92 Corolla High HC Emissions

Every two years here in Calif I have to do an emissions test on my 92 Corolla (4AFE engine). Every year I just barely pass the HC limit. I just had this test done last week. So I should say “Every year I barely pass, until now”. Here’s the emissions data showing I failed:

HC’s at 15 mph measures 140, max allowed is 130. FAILED.

HC’s at 25 mph easily passes with 67 measured, max allowed 105. PASSED.

CO and NO easily pass at both speeds. PASSED.

%CO2 =15, %O2 = 0.1 to 0.2 (Not part of pass/fail for Calif test, just fyi).

The HC/15 mph has always been the part that just barely passed on prior tests. In 2012 it measured 128.

Here’s what I did/didn’t do just prior to taking the test:

  • Replaced original O2 sensor with Denso version. Old one looked pretty carboned-up.
  • Measured valve clearance. All 16 valves within specs. 15 well within. One exhaust valve is right at the max limit, .3 mm. Exhaust spec is [0.20 to 0.30].
  • Replaced air filter.
  • Replaced fuel filter.
  • Replaced thermostat. Why? Below normal engine operating temp. Old stat had faulty internal seal. New thermostat solved under-temp problem.
  • Didn’t replace spark plugs. Last replaced 2 years -15,000 miles ago. Pulled each plug, looked ok, & gap measured correct, re-installed.

I have 30 days to take corrective action and repeat the test. Any ideas appreciated.

It’s probably time to check the compression. How many miles are on it?

What is the oil consumption like?

The catalytic converter may be losing efficiency if it’s old, and if it’s relatively cold during the emissions test it won’t scrub the hydrocarbons as well as it would at a higher operating temp. Do you drive straight from your garage to the inspection station, and how far is that? I would try driving on the highway for 30 minutes or so to get the cat good and hot and try the test again.

Since you’re just on the ragged edge of pass/fail, getting the cat hotter might just do the trick.

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Ignition misfires or lean misfires cause unburnt fuel in the exhaust which is HC. It seems you massaged everything except what might be relevant (plugs, wire, cap , rotor). Spark plugs can look good but still have an internal crack causing an ignition misfire.

Vacuum leaks cause a lean misfire. Be certain there are no vacuum leaks from hoses or gaskets.

You might want to try retarding the ignition timing slightly, I believe this has adjustable distributor. The greater the timing advance, the more HC you produce.

As a last resort you may want to change the catalytic converter as a brand new one generally works better than an old one. The catalyst reduces HC.

Of course what @cigroller says above is spot-on. Weak compression can also cause a lean misfire if the engine is beat.

One spark plug could be the cause of the problem so you need to consider new ones. The eyeball test is not indicative of whether or not a plug is good.


George lives in California. They check everything here for those older cars

If his ignition timing is incorrect, he’ll fail. They don’t give you much leeway. I believe it’s only plus, minus 2 degrees


We all know a warmed up cat is the way to go

That said, all the smog technicians I’ve run into always tell me something along the lines of “Park it over there. Shut off the engine.”

And then they get to it SEVERAL minutes later

What would you suggest?

A combative attitude?

“No! I demand you drop everything and start on it immediately!”

“S . . . w you! I’ll bring my car to somebody who will give it a chance to pass!”

. . . ?

We’ve been through this before. The guys are literally following written down guidelines.

And I’ve posted information which spells it out

@db4690, no combative attitude. :wink:

Sorry, I didn’t realize they make you cool the car off before the smog test.


I know we talked about this before

Perhaps you weren’t on the website that day?

@db4690, yeah I must have missed class that day. :wink:

I would throw in a fresh set of NGKs and a Denso cap and rotor if they are more than 15, 000 miles old. This engine uses copper core plugs that become suspect after 12, 000 miles. $30 for these is a cheap investment.


You missed a good session

Quite a few of us didn’t see eye to eye . . . AT ALL

Good idea @cigroller , a compression test is now on my agenda. The car has 200 K miles and uses one quart of oil in 6,000 miles. The engine idles and runs perfectly, as good as the day I bought the car.

About the cat temperature @‌jesmed . I drove it about 5 miles to the place at 50 mph. The coolant temp guage was at full operating temp. I then sat in a nearby parking lot and idled the engine until the radiator fan came on. I was able to pull right into the stall and the test started within 5 minutes. The tech there started the car and let it idle for 10 minutes before he even put the probe in the tail pipe. The cat temp probably wasn’t the main reason.

