D-Jet Woes

[Tim Polzin]

I have a 74 2.0 L engine with D-Jet fuel injection. I rebuilt the engine over the winter. Prior to the rebuild, I had some bucking at constant throttle that I assumed was caused by a worn throttle switch. Otherwise the throttle response and mixture was good. Now after the rebuild, with the throttle switch connected, the engine runs very rich, even stumbling a bit when I give it gas. If I disconnect the connector at the throttle switch, I get what appears a normal mixture, reasonable throttle response and a minor increase (200 to 300 RPM) in idle speed. I took the throttle switch off to clean the contacts and visually checked it to make sure nothing was grossly wrong.

I then checked the regulator the usual way (sucked on the hose to see if it held vacuum, it did) and tried a spare regulator as well. There was no difference between the two regulators.

Any suggestions?

[Dave at Pelican Parts]

Sounds like you've got the diagnosis narrowed down to a faultyor misadjusted throttle switch. Try adjusting it. If that has no effect, see if replacing it has any effect.

The throttle switch serves two functions. The first is signalling the ECU when to go into "idle" mode. The second is telling the ECU to squirt extra fuel when the throttle pedal gets pushed down. With the switch disconnected, neither of those two functions is active.

The "idle mode" of the ECU is where the FI actually pays attention to the knob on the ECU. So you can also try playing with that knob. CW should be a richer idle, CCW should be leaner. Adjust the switch first, though.

To adjust the switch, you check for continuity between two pins on the switch. I use the "beep" function of my multimeter for this. I'm never sure I remember which two pins are the right ones to put the probes of the multimeter on, but I think it's the center pin and the pin next to it on the left--as seen looking directly at the switch. That would be to the right-front of the car when the throttle is mounted on the engine.

You loosen the two screws holding the switch onto the throttle. Then rotate the switch CCW (as looking directly at it) until you don't hear a "beep". Then rotate it CW until you do. Find the "edge" between the two, then rotate CW by one mark on the screw holes (about 2 degrees). Tighten the two screws. Now work the throttle lever by hand. You should get a "beep" only when the throttle is closed, or open a really tiny bit.

That's it.

--DD

[Tim Polzin]

Looks like you were pretty close Dave. When I had the engine on the stand, I eye balled the idle contact closure. What I didn't realize was that I did not have a good contact (actually none at all) on one of the directional switches. Cleaned all the traces (again) and burnished them and the directional switches with some very fine wet/dry paper. I then installed it and set the switch for proper idle contact closure. It works!

I'm still don't understand why my engine ran so rich under these conditions, but now it's probably an insignificant point. Thanks again Dave.

[Wayne 962]

Hmm, my guess would be that the ECU, since there was no feedback from the switch, assumed that the engine was not at idle, and therefore adjusted the mixture for driving (richer).

Just a guess...

-Wayne

[Tim Polzin]

Sounds reasonable, except why would it not run rich when the switch is disconnected was well?

[Wayne 962]

Hmm, not sure. Maybe there is a contact in the switch to tell the computer that it's connected. When it's connected, then it looks for a reading? If it doesn't get one it assumes that it's between throttle positions, which then means that the car is running at a higher RPM with load?

Maybe? Not sure...

-Wayne

[Dave at Pelican Parts]

I have two guesses, but they are only guesses.

First guess. The mixture an engine needs at idle isn't necessarily the same mixture it needs under load. That's why there's a separate "idle mode" that the switch activates.

Second guess. The switch was sending a "fake" acceleration signal to the ECU, which was then putting out extra injector pulses, causing the rich mixture.

I don't think the ECU knows that the switch is connected or disconnected. I don't think there's any set of contacts that get bridged all the time the switch is plugged in.

--DD