D-Jetronic MAP Sensor - Any tricks to knowing it is calibrated?

My ‘76 XJ12C is back together and running well. I had the injectors cleaned and flow tested by SD Faircloth in Florida. Amazing how different exhaust note is and how much smoother the idle is.
I am tweaking on the idle trim screw on the ECU in the trunk to find a sweet spot and feel I’m almost there. Little rich, little lean turning back and forth trying to zero in.
Regarding the MAP Sensor on the D-Jetronic system, I’ve read rather extensively on various websites such as jetronic.org and other marque’s discussion forums and of course, this forum. Mine is not in its factory setting. (These are considered Type 3 MAP Sensors)
But gasoline has changed since the 70s. Mine has been turned, yes including by me.
What I do know - if you turn the hex adjustment screw clockwise, it will create a flat spot upon heavy/or full acceleration. So I was backing off the screw (CCW) and have reached a point where full acceleration is much more linear/smoother.
But is there a better way to calibrate these? How do I know it is right? Am I leaving some full acceleration on the table? Or part throttle load on the table?


You can choose between lean best or rich best, or go anywhere in between, you know that the ‘map’ is slightly incorrect so maybe you just have to go by fuel consumption and as lean as it likes, before it starts to suffer. Or go by CO values. It’s always a compromise. As you have done is going to be the best you can get. Eliminate flat spot and not any leaner.

Another thought I haven’t figured out - does the idle ECU circuit basically “override” the MAP sensor at idle, or is the MAP sensor input still being used at idle?

In the past few days, I have tweaked back and forth, the MAP sensor, the idle trim screw on the ECU, and the idle air valve screw (rpm control). I have bounced back and forth between all three trying to find a sweet spot for the smooth idle. It idles wonderfully. Always allowing 10-15 seconds after a change to see how engine reacts.

I’ve basically got the MAP sensor screw where if I turn 1/8 turn clockwise or 1/8 counter clockwise, the engine gets worse, so feel it is pretty dialed in.

The idle air valve screw easily moves the rpm up or down, doing its job.

The ECU idle mixture trim screw can change the exhaust note to very heavy sounding (clockwise) to lighter note and a pick up in rpm as adjusting counter-clockwise, always giving 10 seconds or more between clicks on the adjustment. Currently positioned much closer to full CW vs. full CCW.

The engine idles wonderfully at 850 rpm, gearbox in park.

But put a load on the engine with foot on the brake in gear, and my idle is not smooth and is not acceptable.

Other thoughts?

Didn’t get to spend much time on the car yesterday, but did start the engine. idling pretty dang well while cold. Put it into Reverse and it idled rather smoothly.
So when fully warm, not idling smoothly in gear.
Obviously, the engine is in an over-rich situation while trying to warm up.

My temperature sensor for the ECU is not exactly in range, giving a little low reading across the temp range, according to my laser temperature sensor shooting the aluminum boss around the sensor. I have a large variable resistor, should I try to build in additional resistance?


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