Questions and Answers

High NOx Readings Q. Vincent: I'm hoping you can shed some light. I have a 1988 Toyota Celica GTS, 135,000 km. This past spring the car started running rough so I took it to the Toyota dealer, I wanted the spark plugs changed etc, tune-up. At this time, the spark plugs were changed, fuel injectors were cleaned and throttle assembly cleaned.

Car was still running rough, as it turned out there was a crack in the air intake hose. Air intake hose was taped up to cover the crack ($100 for a new one), car running okay.

The car passed the emissions test two years ago, no problems.

Now the car was due for another emissions control test. I took the car in, it passed the HC test (20), CO test (.02), but failed the NO test (1700). Thus I did the standard checks, plugs okay (denso platinum), changed the fuel filter, oil change, changed the air filter, cleaned the EGR modulator filter, replaced the air intake hose as the tape was allowing air in through a crack in the bottom.

I then did a re-test, still the NO failed with a rating of (1450). Thus the shop did some analysis and said the oxygen sensor needed replacing. Replacing the oxygen sensor and re-test indicated an NO rating of 1990!!!! (HC rating 12, CO rating .01 - slightly better) This was worse than before, they had thrown out my original oxygen sensor (so they say), but stated that since the cost of the sensor, analysis, re-tests etc was $400.00 the car got a conditional pass.

So, my guess is that the original oxygen sensor was okay. What are your thoughts? Where should I look next? I realize I've got a couple of years, but would rather do some pro-active maintenance.

Thanks in advance.

A. I would have to start looking at the EGR system for the problem. This system is used to dilute the incoming air/fuel mixture with recirculated exhaust gases. Dilution of the incoming mixture lowers the peak flame temperatures and thus limits the formation of nitrous oxides (NOx).

The most common problem with this engine is the EGR passages get clogged up with carbon. What you can do is while the engine is running, reach under the EGR valve and manually lift open the diaphragm. The engine should stumble and stall. If it doesn't you have either a bad EGR valve or clogged passage.

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