RickP

Hey guys,

My SOs 94 accord massively failed the CA smog check this year. It's running incredibly rich even at idle.

Smog failures were:
HC
15mph: 269 (max allowed 61)
25mph: 280 (max allowed 36)

CO
15mph: 6.67 (max .51)
25mph: 7.11 (max .49)

NO was way under the limit (even before we cleaned the EGR ports) and O2 is 0%.


No codes are being thrown. The car idles and otherwise runs great. It's got 180000 miles on it. We changed the O2 sensor about a year ago along with the entire ECU (old one had leaky capacitors). I checked the timing and distributer cap, changed the spark plugs, cleaned the air filter, checked the O2 sensor, coolant temp sensor, and cleaned out the EGR ports (they were completely clogged with carbon, but it didn't change the emissions at all.)

I don't have an oscilloscope but I backprobed the O2 sensor with a regular multimeter and it appears to be constant at .9v (I understand normally it should be oscillating between .9v and .1v?), which I believe means the exhaust is running super rich, which matches the emissions results. It does drop down pretty fast when you rev the engine, so I think the sensor is working.

The only other thing I can find at all weird is according to the service manual, two of the three pins on the plug to the MAP sensor should have a 5 volt reference. Measuring the voltage with the key on engine off, I get 5 volts on one pin and 3.5 on the other. Does that mean the wiring or ECU is bad? I wasn't able to backprobe the MAP sensor itself, I need to find some thinner wires to jam in there.

Everything I've tested on the car seems to be working fine, except for that one weird voltage.. Anything else I should look at?

Thanks!

MAD_MIKE

Input voltage from the ECU will be 5V, output voltage with no vacuum will be in the 3.5-3.0 voltage range, the third wire will be ground with 0V. With the engine off, your 3 volt reading is correct.

Have you verified fuel pressure at the rail?
If there is an over pressure condition it will cause the engine to run rich.

Pull the O2 off and make sure it does not rattle, if it does the ceramic has busted and will stay stuck indicating a lean(causing a rich) running. Older OBDI systems don't always pickup on this fault. There should be some switching.

What kind/brand of O2 sensor did you use?
Bosch 'beer cans' are inherently lazy. Lots of cheaply made units are like these older O2s, avoid them.
Denso 'pencil' O2s are what you want.

RickP

Thanks, the O2 sensor is Denso. I may pull it again just to make sure it's working. Testing fuel pressure is a little beyond my limited car knowledge but I'll read up on the service manual.