Surging and Unsettled Idel after MPS Rebuild, '74 2.0L D-Jet Options

[914four]

Running down a bucking issue and found my MPS to be leaking. After rebuilding the MPS and verifying a good seal, everything went back together okay. When I started the car, the engine surged up and down and would not idle.

The electrical connector socket on the MPS does not appear to be keyed and using the diagram below from Clay's D-Jet troubleshooting guide it seems I may have plugged in the MPS wire incorrectly. I ohmed the wires from the ECU connector to the MPS socket and made sure the wires are in the order shown in the diagram.

The wires look to be looping back from terminal 7 to 15 and 8 to 10 so that the socket could be plugged in either way and work correctly. Could plugging in the MPS wires incorrectly have damaged the ECU? If not, what should I be looking for that would cause the surging and lack of idle?

[JeffBowlsby]

The MPS may be calibrated too lean for your engine. It may be possible to compensate for that by bumping the fuel pressure slightly. Use a fuel pressure gauge and increase 1-3psi to see it idle and driveability improves.

[pbanders]

The MPS may be calibrated too lean for your engine. It may be possible to compensate for that by bumping the fuel pressure slightly. Use a fuel pressure gauge and increase 1-3psi to see it idle and driveability improves.

I agree with Jeff but suggest that you test this before making any changes. Either borrow or obtain a CO meter that can be used while driving on a flat road at part-load, and check your CO level. If you're seeing 2% CO or less you're too lean. I believe mine is set to 2.5%, Jeff can give you better numbers.

Jeff brings up a good point which is that the order in which mixture settings are done on the 2.0L and 1.7L D-Jet engines is critical. The first setting should be the part-load setting (center screw on the MPS), followed by the full-load setting (full-load stop), then the transition setting (outer screw). Once all of these are set, then the idle mixture is set using the ECU knob. All of these settings need to be done with everything else fully checked-out (fully-charged battery, good alternator/regulator, fuel supply, fuel pressure, ignition timing & dwell, proper plug gap, valve clearance, no vacuum leaks, etc, etc), and with a fully warmed-up engine.

914's were originally build with stock Bosch MPS calibrations locked into the sealed unit. The only mixture adjustment that could have been done at the factory was the idle mixture, and I doubt that was done, as the ECU's have a mark for the factory setting, and I suspect that's where it was left at. Only thing I believe was actually done on any of the motors is setting the idle bleed screw. If you want to do optimal tuning of the MPS for your engine, by following the procedures on my page, you can match the MPS more precisely to your needs, but it requires access to emissions measurement systems and a good understanding of MPS adjustments.