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2 Puzzles: Deciphering Calif smog inspection results for 1990's Toyota Corolla

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?

Puzzle #2:

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

2004 26
2006 129
2008 28
2010 120
2012 128

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?

The HC could merely be the engine/cat converter being cold on those test dates. And that is what would make sense since it comes and goes. Unless you are changing oil weights and causing major fluctuations in oil consumption.

When you check and adjust the ignition timing are you doing this with the engine fully warmed up and a jumper wire on the correct test terminals?

You say the timing is very late (meaning ATDC) and timing that is late will effectively richen the engine and drive the HCs up. Advancing the timing will lean it out and HCs should drop.

If the timing is set correctly and it keeps varying as it does by year then I’d have to think there’s a problem with the ECM, ignition module, or a poor connection in the distributor/ECM circuit.

When you adjust the timing on your unknown year car, be sure to disconnect or not disconnect a vacuum line from the vacuum advance if you have a vacuum advance. The instructions may be specific to whatever year your car was built. Without the year, nobody can google it and be certain, or like me will lose interest in trying to find out. Cars are very year specific and sometimes date of manufacture is also needed.

Is the Corolla OBD-1 (1995 or earlier)? If so, the car has to be fully warmed up before the inspection. I had similar problems getting my 1993 Caprice to pass NJ emission standards in 2010. Here is a link to a discussion I initiated on this subject.

Ed B.

The only three things I can think of that I might be doing differnetly year to year are

  1. the date the engine oil may have been changed; this year I changed the oil about 4000 miles ago;
  2. the amount of fuel in the gas tank; this year the tank was close to empty;
  3. whether I’ve driven high speed on the freeway on the way to the place; in 2010 and this year I didn’t drive on the freeway, but not sure about the other years. It is possible that I did drive on the freeway when I got the low readings. Is the freeway driving warming up the cat better I wonder?

It’s a 1992 Toyota Corolla Base model. It has fuel injection, electronic ignition, with a distributor. You change the timing by rotating the distributor slightly.

This year like all the others to warm it up I drive it up and down the street at about 35 mph in front of the emissions testing place for about 20 minutes, then I pull into a nearby parking lot and let it idle until the cooling fan comes on, at which time I drive over to the test place and do the test. So it should be pretty-much warmed up, unless that warm-up routine isn’t enough to fully warm the cat.

The Toyota shop manual indicates to check the timing, you turn the key to “off”, then jumper the test connector, then start the car, and warm it to normal operating temp. Adjust the rpm to 700, and read the timing with a timing light. The only other thing it says is to make sure no accessories are running, including the cooling fan, so if the cooling fan comes on, I wait for it to go off before checking the timing. That’s exactly what I do. I set it as spec’d, 10 deg BEFORE TDC. Usually, as was the case this year, I don’t need to rotate the distibutor, as the timing is already correct.

There seems to be some confusion here about the “before” vs “after”. I set it BEFORE, but the guy is reporting AFTER. The car is spec’d at 10 deg before. That is exactly what I set it at. But the person who does the test , on his test report, is saying the timing is 10 deg after. But he’s clamping off that unknown hose. I’m thinking when he clamps off that hose, whatever hose it is, it is retarding the timing from before to after. Maybe that’s just the way they do the test, but I can’t understand why, or what that hose might be he’s clamping off that is changing the timing.

Anyway, I appreciate your comments. What I’m taking away is that next year I’ll drive the freeway, go on a full tank of gas, and change the oil right before doing the test. Any other ideas? Check the PCV? Check t the EGR?

Any chance of finding out what that hose is that the guy is pinching off? Your car should not have a vacuum advance on the distributor and I’m not familiar with CA emissions testing.

I understand what you’re saying about the before and after thing and believe that you set it at 10 before using the prescribed method.
It seems to me that any engine that has the timing yanked back for whatever reason to 10 degrees after TDC is going to be total garbage performance and emission wise. Any car with timing that is retarded that much is going to suffer.

