92 Corolla High HC Emissions

No @Mustangman, haven’t replace the spark plug wires. It’s a good idea though. The HC problem is at 15 mph and not at 35 mph, and something like an occasional misfire due to faulty spark plug wires will be harder for the ECM to correct for at the lower rpms.

@db4690 mentioned a fuel injector cleaner, which also makes since sense. If the fuel injectors were slightly plugged, it would be harder to meter out small amounts than larger amounts of gas, and so an overly rich mixture could well result at low rpms and not at higher rpm.

@db4690 … you’ve mentioned before that you get best results by not putting the cleaner into the gas tank, but injecting it directly into the fuel rail. Could you provide a brief tutorial on your set up? On the Corolla I have access to the fuel rail by removing the cold start injector feed tube. But I’ve never thought about how I’d inject fuel injector cleaner through it. I can’t just put a tube in a bottle and let it suck the cleaner into the fuel rail, since the fuel rail is pressurized.


Hooking up the fuel cleaner to the rail is possible . . . but difficult . . . on Toyotas because there is no test port

This is the idea
Disable the fuel pump . . . pull the fuse, relay, etc.
Pour the cleaning solution in the "machine"
Hook up the machine to the car, adjust pressures as needed
You need an air supply, I might add
Start the engine . . . it’s getting its “fuel” from the machine
Let it run until it stalls out
Remove the machine
Put the fuse and/or relay back

Here’s the cleaning solution I use


Here’s my setup


The cleaner pours into the red canister on the machine

It’s more effective and more direct than pouring weak generic fuel cleaner into the fuel tank

Thanks for the good explanation & useful links @db4690.

Eureka! Thanks to the ideas presented here by forum contributors, just today my 92 Corolla 4AFE equipped finally passed the Calif emissions test. Just barely I might add. Measured 127 ppm HC at 15 mph treadmill test, 130 ppm max allowed. As before, all the other tests at 15 mph, and the entire set of tests at 25 mph easily passed. For some reason the 15 mph HC emissions remains close to the limit.

Anyway, as a summary FYI, besides what I did as posted above for the initial test (where the result was it failed with a measured HC of 160 140 ppm at 15 mph), here’s what extra stuff I did for this second test:

  • Filled tank with (edit: 91 octane) ethanol containing gasoline.
  • Added one liter fuel injector cleaner (Clean Power) to full tank.
  • Waited until tank was less than 1/4 full before taking test.
  • Several 30-40 mile freeway speed tonic sessions, including one immediately before test.
  • New spark plugs
  • New distributor cap and ignition rotor edit: and spark plug wires.
  • Tested compression – ok
  • Ignition timing retarded 2 degrees.
  • Removed 1/2 quart of oil from crank-case.
  • Edit: Cleaned fuel vapor canister

I don’t notice any difference in performance, but whichever of the above it was that brought down the HC’s a little, it was enough to pass. Thanks to all for the good ideas.



Forgot to mention, when I filled the tank, I used premium gasoline … 91 octane.

Just a sidenote, but retarding the ignition timing has a tendency to richen the fuel/air mixture.
It would be preferable to have 2 degrees more advance in it as compared to 2 degrees of retard.

Someone here maybe a year ago posted a link to a study done to experiment various tricks to reduce HC emissions. In the link they tested various methods to reduce HC’s on a modern electronic fuel injected econobox. Fuel additives, fuel octanes, spark plugs, ignition timing, heating the cat using fast driving prior to the test, changing the oil, air filter, etc. Of all the tricks they tried, other than perhaps installing a new cat, retarding the ignition was the only one that made much of a difference in reducing HC’s at the tailpipe.

I’m not doubting what you are saying @ok4450, that retarding the ignition timing richens the mixture, but in a computer controlled electronic fuel injected car, somehow the ECM must compensate enough that by the time it comes out the tailpipe there must be a little less HC when you retard the timing.

Retarding the timing raises the gas temperature late in the exhaust stroke.
This gives the mixture more time to burn more completely before the flame quenches out.
Back in the '70s some designs kept the reaction going in a baffled exhaust manifold to reduce HC.

