Failed the Smog Test: What's the next step?

Won’t a tuneup fix the timing? I thought setting the timing is part of the tuneup process. It might be as simple as that.

As for government aid, it depends on your state. What state are you in?

@2davidc8 California!

My air filter is about 2 years old.

What does Ignition Timing: 15 BTDC mean.

Ignition timing means the spark must spark when the compression stroke is at it’s maximum. A matter of setting the spark plug in the proper angle to the compression chamber. Or, somewhat like that.

Easy fix with your tune up.

Oh for the record just had spark plugs and wires replaced about two months ago! I had rough idle and the tune up fixed it. Also got an oil change and new coolant same day.

Ignition timing is adjusted during a tune-up.

The simple version (the one that doesn’t get into a full description of the operation of a four-stroke piston engine) is that ignition timing is the crankshaft’s position at the time that the spark plug fires and ignites the fuel/air mixture in the cylinder. As the ignition is not instantaneous, you want the spark plug to fire before the piston hits Top Dead Center (TDC), though how far before varies from engine to engine.

Improper timing can cause all sorts of issues, depending on how far and which direction it’s off. Many of those issues are related to an incomplete burn of the fuel/air mixture, and an incomplete burn will usually cause you to fail emissions. There are other things like power loss, lower MPG, and (if it’s really off) mechanical damage, but those are beyond the scope of this question.

BTW, I assume that we are talking about a 240SX with a KA24DE engine? The “SX” moniker was used quite a bit by Nissan :/

Sorry @jerv 200sx nissan! 1995

Stock timing on the KA24DE is 20 degrees BTDC

200SX… mostly the same car, but with the SR20DET engine. Unlike the KA24DE, the SR20DET is supposed to be 15 degrees BTDC. That should’ve been a clue…

In any event, ignition timing is easy for a trained monkey to set, if they have a timing light. And, as mentioned previously, any competent tune-up includes a trained monkey using a timing light to make that adjustment to your ignition timing. Get a tune-up from any reputable garage and you should be fine.

Yes, I’m a little bit of a geek when it comes to well-known Japanese street-racing cars like your Silvia S14 clone :D

As @jerv noted, “15 BTDC” means “15° Before Top Dead Center”, but I don’t think this means quite what he said about the pistons.

“Top Dead Center” is (I believe) a scribed mark on your timing chain or belt drive wheel, and the reading that you got shows that the timing on your engine is adjusted to fire the spark plugs at 15° before that mark is reached (visible with a timing strobe light hooked up to view your timing; the strobe fires at the same time as the coil wire to the spark plugs, making the mark visible on a wheel that otherwise turns so fast that you’d never see an individual mark on it in plain light).

I’d defer to @jerv on every other aspect of the question and response (and I’d be willing to accept criticism and correction of my off-the-cuff process description as well). Pistons don’t have “top dead center” marks; they slide in cylinders from top to bottom; “center” has no meaning to a piston, but it does to a wheel.

So the consensus is we all have a good feeling that a timing ignition fix will do the trick? Do the numbers in the photograph jump out as extremely surprising?

@CWOTUS The terminology is funny. Basically TDC has the piston at the top of it’s travel. If the piston were pushed down from TDC, the crank would not rotate as the force transmitted through the piston rod would be perfectly aligned with the center of the crank. Instead of rotating the crank left or right; it’s dead, the crank is centered between advanced and retarded, and (as mentioned previously) the piston is at the top of the cylinder.

The scribed mark on the crank pulley is merely an indicator of where the crank’s position when the #1 cylinder is at TDC as it’s far easier to see a mark on the crank pulley than to see inside a cylinder. However, most engines have various marks on the block as well, denoting various positions between usually 0 and 20 degrees BTDC.

For the benefit of those who don’t know how to set ignition timing, it’s really simple for older cars. Hook a timing light up to the #1 spark plug wire and the light will flash every time the #1 spark plug fires. That will always be when that scribed mark on the crank pulley is in the same position. Compare the position of that mark to the marks on the engine block and you can see where your timing is. For an SR20DET, that light should fire right when the scribed mark on the crank pulley is aligned with the 15 mark on the block. If it’s not, loosen the bolts on your distributor and turn it slightly until the marks line up.

For a car with misadjusted timing, I am not surprised at all.

My second suspect would’ve been plugs and wires, though you’ve replaced those recently.

One thing that may complicate matters is if your engine doesn’t have a distributor, but rather, uses an Crank Angle Sensor, which is inside the engine and nowhere near as easy to adjust, as you can see here. With the SR20DET, it’s hard to say as I’ve seen those go both ways. It’s a popular tuner car, and some people have retro-fitted a distributor to certain engines originally equipped with a CAS.

But just take it in for a tune-up and pay someone else to deal with the technicalities :D

Nice, well the good news in all of this is I have 1 free smog retest. I’m really hoping it’s just the ignition timing

I’ve seen many older cars fail once, crank their distributor a bit, and pass with no other work.

@jerv Super encouraging that’s why I sent it to you bud.

Thanks all for the info and support keep the info coming! Tell me how you failed your smog and overcame it

Okay then. Compromise position on the wording: TDC marks the top dead center of the crankshaft, which is the same as “top position of the piston” (because it forces at least one of the pistons to its top location at that spot) and which mark is transcribed to the crank wheel to mark (on the wheel, in a more easily visible location to the mechanic) when the engine internals have reached that point.

Try CRC Guaranteed to Pass gas treatment. My truck failed and this product helped it to pass the emissions test for several years. Also change your oil/filter and use and oil treatment like STP.
Good luck!

@CWOTUS TDC generally refers to the #1 piston. The timing marks on the ignition system and cam shafts are placed accordingly, so unless otherwise specified, assume they are talking about the #1 cylinder. Aside from that, you got it.

@pleiades One thing I almost overlooked. Change your air filter. More air means more oxygen to combine with the fuel, which nets a more complete and cleaner burn. Again, a normal part of a complete tune-up. .

I understand that it is also helpful to have your engine fully warmed up (by travelling several miles) before you go in for the smog check.

If after trying the remedies above, your vehicle still fails the smog test, in California you can get assistance in several ways. Here is the link for some of that info.

Basically, if you can prove that you spent at least $650 in repairs to get your vehicle to pass smog and it still doesn’t pass, you can try to get a one-time waiver. You don’t have to worry about the smog test for another 2 years, but you can only use this waiver only once.

The other assistance is you can get $500 for smog repairs if your income is below a certain level. Again, check the link above.

Finally, you can sell your vehicle back to the state for $1000 (or $1500 if your income is below a certain level).

In summary, if you can get the waiver, then the next time a smog test is due, your car will be 20 years old. I don’t think it would be worth $1000 if it doesn’t even pass smog. Turning it over to the state for $1000 seems like a good idea.

UPDATE FAILED AGAIN!!

The only part that failed this time around on the emissions test was the NOx. It was extremely high on the 15mph test. The 25mph test passed.

It’s gonna run me 75$ for a smog specialist diagnostics test.

Well, it’s progress.

Huge progress, High NOx

So after 4 hours of online research, I’m thinking I can rule out the catalytic converter being the problem as the HC levels were pretty good and passed.

Most problems described with High NOx seems to be bad EGR, dirty EGR as well as bad Oxygen sensor…

My car runs smooth…