@rattlegas etc on spark plugs … good ideas, I agree. I’ll just put in new spark plugs, cap, and rotor and be done w/it for the next test. Inexpensive and easy to do. Esp since I only need to reduce HC’s by less than 10%, from 140 to 130.

@db4690 etc on timing … the ignition timing is spot on at 10 BTDC. As is the idle speed. But I could back the timing off to 8 BTDC for the next test.

Here’s some other ideas I’m considering …

  • I wish I had some test like a radioactive tracer or something to tell me where the HC’s are coming from? The gasoline? The exhaust gasses? The crankcase?

  • PCV … .I replaced that about 6 months ago as part of routine service. Testing by standard method shows it remains ok. Not planning on replacing the PCV again.

  • I was thinking I might drain the crankcase so it is about one quart low. Less oil splash. Just for the test I mean.

  • Also was thinking for the test I’d switch from 10-30 W to 30 W, the idea being that pure 30 W is probably a better oil than 10-30 when the engine is warm. Grasping a straws?

  • I replaced the thermostat just prior to the test b/c the coolant gauge was consistently reading low and has been for over a year. I’m thinking the engine internals may be gunked up from that and some high speed driving may clean it out a bit.

  • EGR … on a 22 year old car the EGR probably has some carbon build up. It does pass the test where you activate the EGR and the car stalls. So it is not completely stuck. But it might be pried open a tad and allowing a small amount of exhaust gas into the mixture. Could that cause a boost in HC’s? I’d just replace the EGR except it is in a very difficult location access-wise, at the back of the engine, halfway down and underneath the overhang of the intake manifold, and wedged close to the firewall. At this point I’m thinking the EGR is probably not the problem and I won’t replace it.

  • Fuel pressure. The FPR looks ok, no signs of gas leaking into the vacuum line. Would a fuel pressure test still make sense? I’ve never done one on the Corolla but I did it on my VW Rabbit many times. I could do a leak-down test too to see if the injector are leaking. I use the cold start valve port for the FP gauge I think. I was thinking I’d just disable the cold start valve (plug it off) at that point for the test, to eliminate the chance it is dripping fuel.

  • Fuel additives? I’ve never used fuel additives as I’ve never had a fuel related drivability problem. But I’m thinking of running some Tech-ron through the tank a couple of times prior to the test, and maybe for the test. Grasping at straws?

  • I plan to retard the timing to 8 BTDC from 10 for the next test.

  • There’s no signs of vacuum leaks. I did a fairly thorough inspection for this last year while solving a high idle problem. (Turned out to be a faulty IAC.) As evidence, I can turn the idle adjustment screw full off and engine stalls.

  • I concur replacing the cat would likely solve the HC problem, but it is sort of an expensive sol’n to a less than 10% over-HC problem. I’ll keep that as a back-up in case I fail the second test.

Thanks to all for the good comments.

Mission statement: 130 or bust!

Why don’t you suck some seafoam in to the intake, while it’s idling?

Several of the guys on this website say it’s good stuff

One more thing I forgot to mention was about the exhuast manifold … when I was replacing the O2 sensor I had a good opportunity to remove the shield and inspect the exhaust manifold (and upper exhaust system) for cracks. I couldn’t see any cracks, but I’d have to completely remove the EM to be certain, as I can’t see the back side of the EM with it bolted in place. But no cracks on the front surface anyway. But I did notice that 3 of the 5 bolts holding the exhaust manifold to the block were loose. I could tighten them easily a half turn, one maybe 270 degrees, with just an 8 inch ratchet. I just tightened them all to their torque spec, but wondering if there could still be a problem there?


I advise you to not remove the exhaust manifold

That era of Toyota exhaust manifolds . . . on the 4A-FE and 7A-FE engines . . . were known for cracking

The manifold may be borderline, and “disturbing” it may put it over the edge

Tune up and backing off the timing to the minimum is what is going to get the car to pass. Remember he only needs a little reduction to gain compliance.

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I believe the ignition timing is only allowed to be off plus/minus 2 degrees

Thank @db4690 … that’s useful info.

All good comments. I’ll add one more, have you replaced the spark plug wires lately? That can also cause a misfire at idle thus raising the HC.