California ASM Emission Test Results - Puzzling

[tonerbreth]

This past weekend I passed FOR THE 3RD CONSECUTIVE CYCLE, my smog test by the hair on my chinny chin chin.

OK... so I'm being a little corny...

What's puzzling is the 25 mph test results are stunning for a 91 4wd Explorer with 106k miles on it.... with most everything still 'original'.

Here are the numbers:
15mph
1354 RPM, CO2=14.46, O2=.15, HC=83 (max allowable); avg=29, CO%-.19 (max=.68%), NO (PPM)= 111

25mph
2258 RPM, CO2=14.59, O2=0.00, HC=27 (max = 98 w/avg @23), CO%=.15 (avg = .15), NO = 15 (w/avg=355)

It's been awhile since I've tuned it up, but have only put on 3-5K miles in last 5 years.

I think the low NO might be a clue.... Feel free to chime in....

 

[Maniak]

a 1991 with only 106k miles is pretty rare and with so few miles the Cat is probably still working well..

I haven't put our X through emissions in 15 years (not "officially" a DD).. We have 3 other vehicles that are considered DD and they get tests (well, not the motorcycle)...

anyway, at 25 mph, what gear is it running in during that test? I'd expect it to be some pretty low rpms and as long as things are running well (good plugs/wires/coil) and you don't have air leaks it "should" still give some good results..

My first impression of the 15 mph test makes it look like it was more loaded (maybe lugging) since the HC is higher which is more unburnt fuel..

Even when I ran the X through emissions the dyno was just a single 30 mph or so "hold speed" test plus idle before and after.

~Mark

 

[Rhett]

I'm glad you passed. Where I live, it's rare to see a 25-year-old vehicle that still has to undergo emissions testing. Most people don't daily drive such old vehicles. In my state you could apply for a Historic Vehicle registration, and even if you lived in a city like St. Louis (where they do have testing) your truck would be exempt.

My thinking is, these engines do not pump much air compared to modern engines (1.8T, 2.0T, 2.3T, 4.6, Coyote 5.0, for example) and with typical 1st Gen Explorer gearing (3.27 or 3.73) you aren't going to be turning much rpm. All of which, with a good working cat, might mean pretty decent results.

 

[tonerbreth]

Yeh.... good ol' CaliforNIA... Cars 1975 and earlier are exempt here...
According to what I could find regarding gearing for the low speed test.... the policy says to try to attain the target RPM in second gear... and down shift if that can't be attained. The smog tech follows prompts on the test console... Like you say, at least it passed... this time!

 

[tonerbreth]

Not sure what gear the 25mpg test is done in. That's a very good question! Again, I think the operator follows prompts from his/her console. I have a cat I took off a 94 I bought for its tires... and anything else I could unbolt and store without causing too much of a mess.

Not sure why there's no 'hold' only test here.... they call it ASM (Acceleration Simulation Mode) in Cali.... Thanks for the input....