Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Corolla Barely Passing CA smog

Hi folks, I recently got my 1996 automatic Toyota Corolla smogged. It had a measured HC (ppm) of 61 (passing value is also 61) at 15 mph. All other parameters (CO and NO) passed easily. Prior to the test, I had replaced air filter, spark plug wires, and spark plugs (within a month). I noticed that after these changes, my fuel economy decreased by a couple mpg and the engine seemed to rev higher before shifting. The mechanic at the smog test had suggested that I replace the new spark plugs with platinum ones (I have the copper NGK) since it is hotter ignition and would burn fuel better. However, I had tried some Bosch Iridium plugs with same results. This is only the second time that I’ve replaced my plugs and first time for wires-- perhaps I messed something up? Thanks for the advice!!!

It’s possible you knocked off a vacuum line, or it was cracked and you knocked it and now it leaks. Its also possible the cars computer has not relearned it’s new fuel curve. Try dusconecting the battery for 15+ min to clear the computer out. Also check your vac lines.

I have an early 90’s Corolla and have a similar problem with HC’s close to the limit. I’ve always passed, but just barely. Here’s some ideas.

Bringing all the routine engine maintenance up to Toyota’s schedule is the first place to start. Installing a new air filter is an excellent idea. This can be a make/break thing itself for passing. Since you didn’t quite make it, you’ll have to come up with some other things to try. Were the new plugs properly gapped? Doublecheck that the gap is correct. And make sure the plug you use is one recommend in your owners manual. An exact replacement. I use the NGK verion my owners manual says and get good results. It seems unlikely to me the plug would make much of a differenceas long as it is one of the ones recommended.

If your car has a distributor, you might try replacing the cap and rotor also though. And verify the PCV system is working correctly. No harm done in double-checking that the EGR valve is working correctly. Although if it isn’t, it would usually turn on the check engine light. An oil change just prior to the test can help with lowering HC’s too. It might be worth it putting in a thicker oil than you normally use too. 30 weight rather than 10-30 say. Try to schedule the test on a warm dry day, low humidy if possible, and make sure your car engine is at operating temperature when they do the test. I idle the engine in the next door parking lot until the fan comes on, before driving over to the test site.

Google “How to pass an emissions test”, there’s a couple of web sites there with some very good ideas on common sense things you can do to improve your chances of passing.

If you tried all that and still slightly are above the HC limit, no worries, but hold on to your wallet, you’ll probably have to install a replacement catylitic converter. It may very well be that the reason you are failing is becuase your cat is simply worn out and not working. Replacing the cat just prior to the test should easily get you over the mark. Best of luck.

Oh, one more thing, if you do what the above poster suggests and disconnect the battery, make sure you do this several days, best if it is a week, prior to the test. You engine computer knows that the battery has been disconnected, and will tattle on you when you go for the test. They won’t test your car if the battery has recently been disconnected, until the complete driving routine the engine computer needs to re-calibrate the emissions equipment has been repeated. And that usually takes a week’s of driving.

Forget that bit about the platinum plugs causing a hotter igntion as that is simply not true.

There are a number of things that could cause a problem like this. Lowered engine compression, any oil consumption that may exist, partially clogged converter, or ignition timing that is off.

Info about mileage, compression, oil consumption, any history of the distributor being removed or timing altered, etc. might help in making at least a guess.

JMHO, but anytime spark plugs come out of anything the compression gets tested no matter how many miles are on the engine.

If everything appears to be on the up and up (compression fine, no codes, etc, etc) then you might consider advancing the ignition timing a few (as in 2) degrees to see if that will help.

How old is the thermostat?

Excess hydrocarbons come either from excess unburned gasoline in the exhaust or from oil being burned.

It’s important for starters to be sure that all of your ignition componenets are in tip-top shape, including the plugs and wires (which you’ve already changed)/ In your case, the engine is distributor based, so it’s also important to change the distributor cap and rotor.

When you change your spark plugs, it’s also good to “read” them. If you have one that’s wet and smells fuel you may have an injector not closing properly. Black soot coating could be carbon from a slightly rich injector (or poor pattern). I could be wrong, but I believe your engine is multiport injected. You could have the injectors bench tested, or replace them if you suspect one (probably cheaper). If the plugs are black goop, that’d be oil.

If you know you’re burning excess oil, that’ll cause high HC readings too. Oil is a hydrocarbon.

Different sparkplugs do have different “heat ranges”, but that’ll be controlled by the design of the plug, mainly the length and ceramic. A plug of a different electrode type that’s listed for your car will have the same “temperature range” but wear at a different rate.

Regarding th eoil burning? are you burning oil? If you’re unsure, you could always have a wet/dry compression test done.

You should always get an oil change just before the smog test. Other than that, a good drive before the test so that the engine is fully warmed up can help. The one thing I didn’t see mentioned here though is the PCV valve. Its cheap and easy to replace

Maryland suggests taking the car out for a half hour ride on the highway before having a smog test run. Did you do this? If not, you might try it next time.

Getting the engine up to operating temp with a half hour drive on the highway just prior to the test is a must. The coolant temp guage on the dashboard should show it at the midrange of the scale, where it is after it stablizes on a long drive. I also take my car for a long freeway drive in the week prior to the test, in hopes some carbon buildup will be removed. I’m not sure it helps, but it doesn’t seem to hurt. I’ve always passed, although sometimes just barely on the HC’s.