Air filter and smog test

Is it better to blow out my used air filter or put in a brand new one before having a smog test on my '97 Tacoma? I've read somewhere that a new filter lets too much air through and negatively impacts the test, but I can't find where I read that.

 

I've never heard that before. If your air filter is dirty then I would change it, not for the test but because it's always better to have a clean air filter than a dirty one...and they aren't expensive.

Every time I've smogged my 04 I've never had a brand new air filter in it and it's always passed, I'm in SoCal too. The only thing I could see a dirty air filter doing is possibly causing a CEL to come on if its so dirty that it's messing up the MAF sensor. If you don't have a CEL on then you're fine.

The best thing to do is make sure you drive it a bit and take it to the smog station while it's fully warmed up. Smog testing a cold vehicle is a guaranteed way to make you more likely to fail.

If you have a OBDII scanner of some sort, you can pull up your Emissions Readiness screen and make sure it's all green. If the emissions system isn't ready then they will tell you to come back when it is. It works on a cycle, so for instance if you recently replaced your battery or had it disconnected then your system might not be ready yet. Usually takes a couple hundred miles for it to complete the cycle.

As a general rule, if your check engine light isn't on then you probably don't have anything to worry about. These trucks (even 1st gens) have so many sensors on them that if anything is off the CEL will come on, and that will make you fail.

 

The fuel/air mixture is computer controlled so it works with the amount of airflow available. (In theory)

With a dirty air filter, the low speed emissions might be affected?

 

We're not talking about a dirty filter. Is there no advantage of using an older, yet clean filter over a new one? I can either put the new one in before or after the test.

 

for purposes of this discussion... if a clean air filter is better than a dirty one then no air filter is better than a clean one, unfortunately, for smog purposes it doesn't make any difference unless your air filter is so plugged that no air gets by it. You are in CA with 97... it is going to be a dyno and tailpipe test anyway as opposed to a OBDII scan only.

 

about the only time an air filter will hinder a smog test is on a carb'd vehicle that will mess with air fuel ratios. Your 97 will pass just fine if everything is in order regardless of air filter state. Dont change a thing before taking it to get smogged IMO.

 

Thanks for the feedback. I just need to finish this tank of gas (with the fuel system cleaner in it), and then I'll change my oil and try to find a place that hasn't shut down for the pandemic.

 

I had an 86 turbo 4runner that died during the SMOG test due to a faulty distributor.

It still easily passed.

The presence or absence of some air filter is not going to make a hill of beans difference in a Toyota passing a SMOG test. I see all these type of things on Jeep forums where you have to do all these magic tricks to get them to pass smog.

It'd have to be running REAL dirty not to pass on the sniff test. All you need to do is make sure it doesn't have any CEL codes, and the emissions readiness codes are in the green. I'd bet if it were running on 5 cylinders it would still pass smog if it didn't throw a CEL code...