High NOX scores are due to excess combustion chamber temps. Find out why you are running too lean. I'd bet your plugs are almost white coated. Check the EGR system. MK
Yes, check your EGR system components. Your truck has a vacuum actuated EGR valve, a EVR solenoid, and a DPFE sensor. Most likely your EGR valve is sticking, the vacuum lines are broken, or the EVR solenoid is not working. is your check engine light on?. E-mail me with any more questions.
Rob
my f100, considerably older than yours (1982), also failed this test. a pretty good mechanic diagnosed it as EGR valve.
Although I replaced the EGR valve, it still failed NOX test. I was able to get 1 year exemption for inspection (Texas).
I now believe the EGR valve is not working properly because of carbon buildup. I have been quoted $300-$500 to remove and clean intake manifold.
I think, after my 1 year exemption expires in October, I will try using the 'Guaranteed to Pass" products I see in auto supply stores. These might clean out some carbon buildup. My truck is too old to spend a lot of money on.
1982 F100
Check out this site it might be of some help.
Verify your EGR system is working.
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