JohannB

My '90 NA failed emaissions test again this year. Carbon monoxide was about twice the allowable limit. It failed last year too, and I replaced the air filters, plugs, cleaned the throttle bodies, and finally, when I changed the oil, it passed.

I have since put a new throttle position sensor on, and I just changed the oil last week.

I have heard it will help if I retard the timing a few degrees before going back through???

Any help is greatly appreciated.

Thank you,
Johann B

JohannB

Thanks for the suggestion... I hope I can get by without the expense of a new cat. I have till the end of the month to sort it out, so maybe I will be able to find another way to get the emissions down.

Thanks again,
Johann B

Riz Z Speed

EGR valve might be clogged and/or in need of replacement. Check the valve and all vacuum lines leading to/from it.

CanyonCarver

It could be as simple as your oxygen sensors. they can be tested with a multimeter. If they're old just replace them.

Is the ECU showing any codes?

On the EGR, it doesn't open until 2000 rpm or so. If the test failed at lower rpms this probably won't be the problem. A clogged EGR passage will cause high HC at higher rpms. A bad EGR diaphragm or vaccum line, or a dirty EGR valve that won't close all of the way, will cause a lean condition which usually increases NOX. Since I'm too cheap to buy the good testing tools like a vaccum pump, I check vaccum diaphragms, EGR or otherwise, by attaching a long piece of hose to the diaphragm and then sucking on it (no smart-*** remarks please), if I can continually pull air on the diaphragm then I replace it.

More on killing cats. they go bad.

I've seen them actually clog up to where a engine won't run anymore. A simple test for a clogged cat is to find a similar vehicle (engine size) and compare the amount of pressure coming out of the tailpipes with your hand. A lot of oil in your intake tract is a good sign of an exhaust blockage since the exhaust has to go somewhere, it'll go past the piston rings. I've seen where people pay for a new engine because it ran so bad, and it still ran bad with a new engine because the cats were clogged.

And still, if you drive for a long time with bad oxygen sensors, or dirty air filters, or some other malfunction to where your engine runs rich, the carbon deposits will build up on the platinum plated baffles in the cat preventing the natural reaction between the platinum, oxygen, and unburnt fuel.

Before I bought my Z, 5 of the first 6 that I looked at on dealer lots all ran pretty rough at idle. I couldn't believe that dealers would be selling cars with engines that ran like that. A nissan tech on another forum told me that it's a pretty common situation on olders Z's and the EGR just needs to be cleaned.

So before I get flamed, I am a Z noob. Hope this helps

JohannB

Thanks to everyone for your suggestions. I finally passed the emissions test and I'd like to pass on this info... maybe it will help someone else. This is what I did to pass:

(1) I Cleaned the throttle bodies with spray carb cleaner.
(2) I tweaked the timing.
(3) Ran (1) tank of gas with injector cleaner, then refilled with fresh tankfull.
(4) Changed oil, & added (1) can of Bardahl 'No Smoke'
(5) Ran hard on freeway & went directly to inspection station.

Hydrocarbons went down 2,500 percent, and CO down 500 percent!!!

Try to test when temp and humidity are low for best readings.

Thanks for your help!
Johann B