weird smog test numbers

I have a test only smog coming up for my '90 XJ. I was going through the papers i have for it and i found a smog report from right before i bought it in april of last year. Curiously, the report doesnt have numbers for NOX, anywhere. It was also an idle emission test done at 904 and 2394 rpm. Where are my NOX numbers? Does the idle test not measure NOX?

CO2 was 14.3 (idle) and 14.8 (2500 rpm) , O2 was 0.8/0.0, Hydrocarbons came out at 34/12 (max: 120/180 ave: 30/20) and CO was 0.01/0.04 (max: 1.0/1.0 and ave: 0.1/0.1)

Its passed 7 smog tests since 1996. Failed one in 2000, but passed the next one taken only 2 hours later. Seems like they might have replaced a sensor (o2) maybe? Should i be worried about this one? Its my first smog in california, and the 'test-only' requirement has me a bit nervous. I was going to finally install my new exhaust ive had sitting for 5 months, and clean out the throttle body and IAC but didnt want to do too much more until i get the test. Since ive got it ive replaced the CCV in front and back of the valve cover, swapped out the plugs and have new dist cap and wires. Cat may/may not need replacing but i cant tell from those numbers.

Also, ive heard people mention to have the dynamo test done in 1st and 2nd gear, not in Drive. Whats the reason for this, and will the tech know to do this?


Thanks for any advice

Cheers

So, i took the test today. Failed on high NOx, but everything else was way below average. Lambda calculator says that im right in the green for air/fuel mix and the cat is doing its thing. Tech says its probably the EGR valve.

Numbers are for NorCal

15mph rpm1611 CO2 14.6, O2 0.2, HC 123/41/21 CO 0.73/0.14/0.01 NO 711/332/1111
25mph rpm1577 CO2 14.6, O2 0.2, HC 103/33/21 CO 0.93/0.15/0.01 NO 613/258/1118

In the past couple weeks i put on my new muffler, cleaned the TB and the IAC, ran a bottle of GTP additive, and replaced the CCV grommets and vacuum tubes. Overall, ive gotten the mileage up from 11mpg when i got it (13 months ago) to 16mpg on the last tank.

Should i trust the tech and switch out the EGR? I have a free retest within 30 days and my registration is due on the 26th, so i have time to work on it myself, if its a job i can do easy enough. I feel like i should leave emissions stuff to the pro's but if its a simple valve replacement, im sure i couldnt screw it up too badly.

Cheers for any insight!

Did you post this in other forums under a different handle? No problem if you did, just want to make sure you get the same answers.

On the Renix years, high NOx can be caused by a bad EGR. However, they are expensive. You can search for Renix Sensor Diagnostics and test the EGR valve and solenoid. Sometimes they just need a good cleaning and new gasket.

yeah, i did, as np251. I didnt realise my handles were different.

Thanks for the info. Is rthe EGR the only cause for high NOx? I understand its due to high cylinder temp, and the EGR reduces the temps, lowering NOx. But is there anything else i should be checking?

Also, those max allowable numbers for nox are really low- am i even going to be able to reach those levels in my '90?

Cheers

High NOx is the result of too high combustion chamber temperatures.

The EGR is the usual suspect for vehicles so equipped.

TEST your EGR system, info from 90 FSM:

Warm the engine to normal operating temperature.

Operate engine at idle speed.

Check vacuum at solenoid vacuum source.

Disconnect the hose and attach a vacuum gauge to it.

Vacuum should be at 17 inches.

If vacuum is low, check the line for kinks, twists, or a loose connection at vacuum connector on intake manifold.

If vacuum is ok, remove gauge , reconnect the line and proceed to next step.

Check vacuum at solenoid output port.

Disconnect the line and attach a vacuum gauge to output port. Vacuum reading should be 0 at this side of solenoid.

If vacuum reading is 0, leave the gauge connected and proceed to the next step. However, if vacuum is present check solenoid/ECU operation with the DRB-ii tester.

Disconnect electrical connector at solenoid and note vacuum at output port. Vacuum should now be present at output port.

If vacuum is present, proceed to EGR valve test. However, if vacuum is not present, replace the solenoid. Test the EGR valve as follows:

Leave solenoid electrical connector disconnected. Note engine idle.

The engine should idle roughly or stall. If this occurs the valve is ok. If the idle does not change, proceed to next step.

Disconnect hose from the EGR valve and connect a hand vacuum pump to EGR nipple.

Apply a minimum of 12" of vacuum to the valve and note engine idle. If engine now idles rough inspect the vacuum line between the EGR valve and the solenoid.

If the idle did not change, remove the EGR valve and inspect the valve and the exhaust passage in the manifold for blockage, repair as necessary. If no blockage is present replace the EGR valve.

TRANSDUCER VALVE:

Disconnect the transducer vacuum lines and the back-pressure line (bottom). Remove the transducer. Plug the transducer output port. Apply 1-2 pounds air pressure to the back-pressure port. Apply a minimum of 12 inches of vacuum to the input port. Replace the transducer if it will not hold vacuum.

This might seem rudimentary and I can't check my papers because I am at a benefit right now but when I had high nox probes w/ my 89 I was able to get it down by running some fuel injector cleaner and changing the fuel filter. Combustion temps will go up if the mixture becomes to lean and sometimes changing the egr can make things worse. You may want to check this out if neither has been done in a long time. I check my papers out in the morning and get back to you. If nothing else it'll be a lot less money invested/ lost initially and if it solve the prob hey great.

I recently failed smog three times. The State paid for a new Magnaflow catalytic converter and the NOx went from 1200 to about 30 ppm.

Granted, mine is 92 and therefore has no EGR. But it's something to think about.