Running Rich After Rebuild

  • #1

    Running Rich After Rebuild

    I recently acquired two 80s vintage 300s, an 85 and 86, both non-turbo. They are carbon copies of each other except the 85 is a 5 sp while the 86 is an auto. Neither has been smoked or registered in about 6 years. Because the 86's body is in better shape, I am using the 85 as a parts car for the 86.

    I rebuilt the 86 engine. Compression, cylinder leak down all checked good. Also put the 5 sp from the 85 into the 86 and, because the 85 ECU is compatible with the transmission, put the 85s ECU in the 86.

    Ran the diagnostics on the ECU and came up good with flashing 44s. But the engine runs extremely rich and doesn't come close to passing smog.

    In reading these posts I've seen references to Nistune which may be the answer but I'm not familiar with it.

    Any thoughts appreciated.

    Last edited by dpowers6059; 07-10-2018, 11:25 AM.
  • #2
    Shouldn't need Nistune, since everything is factory.

    Check for vac leaks, might be a hose dangling somewhere.
  • #3
    Thanks for the thought. I’ve had the engine checked by a real mechanic for leaks and he didn’t find any, he claims the problem us engine management although he’s not familiar enough with the z31 to pinpoint it. I’ll check again.
  • #4
    Are we talking 'failed smog' rich or 'my rear bumper is black' rich?

    Check here: http://www.xenonzcar.com/z31/ECUdifferences.html

    If you're using the 86 harness and sensors and the 85 ECU, you've got the wrong O2 sensor in there.
  • #5
    Hi Frozen - No black smoke but car fails smog big time, i.e. over twice the gross polluter level in HC and CO per Cali standards. Thanks so much for your reference, I think maybe this has nailed the problem. I'm going to switch out the O2 sensor and see the results. I'll keep you posted.