Hi all, brand new to the forum and apologize in advance for any newbness to my post.

I have an '85 Cherokee with a 350 TPI swapped out of an '86 Vette (L98 heads). The engine has been over bored 0.030", I run a mild cam and around a 10 to 10.5:1 CR.

I had been chasing a "MAF Low" trouble code for forever and just recently traced it to a bad ground connection at the MAF burn off relay (after 3 MAFs, 2 ECMs and countless hours of frustration).

While the trbl code was on, the engine was still running fine. I assume it was running in a limp mode of some sort.

While this was going on, my registration had expired and I had to go through an emissions test. I failed on high NOXs. I run true dual exhaust with fairly new Red/Ox cats. I don't think it was the cats fault.
I suspect that the high NOXs were a result of high combustion temps.
What I'm guessing is that while in the limp mode, EGR is disabled and there was no re-injection of non-combustibles into the intake, and so the combustion temps were high.

Anyone know if that is correct?

My only other thought was that it could have been running lean do to the engine mods requiring more fuel than the limp mode is calibrated for. But...... while on the diacom and before the MAF problem was solved, it WOULD go into closed loop and toggle between lean-rich at the O2 sensor.

Thanks in advance for the help.

 

There are various levels of "limp home" modes. I like to reserve the term "limp home" for actuall limp home mode, which is what happens if you yank the chip out of your ecm and try to run the vehicle, not pretty at all, but it will get you home. When the SES light comes on it just alerts you to a problem. The ecm usually only shuts down that particular sensor or other related routines, it does not completely shut off and rely on the "limp home" circuitry. In your case yor MAF was reading low, which in turn made you run lean which causes high NOx. Give her another whirl I bet it'll pass now.