Anyway...what would be the cause for high NOx emissions from a vehicle?

My friend's 1988 Benz failed emissions solely on that reading. The hydrocarbons were well below the cut-off, as was the CO readings. But the NOx reading was *just* over the limit. The limit for NOx emissions is 2.50, her car tested at 2.58.

Her car calls for premium gas (even though it's a 4-cylinder). She just runs regular in it...yeah, I know, I nearly hit the floor when she said that. I told her to run a few tanks of premium through it, and go get it tested. I'll find out the readings after that test and see what happens.

Parts that were "recently" replaced were the oxygen sensor, muffler; throttle plate was cleaned a little while back. That was all done about 2 years ago.

NOx is formed under extreme pressure and temperature. Its possible to generate more NOx if you run lean, or increase compression, or advance timing. Think along the same lines as the type of conditions that cause detonation (extreme pressure)

So running regular instead of premium, probably goes a long way towards explaining the problem, timing may be a little advanced, or it could be running a little lean for some reason.

In contrast the high temps and pressures that lead to NOx emissions usually result in the low CO and HC emissions. Its a trade off. So if the gas doesnt fix it, and she richens up the mixture you might see the NOx come down but the HC and CO go up. HC and CO are of course products of incomplete combustion - and can be caused by low temperatures/pressure and rich mixtures.

from: Emissions Testing: Exhaust Analysis
by Larry Carley c2005

...Harder to diagnose are elevated oxides of nitrogen (NOX) emissions. Causes here may include a defective EGR valve, EGR vacuum solenoid or motor, plugged EGR ports in the manifold, over-advanced ignition timing or engine overheating.

Pretty good read at:
http://www.aa1car.com/library/2000/ic60032.htm

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-Paul

70 Mach1 351C/FMX
79 Ranchero GT smogger 302/C4
97 Expedition XLT 5.4
05 F150 Lariat SuperCab 5.4 4x4

quote:

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Originally posted by BlueMule:
to add to Phil's reply,

from: Emissions Testing: Exhaust Analysis
by Larry Carley c2005

...Harder to diagnose are elevated oxides of nitrogen (NOX) emissions. Causes here may include a defective EGR valve, EGR vacuum solenoid or motor, plugged EGR ports in the manifold, over-advanced ignition timing or engine overheating.

Pretty good read at:
http://www.aa1car.com/library/2000/ic60032.htm

---

I saw the results and just couldn't believe how marginal she was over the limit. It couldn't have been anything too major, so I definitely suggested premium gas for now. The HC and CO readings were way, way under the limit...so unless those would go drastically up, she'll be safe with that.

If the premium doesn't work...well, off to the mechanic again for it, much to her dismay.