emission failure 1991 Mazda PU

dckozak

Iv just received my NJ "ASM 5015" enhanced emissions test and safety, and failed.
The following results were given.
Gas Standard Reading Result
NOx 1146 2051 Fail
HC 168 60 Pass
CO% 1.42 .56 Pass
CO2% 14.3
O2 0.6
RPM 0

The truck has a fairly recent (less than 15000 miles) "tune up" and a brand new exhust.
I would like to do this work my self as I figure it to be fairly straight forward. What I'm unsure of is what to replace. I know I could do an ERG valve, PVC valve and plugs. But what realy needs replacing? I'v also seen and noted the location of the O2 sensor on the exhast mailfold and would be willing to take a shot and it, if its required. Any suggestions??

Don

Mr_Shiftright

Yeah, I would try the 02 sensor and also there are additives that are supposed to help you past the test. I've had 2 situations of a car failing smog, and when I replaced the 02 sensor and put in that additive, both cars passed...not by a hell of a lot, but they passed.

These two items are not too expensive or troublesome, and are a good start and a good guess. It could be other things, but you can start here. Also check your spark plugs. That's another easy thing to do.

alcan

NOx (oxides of nitrogen) are formed when combustion temp exceeds approximately 1400 C. The most common causes are defective or inoperative EGR system, lean fuel mixture, high compression due to carbon deposits in combustion chambers, in that order.

Mr_Shiftright

So alcan, you dont' think replacing the 02 sensor in this case would do any good?

alcan

Only if the O2 sensor input to the ecm was causing the ecm to drive the injector(s) lean, which would increase burn temperature. The other tailpipe gases look OK, but something's causing high combustion chamber temp. That's the only way NOx can form, as some of the nitrogen and oxygen in the air recombine. EGR uses about 5% spent exhaust gases to reduce combustion chamber temp. I'd look there first.

dckozak

richcolo

I agree that EGR is the most likely suspect, but would check for over-advanced timing first (no disassembly required). A little preignition can make a lot of NOx (and knocks...hehe).