UPDATE: 1991 Volvo 740 not passing SMOG
Greetings!
I recently made a post about a non-turbo 740 from 1991 not passing smog due to high NOx at 25 mph, all other tests were fine. My latest test was 701 ppm, needs to be below 507 ppm.
Since then, I've replaced the O2 sensor, replaced the fuel filter, fixed an exhaust leak at the flange between the manifold and the pipe, and fixed a minor leak in the intake. Additionally, I addressed the heated air intake - the thermostat was indeed stuck open, causing the main intake to be permanently closed and the car only operating on hot air. Lastly, I ran a bottle of GTP in just for good measure. Before the test, I filled it with 91 octane, aired up the rear tires, and drove for about 20 min to heat up the cat.
All of those actions dropped my NOx to 600 ppm, which is still quite a bit above the limit. I hope that it will pass on my 4th attempt :( Do any of you have suggestions for what else to check? The last things that come to mind are fuel pumps and maybe the cat, though it was replaced in 2015 so I was hoping it would be good as it easily passed 2 years ago.
Thanks for being part of this awesome community!