I have a 1993 Del Sol Si with a D15Z1 swapped into it. My first of several Del Sols When I first got it the O2 sensor (5 wire) was not connected and the car's harness had a 4 wire connector. My solution was to put in a 4 wire sensor and it worked fine (no CEL) until I noticed my Gas mileage was not as high as I expected and The VTEC seemed to kick in at about 4,800 Rpm. I checked the ECU and found it was a P28. I just happened to have a P07 on the shelf with a spare engine and plugged that in. Now I get a CEL code 41.

The guy I bought it from had mentioned the O2 sensor and ECU when I bought it, but at the time I had no idea it all meant.

My goal for this car is to get max gas mileage. I think I need to modify the harness to use the 5 wire sensor, but really I'm not sure and looking for suggestions. The best info I can find on wiring in the 5-wire sensor is here:
https://honda-tech.com/forums/showpo...3&postcount=22 . Are there more complete instructions/FAQ?

 

Update:

I made a sub-harness to connect the 5 wire O2 sensor to the existing 4 pin connector, plus 3 wires direct to the ECU and 1 to ground.
The car is now a functioning Del Sol VX model.

I just connected it all up this evening. I have not had a chance to check my gas mileage, but it ran great with no CEL. Lower end power seems better, the VTEC-E actuates at about 2.8K RPM, and the gearing seems to make a lot more sense now than it did running with the p28 ECU.

This thread was great in helping me to figure it all out:
https://honda-tech.com/forums/showth...3#post43799033
I think that the pin numbering on the O2 sensors is wrong in the diagrams, but everything else looked good.

I looked at some pin-out diagrams of OBD1 ECUs to verify what goes where. Most of the forum threads related to this are about how to put a 4 wire sensor in a VX harness.

I needed two connectors, a male for the 4 wire O2 connection (Cut off an old sensor) and female 8 pin (Cut off an old harness) for the 5 wire O2 to plug into.

The wires going to the ECU have to have ECU female pins. All 3 were open slots at the ECU connector. At first I made it with similar looking pins, but found out they were just a little bigger than the ECU connections.

Here's a simplified sketch of how my harness is laid out:


I don't expect there will be a lot of other people looking to do this, but you never know. Performance is about the same as my 'S' model, but I'm expecting about 50+ mpg with this one.