Re: 1991 Ranger with high Nox
FOUND THIS FOR A DIFFERENT CAR BUT PRINCIPLE IS SAME.
NOx emissions are the result of high combustion chamber temps pure and simple. NOx readings go up whenever there are hot spots that exceed about 2400 degrees. Things like high compression and lean mixtures contribute to high chamber temps, as does over-advanced ignition timing.
Poor fuel quality contributes to detonation, a favorite cause of high chamber temps. Got a load of pretty poor gas? That will do it easily. Worn plugs allowing a little misfire? Not a problem unless the worn electrode is glowing and causing early ignition, and none of us true enthusiasts would leave old plugs in the engine. Oil contamination of the intake charge can cause an effective reduction in octane rating of the fuel.
The cat depends on some reasonable operating temps to get a good reaction, and a bout of sitting in traffic or waiting in line for a pass at the rollers might allow the cat to cool enough to stop the NOx conversion process. The principle component that heats the cat is HC, where the raw fuel that passes through the engine unburned finally gets a chance to do some work heating the rest of the catalyst media. With the excellent engine management and the feedback loop from the oxygen sensor keeping the HC and CO in the pipe to a minimum, the NOx reaction is probably reduced.
-- Use one of the proven octane boosters next time you test.
-- Have the tester do the rolling tests with the trans locked in first gear. Might be enough extra charge flow to get better chamber cooling. My tester guy didn't want to do that, but maybe a PKS would do the trick next time.
-- Make sure the oxy sensor is working and relatively new. I haven't changed any on this car, but know that a new sensor pair on my Explorer paid off in improved mileage in less than eight tanks of fuel with 60k on the original pair. Payout might be even quicker with the higher gas prices lately.
After those simple things, you may be looking at needing a new catalyst.