I have 91 Z28 5.7. I just had it smogged and it passed, but one of the measured values concerned me. The HC were maxed. I was wondering what this could mean.

%CO2 MEAS
15 MPH 13.3
25 MPH 13.3

%O2 MEAS
15 MPH 1.6
25 MPH 1.9

HC MAX AVE MEAS
15 MPH 108 31 108
25 MPH 83 20 83


CO (%) MAX AVE MEAS
15 MPH 0.71 0.1 0.29
25 MPH 0.58 0.09 0.06

NO (PPM) MAX AVE MEAS
15 MPH 767 237 507
25 MPH 706 199 292


Your input is appreciated. I just want to make sure that this wont haunt me for the next evil smog check.

Thanks
Steve

 

Hydrocarbons are unburned fuel. Running rich. I don't think the catalytic converter will combust excess gas, and since you passed, yours is probably in decent condition. So the only way to lower it is to figure out why it's running rich and adjust/fix it. If you've got an AFPR set too high that could do it. That or improper timing perhaps, unless you've changed the fuel injectors or prom(if you aren't the original owner, having an aftermarket prom that adds in too much fuel and makes you run rich is likely).

 

I just replaced my fuel injectors a couple of days ago with the Accel 21#. I don't have an adjustable AFR. I know there is an aftermarket chip in there, that came with the car when i bought it 6 years ago. The timing is set to 6 degrees. I found my smog test results from 2 years ago and the HC are high then as well.

HC
MAX AVE MEAS
15 MPH 133 30 131
25 MPH 83 12 60

CO
MAX AVE MEAS
15 MPH .71 .14 .32
25 MPH .58 .07 .03

NO
MAX AVE MEAS
15 MPH 1020 206 968
25 MPH 880 194 561

 

I've just been through a smog class so maybe I can help you with this...

HC is unburnt fuel, if you have a lot of HC's it normally means that one or a few of your cylinders aren't running very well (misfire).

The next thing you want to look at is CO, CO is caused by partially burnt fuel and will give you a good indication of wether or not your running rich. Yours is 0.71 at max and anything under 1% is good.

Next lets look at O2. Two ways to get O2 out the tail pipe, very lean or misfire. Yours is at 1.6 and goes up to 1.9. ANything under 1% is good so this is out of spec. (not an emission requirement though).

Your CO2 gives you a good indication of how efficient your engine is. Anything above 15% is good. So yous is low butnot terrible.

Well, you have high HC's (possible misfire), low CO (not running rich), high O2 (lean or misfire), and low CO2 (rich or misfire).

Well the CO and O2 indicate a non-rich condition and the HC and CO2 point to a misfire. So how is your ignition system? What kind of intake do you have, any vacuum leaks? You may also want to check fuel pressure and timing advance? What area do you live in?

 

As far as i know the ignition system is working well. The car has a standard TPI intake with the edlebrock intake manifold. I don't believe there to be any vacuum leaks. I don't have a fuel pressure gauge yet and the base timing is good, but not sure how to test the advance of it. I am in the Chino Hills area.