I purchased a 30-year-old truck, which seems to run well in spite of its age. But when I take it to get a smog check, the engineer clips a timing light to the first spark plug and tells me that the timing is off. He says that I have to change the distributor back to the factory default before I can pass the test. But when I set it to the factory default, my truck runs unevenly and seems laggy. I suppose that there may be a problem with the electronic timing advance (it is a Toyota Hilux pickup, 22R-E 4-cylinder, late 1980s). The measured emissions are always well below the average and maximum for my vehicle class, but I'm a little concerned that the emissions might go up after I restore the distributor setting (as I always have to do after the test) to where the previous owner had set it.

This back-and-forth has been going on for a few smog checks. I've been to different places and I have never been able to get a clear explanation of why it is necessary for the smog check to include a check of the vehicle's ignition timing. If the purpose of the check is to make sure that the vehicle's emissions are reasonable, then shouldn't I leave the timing where it is when I drive it?

I think I have heard an explanation along the lines of "some people change the distributor back to the factory default before they get a smog check, so that it passes, and the government wants to prevent this" but the problem is the second part of the sentence; if the government wanted to prevent people from doing something, then why would they require them to do exactly that thing? In other words, if you wanted to catch people who had changed the timing back to the factory default, then wouldn't you be looking for cars where the timing is at the factory default, not for cars where it is different? And if you wanted vehicle owners not to change the distributor before the test, then wouldn't you tell them not to change it before the test, rather than telling them to change it before the test? Maybe that sounds a little confusing, but I'm not sure how else to express the fact that, according to the justification I have been given, the opposite of the correct action seems to be being taken in every instance.

Maybe the question could be phrased most concisely as follows: What is the theoretical scenario that is being addressed with the check of the ignition timing? If you had to write a short story about a guy who is doing something bad and then he has to get a smog check, and the check of the ignition timing forces him to start being good, what would the story's plot be?

2 Answers 2


The timing affects the point of ignition of the fuel charge during the compression phase as it is usually before TDC.

This point is advanced as the engine speed increases as the fuel charge takes a finite amount of time to ignite and then burn.

If the point of ignition is too late some of the fuel charge leaves the combustion chamber still burning.

The point of ignition will also have an effect onthe combustion process and can cause levels of particular gas emissions to rise according to the conditions. So for the same engine speed, changing the timing could increase NOx or CO or both as an example.

If your advance mechanism has failed then your best course of action is to fix it. As it is checked as to try to ensure the minimum emissions from the engine during operation.

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    You explained correctly why changing the timing can make the emissions worse. However, you didn't answer the question. Perhaps re-read the very last sentence? The smog test already includes a test for emissions. Thanks. Aug 10, 2021 at 5:58
  • @Metamorphic perhaps you did not understand the answer. Anyway you should only ask 1 question and that I answered. This is not a “how to write stories stack” either.
    – Solar Mike
    Aug 10, 2021 at 6:03
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    I apologize if I misunderstood your intent, but upon reading your answer it was indicated to me that you had skimmed my question, and I wanted to give you a chance to correct that. If you were to re-read the third paragraph perhaps? I think that paragraph is the one that your answer most ignores for me. Perhaps we have misunderstood each other. Aug 10, 2021 at 6:15
  • @Metamorphic I focused on the title and paragraph 1 as that seemed to be the “meat” of the problem. The rest I took to be anecdotal or at least peripheral. So rewrite your post with a single question that can be clearly answered instead of the multiple questions you have now.
    – Solar Mike
    Aug 10, 2021 at 6:20
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    Sorry, if I thought the question was skimmable, then I would have made it shorter. I agree with everything you said in your answer, and I think I already expressed this fact in my question, which is partly what made it so long. It feels to me like you're just adding to the noise that people have to read through when they try to understand my question. If you want to try to understand it, then you're welcome to suggest ways to simplify it or make it more readable, but until you understand what I wrote, your help is really not useful to me. Aug 10, 2021 at 6:44

California has the toughest vehicle emissions program in the country and most likely due to smog back in the 40's to 70's before emissions became an issue. If I'm not mistaken, each vehicle has its emissions at the time of manufacture so your '80s vehicle doesn't have EFI and catalytic converters and your state cannot apply 2021 emissions against a vehicle not having up to date self conducting emissions controls (since 1996).

Since the inspector states the vehicle must have its timing adjusted to factory then so be it. The question is will it pass emissions for 80's emissions? Your DMV should be able to provide details when asking. Modern emissions testing doesn't apply to older vehicles so inspection stations must use whatever was required back in the 80's - egr valve, pcv valve, exhaust gas analyzer? If a sniffer is used then the old style carburetor may need simple adjusting for correct factory timing, idle rpm and lean fuel mixtures. Old carburetor style fuel mixture adjustment consists of setting idle and timing then adjusting the idle mixture needle between rich and lean. Rich mixtures will drop rpm. Lean mixtures will drop rpm. The sweet spot is in between rich and lean mixtures with needle adjustment probably less than half a turn between rich and lean. Done by ear. The distributor points should be clean, not pitted and adjusted for correct gap when the cam lobe opens contact points. Old school tuning. Another issue with sniffers may be running unleaded where it may help to run premium or higher alcohol content to help lower CO, HC and NOX. If the engine runs fine but idles lumpy when setting spark advance back to factory, either the vehicle passes or not. If it does, drive home and readjust timing back to whatever works. I doubt emissions people follow anyone after inspection. Remember, 80's emissions control was at the beginnings with all sorts of controls as catcons were added, carburetors came locked too minimize meddling of mixture needles, dashpots, and other things to begin hardening of emissions regulations. Almost every vehicle passes easily with a large margin of error, unlike today's tightly controlled emissions with catcons.

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