It sounds like OBDII VPW.

You may want to think about getting an OBDII to Serial converter and testing with a PC first. I have made several interfaces between these networks and pics. I find it easier to use a converter chip instead of dealing with the protocol myself.

Check out the following for more information.

This company makes the OBDII to serial chips:
www.elmelectronics.com

This company makes assembled units but also publishes pcb's and schematics for personal use:
www.scantool.net

If you want to do it yourself onboard, read the following it talks about VPW and all of its timing requirements:
http://www.circuitcellar.com/adverti...auge%20fuel%22

Let me know if you have more questions.