CO’s are very low, nearly the same as when the car was new. There’s no misfiring, pinging, loss of power, or any other performance loss. That’s what’s curious. The only symptom is slightly higher than normal measured HCs, and only on the treadmill test at 15 mph. HC’s levels at 25 mph (the other treadmill speed they measure) passes easily and is less than the average car of that type. Like most drivers, I rarely drive at 15 mph, but Calif emissions scientists I presume must have a reason to test at 15 mph.
George
Your car goes on the treadmill because it’s old . . . simple as that
My brother’s 1994 Tercel passed the smog, but had sky high HC
My former car, the 1995 Corolla, always passed the smog, but always had high HC, even when it had very low miles
My mom’s 1999 Mercedes-Benz C280, ironically, passed with flying colors. HC was extremely low, single digits, actually
Here’s the clincher . . . Both of those aforementioned Toyotas ALWAYS had significant oil consumption, no matter what I did. The Benz did not. Coincidence . . . I think not
All 3 of those cars had to go on the treadmill . . . high emitter profile, it said
@db4690 … interesting comment, thanks. My Corolla doesn’t use much oil though. Less than one quart in 5-6000 miles. What would be your guess for the CARB scientists motivation to test at 15 mph? My pin-head thinking is that if I wanted to get an accurate measure of how much a car is polluting, I’d test it at a commonly driven speed, like 35 mph, not 15 mph.
George
I suspect your old cat isn’t doing a great job anymore . . . because it’s old
I may be off base, but I think modern cats are much more robust than cats from the 1980s and 1990s . . . because they are required to be
I also suspect certain engines were always fairly dirty, even when new. Nobody’s perfect . . .
Update: Corolla easily passed 2016 and 2018 emissions testing. Not entirely sure why, but these are the differences from the 2014 and before tests.
- Switched to the NGK V-Power spark plug on recommendation of auto parts store
- Used a different testing shop
- Used an o-scope to precisely set warm idle rpm
- Set base ignition timing at 8 deg BTDC rather that 10
- Filled tank w/gas (87 octane) one week prior to test, and tested w/tank 1/4 full
- Forgot to change the engine air filter prior to the 2018 test
- Unusually hot and dry afternoon for the 2018 test
2016 Results: HC @ 15 mph measured 105, limit 130.
2018 results HC @ 15 mph measured 90, limit 130.
Retarding the timing probably had the biggest effect.
Raises the exhaust gas temp, which helps burn up HCs as the gas passes through the exhaust manifold.
Found this out back when I had an '81 Accord with a modified engine.
Friend at a shop let me use an exhaust analyzer to figure out how to get it to pass an idle emissions test.
Back the timing off a few degrees, turn the idle mixture screw lean and raise the idle speed a bit.
The HC would drop by 3X.
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Backing off the timing seems to help, but raising the idle rpm might not work quite as well w/the Calif test method, where they don’t measure at idle, but instead at 15 & 25 mph using a treadmill roller machine. And of course w/electronic fuel injection there isn’t a way to lean out the idle mixture by making a simple adjustment. On this Corolla the ECM has no way to adjust the idle rpm; it assumes the warm idle rpm and timing has been properly set by whoever does the tune-up and uses that as a basis to compute the fuel/air mixture under variable engine loads. If those parameters weren’t set correctly at warm idle, it might inadvertently cause a too-rich condition at driving speeds.
On older cars that actually get the tailpipe test, there is a step where it asks if the timing is within specs
If it’s backed off “a few degrees” that could be an automatic failure, because it’s whatever the sticker says, plus or minus at most 2 degrees
So who cares if the hc is within limits, but the ignition timing is not . . . ?
Failure is failure
Yes, there is a part of the form for the ignition timing. And it passed that section. Apparently 2 degrees off is within the limits, considered within measurement error I guess.
Back when I had the Accord inspection (in DC) didn’t check idle speed or timing.
Only a sniff test at idle.
That’s the way it was.
I think a lot depends on the state you live in
I do smog inspections in California, and they’re pretty strict
But I’ve also read the smog inspector manuals for some other states . . . and some of them do things very differently
The curious thing about these past two tests: Why is the Corolla getting cleaner and cleaner as it ages toward the 30 year mark ? There’s no obvious explanation I can offer.
Depending on how you perform the test, it CAN have an effect on emissions values
It’s quite possible your car should have passed all along
Sometimes a younger guy will smog a car and it will fail
And a more experienced guy will smog the same car and it will legitimately pass
Test method variability then @db4690 ? What about the weather? Do you have any experience hot dry ambient temperatures has a significant effect? I’m thinking about camping. If the weather is wet and cold, the campfire will be more smoky compared to a campfire in hot dry weather.
Let me phrase it a little differently
Some guys are better at getting cars to pass smog legitimately
I’ll now talk about the two-speed idle tests . . . for vehicles too large to go on the dyno
The first part is low speed, up to 1100rpm
The second part is roughly 2200-2800rpm
Certain vehicles in our fleet won’t pass the low speed portion of the test at 600rpm, but they WILL pass at 950rpm, for example.
I’m not suggesting anything, just stating facts
@db4690 … since you’ve done some Calif emissions testing yourself, one aspect of the testing I’ve wondered about is the evap system testing. They appear to connect the re-fueling port to a contraption that fills the evap system with gas (not gasoline, nitrogen maybe) of some kind under pressure, to make sure it holds presumably. But wouldn’t the pressure still leak out from the evap canister and/or the purge valve? How do they prevent that?
We don’t really use that device at our shop . . .
That’s only for vehicles older than model year 1996 . . . and I believe our fleet has few or now vehicles that old which need a smog inspection
2023 Update
The poor Corolla was forced off the road in April 2020 due to the Covid risk of OBD I emissions testing, but is now back on the road. Just today, got the renewed registration sticker in the mail! One note, I did discover an emission’s-related issue: the purge valve was leaking a little, allowing gas fumes from the canister to be drawn into the engine when the coolant is cold. the purge valve is supposed to be closed then. This has no relationship on the actual emissions test results though, b/c that testing is done when the coolant is at operating temperature, purge valve is open.
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When will you replace the purge valve?
How will your truck feel now that it’s #2 in your fleet? 
Congratulations for getting the Corolla back on the road!
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