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Forum > Failed Smog Test > Question

1993 Honda Prelude - Hydrocarbon HC emissions too high.

My Honda has 105K miles. Car failed biennial smog inspection. All emissions well within limits except Hydrocarbons or HC at 25mph. Limit at 15mph is 87ppm and at 25mph 47ppm. First test results: 15mph at 87ppm and 25mph at 49ppm. Failed by 2ppm @25mph. Changed spark plugs, pcv, changed oil, changed air filter, filled up with 91 octane instead of usual 87 & went back to test-only center. Failed again with HC at 48ppm @25mph & 86 @15mph. Improved the 25mph HC by only 1ppm & needed a 2ppm drop. No Check Engine light. I have two thoughts/questions: the original oxygen sensor has never been changed. Could this be the problem? Secondly, I have access to 100 octane fuel at a local gas station. Would putting about 4-5 gallons of 100 octane fuel & thereby raising the overall octane of the current tank of gas higher than 91 help to make the car pass or wouldn't it matter? Would having too high octane a fuel cause CO and/or NOX emissions to increase beyond standards thereby causing another failure of the smog test? I wouldn't want the HC emissions to be good only to have the others increase. What do you think? I've paid the registration fees but want a smog certificate ASAP. Thank you for your cooperation & a prompt response!
Answer:

I'd recommend having a smog check repair center inspect the fuel injection "feedback" system. This will ensure whether the oxygen sensor is working properly, your Honda's emissions computer is receiving the signals from the oxygen sensor, the computer is computing the data properly and sending the correct signals to the fuel injectors to either increase or decrease fuel delivery to the combustion chambers.

 

The feed back test will utilize a 5-gas analyzer (usually a smog machine) and propane. The smog technician should introduce propane to the intake system and ensure the oxygen sensor is reading this introduction as an increase in CO, and ordering the computer to decrease fuel delivery at the instant propane is added to the system. This test should be done after the engine has sufficiently warmed up and while it is running at idle. If no change in fuel delivery is seen we know we have a feedback problem. Then the diagnose should turn to which part of the feedback system is defective. Using a voltmeter the technician should observe the voltage output of the oxygen sensor as propane is added. Voltage should increase. If there is no increase we have a defective O2 sensor. The sensor should be replaced and the test started over. If voltage increase is present at the sensor output the technician needs to ensure voltage increase is also present at the computer input terminal. If voltage is present at the terminal as well next the technician must ensure the computer output terminal to the fuel injectors are lowering "injector pulse rates". At the end of the feedback test it will be know whether the high CO fault is electronic or mechanical, and the technician can further diagnose the fault.

 

You can either go this route, or change the oxygen sensor before a diagnosis since it's long overdue already, and get a third smog test with new results; maybe it passes. Although you've mentioned all other emissions are within limits, even a slight increase or decrease in CO (caused by a defective oxygen sensor) can alter HC emissions.

 

As far as high octane gas... Using it might help lower NOx but only if your vehicle suffers from this failure. High octane burns cooler and slower (keeping NOx low), but it also produces less power which your engine might during acceleration. This could cause other emissions (HC and/or CO) to increase. It is recommended to use only the manufactures required octane at all times, specially during a smog test. Your Honda Prelude's engine compression is designed for optimal fuel combustion with the 87 octane fuel and its emissions computer is programmed to process information based on 87 octane gas.


posted by SmogTips Support

 

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