My "mostly" stock 91 L98 has always passed smog. The smog driver guy has always tested it at ~ 2000rpm (1st gear for the 15mph test and ~1842 rpm, 2nd gear for the 25mph test at ~ 1993 rpm).

This last time ( few days ago), the guy ran the 15mph test in 2nd gear, dropping the rpm down to ~ 1250, and I ended up failing the 15mph test for NOx emissions.

Not much has changed in the two years since the last test.. Runs just as good, only put about 10,000 miles on it since then.

is the load of 2nd gear at 15mph, and the low ~ 1250 rpm enough to "bog" the engine and force a fail because the engine is being bogged? Certainly I never drive it with the rpm that low.

I will note that they ran the 15 mph test for the full two minutes on the rollers, when they accelerated to 25mph it passed that within 10 seconds.

Can I suggest to a smog guy (by taking in my last pass) that he keep the car in the gear of my choice?

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2005 numbers:

15mph
rpm: 1842
%CO2: 12.7
%O2: 2.8
HC(PPM): 59
CO(%): 0.09
NO(PPM): 417

25mph
rpm: 1993
%CO2: 12.6
%O2: 2.9
HC(PPM): 42
CO(%): 0.07
NO(PPM): 516

2007 numbers:

15mph
rpm: 1251
%CO2: 14.2
%O2: 1.1
HC(PPM): 90
CO(%): 0.10
NO(PPM): 1227

25mph
rpm: 1985
%CO2: 14.2
%O2: 1.2
HC(PPM): 58
CO(%): 0.12
NO(PPM): 641

Since the car passed the 25mph test instantly, I have to think that there was a major reduction in NOx almost immediately after the rpm came up, and that perhaps once the combustion chambers/heads/pistons cooled down from being bogged down at 1250rpm, that the other numbers would have cleaned up even more (had they run it for the full two minutes).

thoughts?

 

If I remember correctly from my emissions inpecting days about 2 years ago, dropping the RPM would potentially result in the CO and HC to increase. Where as a higher RPM would increase the temperature in the chamber and burn the "excess" gases. As far as NOX, numbers will increase with higher RPM, bc it's caused by higher temps.

Maybe a little insight for you.

 

the lower RPM can cause your problem.
what Cbras said makes sense, but at the higher RPM the motor isn't loaded as much so the NOx is normally lower, but not always.
also, the EGR valve may not be opening at that low of an RPM.
here in Texas, there is a RPM range the motor must be run in, 2500 RPM is the max with any where from 400 up to 1200 RPM being the minimum.
if he can/will run it in first for the low speed test, i believe you'll pass just fine.

 

Yeah, I concur with checking the EGR, make sure the diaphragm is opening properly and not stuck, that is if it's a mechanical one, if it's electrical, then I don't know what to tell you except replace the thing. That would be the cheaper repair. Alternatively, replacing the CAT helps and we used to do that after trying to EGR repair and it fixed the problem 9 times out of 10. Good luck and let us know.