Thanks @ok4450, I’ll take a look and see if I can see any hoses with a clamp mark on them. If I find one, I’ll post back here. I noted the area of the engine compartment wherehe put the clamp, it was right next to the air cleaner ass’y, but down far enough I couldn’t see exactly which hose.

I wonder, do you think doing some kind of internal engine cleaning process prior to the text, say with a fuel additive, help w/the 15 mph hyrdocarbons?

You are worried about a dog and pony show that is about as meaningless as anything on this planet…Your 20 year old car is good for at least another 2 years…Nothing else matters really…Don’t ask why, just give him the money and say thank you…

Are you sure the clamp is not the wire clamp to ground the electricals. I do not recall any hoses being clamped on my emission tests.

Why don’t you just ASK the technician?

I think ATC vs BTDC is just a typo on his part.
If the timing were really that retarded the car would run very poorly, terrible MPG and overheat.

@galant, no it’s not grounding the timing light. It’t a long clamping forcepts. No wires involved. I am assuming he’s clamping a hose with the forcepts, probably a vacuum hose. I’m assuming it must be part of the procedure. I’m just curious why is all.

@circuitsmith, perhaps it is a typo, but if it is, he made the same mistake three times, in 2008, and 2010, and 2012. You are right that if the timing were set 10 DEG ATDC, I’d know it right away. But the car runs like a top and gets as good of fuel mileage as ever. When I set the timing, I do it by the procedure, to10 DEG BEFORE. I don’t clamp off any hoses. And there’s no way I could set it AFTER TDC anyway, there’s no scale on the AFTER side of the timing markings. The only scale available is on the BEFORE side. He has one of those timing lights with a dial-knob on it. I see him dialing it as he’s looking at the crankshaft pully. I’m assuming he’s dialed it to 10 DEG AFTER when he does the test, and that is how he knows.

I’m guess that by clamping off that hose, it is retarding the ignition timing somehow. That’s the only thing I can figure out. And that is what he’s testing perhaps. Under some condition, the timing should retard the ignitiion to limit emissions. Perhaps it is the EGR control signal. The EGR is supposed to be shut at idle, and open up on acceleration, to limit Nitrogen compounds; maybe it has something to do with testing that.

@caddyman @westernroadtripper, you’re right of course. I’m not loosing any sleep about it. I don’t want to ask the tech anything, b/c the way I figure it, why return to the scene of the crime? I just want to pay my $50 and get the hell out of there without asking questions. Failing the test means a bunch of busy-work for me, which I have no time for. I just squeaked by, and for that I’m thankful. I’d like to know for my next test however anything I can do to widen the margin of error.

For asking questions, that’s not what the tech is for, that’s what CarTalk is for!


I read through this link.

and decided @circuitsmith is probably right, that the tech is simply making a typo on the report when he enters the timing as “AFTER”, rather than what it is, “BEFORE”. Why he made the same typo three years in a row is a mystery. I’m guessing he made the mistake the first time, and the report is partially filled out by the computer with the prior years results, and he’s not catching it. Calif requires the timing be with 3 Degrees of factory spec, so it must be “BEFORE” that he’s measuring. Factor spec is 10 DEG BEFORE for this car.

The hose the tech is clamping off, I think I now understand what that’s all about. I popped the hood and looked at where he’s doing the clamping with the forcep;, it appears to be a vent line from the fuel tank to the chacroal canister on the firewall. He’s not clamping this off as part of the timing test. It’s independent of the timing test. Instead, it is part of the tank pressurization test (as explained in the link above). The fuel system is not supposed to vent to the outside air,; any gas vapors are supposed to be caught in the canister, and burned. So they test it by putting a special gas cap on where you fuel the tank normally, which has a hose which they pressurize the air in the top of the tank. Then they measure to see if it holds pressure. The tech has to clamp off that vent hose, otherwise it wouldn’t hold pressure, as the canister has a hose which goes to the intake manifold.