Auto Emissions Knowledge

[2buck]

I have a friend with a Miata.
It is well maintained & serviced regularly.
It failed the Georgia Emissions with the following readings:
HC ppm      183     allowable 206
CO %        .66            "        1.33
NOx ppm    2629         "        1512
CO +CO2   15.1          "        6.0
Test RPM     2222         "        3000

Someone suggested that the timing be retarded 5 degrees & get it inspected before the engine got warmed up.

Those changes netted the following:
HC ppm     62
CO %        .29
NOx ppm   903
CO =CO2   15.2
Test RPM     2270

I am wondering what this caused to make the engine pass the test? I would have thought that with the timing retarded the combustion temperature would be off & this would mess up the combustion & emissions.
Can anyone tell what was going on?? :?

Thanks I am confused as how this 'fixed' the problem. Is there a long term cure?

[enjenjo]

I hope someone has an answer for you, I don't, but I am interested.

[2buck]

Okay, while waiting on a response I did some surfing and came up with an explanation.
From Wikipedia-NOx production is increased when an engine runs at its most efficient (i.e. hottest) part of the cycle.
By retarding the timing this heat was diminished for the test. The car was driven for the test before it had a chance to thoroughly warm up, as well.

EGR-Exhaust neither burns nor supports combustion, so EGR dilutes the air fuel charge to reduce peak combustion chamber temperaturtes. This, in turn, reduces the formation of NOx.

This tells me that the engine's EGR system is not functioning correctly.

I ran across a U Tube video where a shop removes the intake & unplugs the port from the EGR valve to the intake's passage. That is probably what is causing the motor to fail the test. I will check out that passage with a coat hanger & see if it is open. This car only has 83,000 miles & is a 1990 model.

[papastoyss]

If you have access to a Mityivac type tester simply connect it to the vaccum port on the EGR valve & apply vaccum  at idle. The engine should stumble severely or die if the EGR ports are not plugged. If no change then the EGR is not functioning, could be ports plugged or defective EGR valve.

[wayne petty]

wow.. thats lucky being able to change the ignition timing on that model..

the crank and cam positions sensors are in the same device on the back of the head...

i did notice that there is a 12 minute video online about adjusting the timing on that one.. must be done with the diagnostic leads jumped in the flip top box.. check the video.. or the manual..


i don't think that model even has an EGR valve.. but i could be wrong..

i wonder if the cam timing belt is on properly...  or if somebody got creative.. and tried to advance the cam.. or just missed out as the cam shifted before the belt could be tightened..


there is a WHOLE lot of info on NOX... also a lot of mis information on EGR Flow...