This is the link I was referring to about an experiment somebody with emissions testing equipment did on various methods econobox owners have tried to reduce HC’s in order to pass emissions testing, including retarding ignition timing, originally posted by @circuitsmith

Interesting article

But in the real world, he would have failed, if had actually taken it to be smogged, and the igntion timing was off by 10 degrees

They don’t allow you THAT much leeway

Down here in Atlanta, Ga, they just check tailpipe emissions on pre-96 cars. They don’t check individual settings, like ignition timing or spark voltage. They just accept the results of the sniffer.

I would still replace the wires even though you passed.

@knfenimore … the spark plug wires are part of the distributor cap ass’y, so they were replaced.

It’s puzzling why, given all the stuff I did, the HC’s are still close to the limit, and why only at 15 mph? I have a few theories, but no data to back them up so really just speculations …

Theory 1. Injector imbalance … if all the injectors are a little mis-calibrated and squirt more gas than expected, the O2 sensor reading would allow the ECM to correct and reduce the injector pulse-width. But if just one of injectors squirts a little more gas than the others, that’s harder to correct for, and to get the correct O2 reading, some extra HC’s may have to pass through.

Theory 2. The HC measurement is incorrect at low exhaust flow rates; i.e. it is a measurement problem and nothing is wrong w/the engine.

Theory 3. It’s inherent in the 4AFE engine design; i.e. most 4AFE’s have this problem and there is nothing that can be done except to keep the engine in as good of tune as possible.

Theory 4. My poor Corolla is the victim of a scam. It’s possible maybe, as the privately owned “test-only” emissions shops have an incentive to show measurements on the cars they test as barely passing. Why? Because that means the car must come back to a “test-only” shop next time. If a car easily passes all tests, then that car will likely be taken out of the “test-only” pool and can go to any emissions testing site the owner desires. Unlikely, but shouldn’t be entirely dismissed.

@GeorgeSanJose‌ , I have that engine in a '92 Toyota Celica. I purchased it from my brother 5 years ago. When I first got it, it barely passed emissions, like yours. But, after a lot of trying this and that, the last emissions test was way below threshold on all tests. The repairs that had the biggest impact was cleaning the EGR and it’s passages, replacing the vacuum actuator for the EGR from the salvage yard, cleaning the throttle body and pipe cleaning the vacuum ports, and replacing the O2 sensor. And that’s on a car that hit 386, 000 miles before that last test. So, I think Theory #3 is untrue. Theories #2 and #4 can easily debunked by using a different emissions shop. That leaves #1, and an imbalance can easily be happen with dirty injectors.

Thanks BK, you are a most excellent source of Toyota 4AFE info. I appreciate you taking time to post. The 4AFE, it’s a great engine, but it does have some quirks I guess. You think it might be the EGR then? hmmm … I didn’t replace the EGR because it passes the test where you activate it and the engine stalls. And b/c the problem is with HC, rather than N emissions. I did clean the throttle body passages, and replace the O2 sensor (per your earlier suggestion). But based on your experience I’ll definitely replace the EGR, clean all the EGR passages, and do the shop manual tests for the EGR vacuum control prior to my next emissions test. Which, like you say, I should do at a different shop.

George, I did not replace the EGR. I did remove it to clean it and the passages, through the cylinder head on the exhaust crossover and into the intake manifold. My EGR also passed the vacuum-stumble test and vacuum no-leak test. The vacuum modulator is what I replaced because I couldn’t figure out how to clean it without damaging it. It had a bunch of carbon crud in the exhaust pressure port. I still had weak vacuum actuating the EGR which led me to the throttle body. That fixed the NOx’s, and the O2 sensor fixed the HC’s. For your car, there is also the MAP sensor, since these cars don’t use a MAF. Probing yours and testing it may show problem affecting your HC’s.

Maybe the aging cat converter isn’t hot enough to work efficiently at 15mph?

Replacing the cat is my back-up plan in case all else fails. I expect a brand new cat won’t allow much in the way of HC to pass by at all. I have another two years before I have to retake the test, so I’ll have some time to work on the EGR and MAP in the meantime. It looks like I may have to learn how to weld on a new cat too … lol …

Re the MAP, I did remove it for a bench test. It is holding vacuum and the resistances measure ok. But the proper way to test it according to the shop manual is with it powered-up, an on-the-car voltage test.

If the MAP were causing the engine to run rich the CO #s would be through the roof.
If running lean you’d feel the loss of power and maybe even misfiring.