or else turn it to mid range and go from there.

The mark on the ECU looks like a little melted area, its not an obvious line.

You will want to make sure the car is totally warm when tuning,

Some good responses here, my input.

You have to set the idle when the car is fully warm (at least 30 minutes of driving in warm temperatures) and with headlights and both blowers on. While the spec idle setting is 950 rpm. I would set it to 1000 rpm or 1050 rpm. A bit higher idle speed has worked for me.

Precursors to setting the idle include the following:

* Valves adjusted to spec
* Ignition timing and dwell (if you have points) to spec
* Proper spark plugs in good condition, gapped to spec
* Verify proper operation of your aux air regulator
* Verify proper operation of your PCV valve
* Battery fully charged (12.75 V) before starting
* Charging system verified in good condition (battery, VR, alternator)
* No vacuum leaks, hoses properly routed
* Verified operation of your decel valve (18 inHg onset)
* Thermostat and flappers working properly (fully open hot, fully closed cold)
* Unmolested MPS, proper part # for your car, no vacuum leaks
* Proper ECU part # for your car
* Verified good CHT and air temp sensors
* Verified operation of your distributor vacuum retard (and advance, if present)
* Verified proper operation of your distributor mechanical advance

I know it seems like a long list (and I probably missed something), but all of these systems have to be working properly before setting your idle.

As for the ECU knob position, as others have noted, there's a mark on the knob surround that from what I can tell represents the Bosch "factory" setting. Start with the stub on the knob lined up with the mark. The proper way to set this is to get the car fully warmed up, turn on the headlights and blower motors, and use a shop quality exhaust gas analyzer to set the CO to an initial setting of 3.0%. Porsche says that CO can be in the range of 1 to 3% CO, but personally, I don't believe any T4 D-Jet 2.0L has ever idled worth a shit with CO much below 3.0%. A local tuner shop should be able to help you out with setting the CO, they'll have a calibrated analyzer. I've been using a Heathkit exhaust gas analyzer that I bought and assembled in the early '80's, seems to produce good results for me. You may be able to do the same with more modern analyzers that are based on oxygen sensors. FWIW, with my Heathkit analyzer, with the ECU knob set as described above, my idle mixture CO is 3.0%.

Once you have the idle set as described above, that's your idle setting, don't futz with it. What you'll find is that with this setting, your post-start idle setting will be high, as much as 1400 rpm, and as your car warms up, the idle will slowly drop, and eventually end up near your idle setpoint. Having the lights and blowers off will raise the idle by 50 to 100 rpm, so in the daytime with no blowers, your idle may be in the range of 1100 to 1200 rpm. As long as your 1st gear synchro is in good shape, and you anticipate engagement at lights by declutching a bit early, this idle setting should work fine.

The simple fact is that D-Jetronic has crappy idle stability compared with modern cars. Modern FI systems use active idle stabilization to achieve a rock-steady idle under nearly all conditions. D-Jetronic is "open loop" and has no feedback in the ECU idle circuit, and no mechanism (eg. like stepper motor control of the air bleed screw) to control idle speed, other than by the secondary effects of varying idle mixture (not a good method). Putting up with idle issues is just something that most D-Jet owners learn to live with .

Don't scare him! That is definitely the exhaustively right way to do it, but you could just get the car warm and give it a go. If your timing and dwell are good and you don't have leaks you should be able to get it rightish.

To recognize it is not right is pretty easy. Unreliable idle, too high when cold or too low, etc. Although I have pretty carefully gone through all of my djet (often with the help of banders writings) it didn't take too much work to get a rock solid idle at 900-950 in my 73. In the end I did almost everything in that list, but yours may not be as messed up as mine was.

I've had this Porsche for four months now, so I don't scare easily