Engine won't idle at 0% TPS, doesn't respond to timing.

So over the winter I've been converting my W124 300E to a standalone EFI system. I'm using the MaxxECU Race, and the engine (if you couldn't tell already) is the venerable 3 liter M103. I've been meaning to wrap up the project, and I'm almost done, but I want it to run stably at idle/low RPM to move it out of the shed, before I give it a course road tune, so I don't have to tow it to the dyno to wrap it up.

So anyways, the engine runs, if I start it with about 5-10% throttle it starts up nicely, it runs nicely between 750-850 RPM depending on how much I keep the throttle open, but as soon as I try to lower the throttle position to down near the 3 percent, it just falls into this 500-550 idle and adding timing doesn't get it out of that zone, only more throttle gets it back to that nice 700-ish RPM area. If I close the throttle further, the engine inevitably dies.

Neither using the idle ignition control nor directly playing with the ignition seems to help. If it is of any help (and it probably is, it might be the main reasonI'm having this issue), I have no idle control set up, as in, no addition hardware to control idle speed.

Am I wrong in assuming that an engine should idle by just playing with the fuel and ignition? Do I strictly need an idle control setup to achieve a good idle? Seems counterintuitive, but the data seemingly points me in that direction. I feel like I'm overlooking something simple, so please add your two cents!

If the throttle isn't open enough for the engine to idle at 750RPM, changing fuel and ignition from optimal is only going to make it worse.

If you don't have any idle bleed or drive by wire then your only option is to open the throttle and use fuel/ignition to lower the RPM as needed.

You need airflow through the engine to allow for it to idle, if you close the throttle too much, and do not have an idle bypass valve, then there will not be enough airflow through the engine to maintain combustion so any changes to fueling and ignition will not make a difference. Essentially you are removing one side of the fire triangle by closing the throttle.

Check the throttle position calibration, then just open it up as much as is required to get the airflow it needs.

I suspect you have some hang-up and fixed idea that it should work at 3% opening - remember, that isn't 3% or the potential, wide open, airflow, but 3% of the angle from closed to fully open, which is practice may be a fraction of a percent airflow.

If it needs 10% to get a reliable idle, set it to that, then adjust the mixture and timing to get the bst idle speed, close the throttle slightly, and repeat.

If you don't have any sort of cold idle speed assistance, what is commonly done is to run the timing a bit retarded when hot, and advance it when cold as the increased efficiency will increase the torque and idle quality.