Getting car to pass smog BEFORE replacing timing belt

I’m concerned about the fact that last time I had my car smog tested, a 1990 Acura Integra, the NO readings were SUPER close to the cut off limit. I am posting this because of some things I read in the following post from around July, 2011:

“Failed smog check, hi NOx, no error lights - What to fix?”

In that post, many of you chimed in about the ignition timing of the engine being off. Is this related to the timing belt?

When I took my car in to the new mechanic (my former one retired) to get the timing belt done, I mentioned that it was due for smog soon, and they told me to get the car smogged first to see if it passed, so as not to dump a lot of money into a car that may have more problems down the road.

Now, if the timing belt is over due to be changed, could this affect the NO readings?

The car has a rebuilt (so I just discovered) Cat converter put in 4 years ago, when it failed smog then, and has about 265 miles right now.
I’m taking it in to the SMOG place today…and praying it passes, but now I’m worried!

Thanks for listening,


The timing belt would not affect the smog test results. They are just trying to save you money in case your car fails smog, ie, might not be worth a new timing belt. Keep your fingers crossed, drive the car on the freeway before the test so it is warmed up.

High NOx readings are a result of high combustion temperatures.

The EGR circuit is designed to allow a small amount of exhaust gasses into the combustion process to reduce combustion temperatures to reduce NOx emissions.

Your vehicle has the OBDI engine management system. And this system doesn’t monitor the EGR circuit as closely as today’s OBDII engine management systems.

So if the vehicle fails the NOx emission part of the test, look for a problem with the EGR circuit, or for a reason that the engine is running hotter. A partially restricted radiator, the engine is running lean, or a thermostat doesn’t fully open.


You’ve gotten good advice from tester. Also be sure your engine is in good tune (fresh plugs and distributor components and wires if you have them) and take a spin on the highway before getting tested. That’ll keep your carbon monoxide and hydrocarbon levels down.

Post back with the results. We do care.

If the ignition timing is overly advanced, that can contribute to higher engine operating temperatures and NO readings too. Better to set the ignition timing on the retarded side a bit. In Calif 2 degrees more retarded than the nominal spec is within the emissions passing limit I think. That will vary state by state.

The ignition timing is not related to the timing belt. The timing belt is for the valves only. As said above, the reason for deferring the timing belt job is an economic one, old or new is unlikely to affect the emissions.

If you want to have some work done before the test, and the car has no particular symptoms or problems otherwise, no check engine light, then replacing the spark plugs and engine air filter would yield the most bang for the buck. After that, and assuming it sports these devices, replacing the distributor cap, ignition rotor, and spark plug wires. Its a good idea to arrive at the test site with a completely warmed up engine. So drive it on the freeway 20 miles or so before the test.

Another idea if you run into difficulty passing the test, try Googling “(Your State) passing emissions testing”. That will usually bring up some good articles.

Hallelujah…Sugar Magnolia (that’s her name) passed! Thanks for all of the great advice, as usual, and clarifying the engine timing vs. timing belt question I had.
I will book mark this so that, come two years from now and at this juncture again, I will be prepared :slight_smile:
By the way, I did run almost two tanks of Premium gas, with some fuel injector cleaner in the first tank, thru the car prior to the test, had the oil changed (although I’ve been told it’s not necessary) and tires properly inflated. I also know the bit about driving a good 20+ minutes on Fwy prior to testing!
Thanks again.

Unless your car requires premium gas, using it will not help anything.its only about $3 to $4 extra per tank, but that adds up over the year. Save your money for other things.

@jtsanders this was just a suggestion by the new place I am taking my car to; as a general rule I just stick with regular unleaded, but the extra $4.00 TOTAL was peace of mind for me, I guess.

If you need a timing belt, get it. Your engine will be toast if it breaks.

@texases that is on the books; it’s whole reason behind my having to find a new mechanic, which I’ve posted about earlier this summer. Normally I’ve NEVER gone beyond the recommended time!

Congratulations and sincere thanks for the update. We do care, and we usually never find out the ending.

Happy motoring.

I will only suggest that when the timing belt job is done that the entire kit is used. This means water pump, tensioners, idlers, and oil seals. The omission of even one of those items during a new belt job can possibly lead to catastrophic engine damage soon afterwards if it fails and which will then take the new belt out with it.

A coolant or engine oil leak in the belt case can saturate a new belt, weaken it, and cause it to fail very soon also.

25 yr old car with 265k miles? Gotta be California. Long rusted away in Midwest.

Before this post gets closed, one more question for curiosities sake.

In reviewing the car’s past smog tests from the last 6 years, the ONLY reading that ever gives me reason for concern is the NO level at 15 mph, which is always very close to the max allowed.
The 25 mph reading is always much lower, although it was creeping up there in 2009, two years BEFORE the failed 2011 smog, leading to the replacement of CAT converter.

Yesterdays 15mph NO level was 799ppm and max allowed is 807.
The 25mph NO level was 250ppm, below the average of 337, not close to the 746 Max.

Why such a difference? I know I could spend time searching this site for reasons, but figured you all might give a quick answer and maybe ways to “fix” this next time around!

Thank you again.


Might want to make sure your EGR is working correctly

May want to have the EGR passages snakes out

However, considering the car passed, it’s probably best to do nothing at all

And if it passes 2 years from now, so much the better . . . !

In reviewing the car's past smog tests from the last 6 years, the ONLY reading that ever gives me reason for concern is the NO level at 15 mph, which is always very close to the max allowed.
I bumped up my timing 3 degrees, and it became evident that the advanced timing was impacting NOx at low-speed idle--not so much at high speed. SO, if and when you fail NOx, clean out the EGR and back off timing a bit.

The engine in your vehicle utilizes a Back Pressure Induced EGR valve.

What this means is the back pressure produced in the exhaust system is used to open the EGR valve along with the engine vacuum.

At lower RPM’s, there’s less back pressure produced in the exhaust system so the EGR valve isn’t forced open as much. So the less flow of exhaust gasses into the combustion process the higher the combustion temperatures and the higher the NOx readings.

When the RPM’s are raised, it produces higher back pressure in the exhaust system which forces the EGR valve open more, which allows more combustion gasses into the combustion process which lowers the temperatures which then lowers the NOx readings.


^Does that mean a more-restrictive exhaust would help the car pass tailpipe emissions? (i.e. more backpressure earlier.)

Yes, good for you. Glad good ol’ Sugar M passed her test.

OP writes above

"had the oil changed (although I've been told it's not necessary)"

Changing the oil is a good thing to do prior to an emissions test actually. It can help reduce HC emissions.

Just curious what your other emissions readings were, besides the NO. I’m in Calif and have a similar ongoing emissions challenge with my Corolla, only for me it is the HC at one of the two speeds. Forget which one it is, but for the past few times I’ve barely passed. Also if it shows on the report, what did your O2 reading say?

Excellent explanation, Tester. So good that I just had to pass along my compliments.
Nice work. Couldn’t have explained it better myself.