I have a 91 Hf with a 89 D16A6 that failed emissions. It passed at 15mph but failed at 25mph due to high NO. (Mods: AEM Intake, 1 Piece DC D series Header)

Timing: 18 BTDC
O2 sensor - a few months old
Cat - Half a year old

Comprehensive Visual Inspection = Pass
Functional Check= Pass
Emission Test= Fail

Emissions numbers:

15mph
HC = 66 (max=134) pass
CO = .31 (max=.82) pass
NO = 833 (max=847) pass

25mph (Fail)
HC= 17 (max=108) pass
CO= .01 (max=.71) pass
NO= 925 (max=786) failed


Any Suggestions? What might be the cause of high NO?

 

Did the car sit for a long time before the test actually started??

 

I drove it on the highway for 15 min to warm up the car...but it sat for about 2 hours before they tested the car.

 

high NO if they mane NOx like in delaware, thats usually when the cylinder temp is too high, usually a stuck closed egr will cause that but since you dont have an egr, try retarding the timming a little bit.

 

I don't know about retarding the timing...the smog tech said that timing was good and I should leave it alone. ..

 

use a higher octane.. NOx is caused by a number of things and low octane is one of them you might wanna probably change out your cat too

 

Trying filling up with premium fuel before the test and drive the car hard on the way there and keep her running. Get that cat nice and hot..