Early 1990’s Fuel Injected Toyota Corolla
A couple of puzzles. I always do the Calif smog test at the same inspection station and this time of year, every two years. It is even the same tech that does the test each year. He’s been there for years. I always put in a new air filter a day or two prior to the test. The car has always passed each year, including this one, but sometimes, like this year, barely, without much margin for error on the 15 mph HC test. This year I also did a tune-up, replacing the spark plugs, ignition rotor, and verifying the ignition timing at the specified value of 10 Deg BTDC a fews days prior to the test.
Puzzle #1: The test results show the idle timing at 13 Deg ATC! Not before like it should be, but after top dead center! I looked at the same smog test results for 2006,08,10 and they are all around 10 Deg ATC. It says this value for the timing passes. This can’t be correct if they do the test the same way I do it, with the engine at operating temp, check connector jumpered, 700-800 RPM idle speed. I always watch the fellow do the test. I’m standing right there by the engine compartment. He does it while the engine is idling and the check connector is jumpered. I notice when he uses the timing light, he clamps off a hose near the air filter housing. I don’t do that. And he may be injecting some kind of special gas from a pressurized tank into the fuel tank too; I forget if he did that during the idle timing check or not.
Anyway, why is he consistently year after year reporting 10-13 Deg ATC, when I set and verify the timing using the Toyota Manual protocol at 10 Deg BTDC? Why is he reading AFTER, when I set it at BEFORE? Does it have sometime to do with his clamping off of that hose? Or the pressurized gas injection?
He tests the HC at two different speeds, 15 and 25 mph, the car on a treadmill. He appears to be injecting a special pressurized gas into the fuel tank while doing this test. The CO and Nitrogen oxides pass without problem at both speeds. The HC passes easily at 25 mph. But at 15 mph, these are the results
HC at 15 mph
To pass the HC must be below 130! So it barely passed in 06, 10, and this year, 2012, but passed easily in 2004 and 2008. The test report says the average for cars tested is 25. I’m not sure if that means the averages of the same make/year/model, or all cars.
What is causing these wild swings in the HC test results at 15 mph over the years? Something sticking one year, and not the next? Something wrong with their test procedure or equipment? Different ambient conditions? Different engine operating temperatures?
It seems like it must be due to a richer idle fuel mixture than it should be. That’s the only thing I can think of that would cause more HC’s in the tailpipe at lower speeds. Wha do you think?