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Smog test and ignition timing

I have a couple of questions about the results of my two recent smog tests and the adjustments to the ignition timing the mechanic made to have the second test pass.

First test, failed because ignition timing 20 degrees BTDC:

TEST RPM %CO2 %O2 – HC (PPM) – — CO (%) — RESULTS


idle 984 7.6 10.3 120 29 22 1.00 0.10 0.00 PASS

2500 RPM 2477 9.1 8.4 140 20 24 1.00 0.10 0.00 PASS

Second test, passed with ignition timing adjusted to 10 degrees BTDC:

TEST RPM %CO2 %O2 – HC (PPM) – — CO (%) — RESULTS


idle 970 12.1 4.3 120 29 14 1.00 0.10 0.00 PASS

2500 RPM 2579 15.2 0.0 140 20 30 1.00 0.10 0.12 PASS

The vehicle emission control label specifies ignition timing 5 degrees BTDC at 950 RPM max with vacuum hoses disconnected from distributor and sealed.

My questions:

- Do the emissions results in the second test (which passed) actually look better than those of the first test (which failed)?

- Given that the mechanic adjusted the timing to bring the timing into compliance and pass the smog test, why would the timing not be set to the specified 5 degrees BTDC?

Looks like spaces in our messages get removed, so posting formatted tables is difficult. Here is a reformatted version of the test results.

First test, failed because ignition timing 20 degrees BTDC:

TESTS: idle | 2500 RPM
RPM: 984 | 2477
%CO2 MEAS: 7.6 | 9.1
%O2 MEAS: 10.3 | 8.4

  • MAX: 120 | 140
  • AVE: 29 | 20
  • MEAS: 22 | 24
    CO (%):
  • MAX: 1.00 | 1.00
  • AVE: 0.10 | 0.10
  • MEAS: 0.00 | 0.00

Second test, passed with ignition timing adjusted to 10 degrees BTDC:

TESTS: idle | 2500 RPM
RPM: 970 | 2579
%CO2 MEAS: 12.1 | 15.2
%O2 MEAS: 4.3 | 0.0

  • MAX: 120 | 140
  • AVE: 29 | 20
  • MEAS: 14 | 30
    CO (%):
  • MAX: 1.00 | 1.00
  • AVE: 0.10 | 0.10
  • MEAS: 0.00 | 0.12

It’s simply that the timing was too far from what was specified. It doesn’t matter about the other readings which were all good. I guess 5 degrees off is close enough for the state to accept it. I don’t know why he didn’t set the timing to where it was perfect. He may have a policy against doing free work or something.

Right, but a more fundamental question is: why specify ignition timing at all when the end result (coming out of the tailpipe) is already being measured, and “correcting” the timing may result in greater emissions.

I paid for the timing adjustment – second trip to the mechanic, fee for the work, free emissions re-test and a fee for the smog certificate.

What happened to the NOx values?
Who “Failed” the emissions test based on the ignition timing, and how did they know what those values were? The mechanic? The State Emissions Test Station?

why specify ignition timing at all when the end result (coming out of the tailpipe) is already being measured

I can only guess. Maybe when the timing is that far off it can cause damage to the system that would result in additional tail pipe emissions later.

NOx values are not tested in the area where I live.

For more information on that, see :

“Emissions Test: The third of the three vital parts of the California Smog Check. This is where the emissions analyzer tests actual emissions from your vehicle, as measured at the tailpipe. Only the emissions test can label a car a Gross Polluter. Emissions measured include Carbon Monoxide (CO), Carbon Dioxide (CO2), Hydrocarbons (HC) and Oxygen (O2). In California’s most polluted urban areas (Enhanced Areas), the emissions test also measures levels of oxides of nitrogen (NOx).”

Note: I don’t live in one of the polluted areas.

The ignition timing test (part of the function check results, not the idle emissions test results) was run during the smog check. This was at a garage that also does smog checks.

Could be.

Another question I have is this: given that the 10 degree BTDC setting resulted in a rich mixture at 2500 RPM (no O2, greater HC, and now measurable CO), I wonder if this would have been even worse at the specified 5 degrees BTDC – in other words, an increasing trend from low emissions at 20 degrees through 10 and then 5 degrees, with 5 degrees being the worst.

Figuring this out could be iffy since there are a lot of factors that overlap so bear with me a bit.
Advancing the ignition timing has the effect of leaning the engine out and retarding the timing the opposite; it has a tendency to richen it.
At some point, and it varies, if the timing is advanced too much this causes a subtle, or not so subtle, tendency to richen the fuel/air mix due to a performance drop off.
Retarding the timing always has the effect of richening things up.

What I would really be concerned about is why the timing was at 20 degrees BTDC in the first place. Who did this?
One can get away with a few degrees of extra advance, or even 5, but it’s a miracle the pistons have not been fried out of the engine.

I’m not saying 5 degrees more advance is the right thing to do here and I’m not familiar with the details behind the State of CA emissions testing, etc.
Just curious about the state’s policy on testing stations, if the mechanic is state certified, what the state has to say about not putting the timing where it should be, etc.

If the underhood sticker says 5 degrees BTDC then I don’t see how setting it at 10 would be within specs. The most variance I’ve ever seen on timing specifications is +/- 2 degrees.

What is the make, model, year, and engine size of this vehicle? Does this vehicle have Exhaust Gas Recirculation and/or a catalytic converter? One reason for setting the timing correctly is to limit the NOX emissions even if there is a three way catalyst. Overadvanced ignition leads to detonation and excess NOX production.

One question to ask about the timing being at 20 BTC is ‘How did it get there?’ Did someone adjust the timing to correct a driveablity problem? Possibly the distributor has a mechanical advance that is stuck. If you rev the engine with everything connected you should see a total advance around 35-40 degrees. Does it do that?

Get back to us with more information. It does help narrow our diagnosis.

I too was struck by the timing having been 15 degrees advanced beyond the manufacturer’s spec.

Can you provide and background on this? Is your vacuum advance inoperative? You should have been preigniting like crazy.

Looks to me like you passed both tests. The readings are almost identical. Overly advanced timing usually causes a rough idle (misfire) which can elevate HC readings. Your HC looks fine. Did you fail for NOx and not list those results??

This is a 1988 Chevrolet Nova, 1.6L, carbureted, with a catalytic converter and exhaust gas recirculation.

It turns out the mechanic did adjust the timing to 5 degrees BTDC prior to the second smog test, but mistakenly wrote 10 BTDC in the paperwork. I had another mechanic measure the timing after the second smog check and it was 5 BTDC.

Here are the relevant events:

January, 2006 / 192004 miles: Lots of work done by Mechanic X on the car, including timing adjustment.

Early March, 2007 / 200047 miles: A bit of work done on the car by Mechanic Y. I told the mechanic that occasionally there is a five-second run-on in cold weather. I do not remember what adjustments the mechanic made to address this, so there is the possibility of fiddling with the timing.

Late March, 2007: Smog check, passed, performed by Mechanic Z.

April, 2009 / 213409 miles: Smog check, failed. Ignition timing measured 20 BTDC. Performed by Mechanic Z.

May, 2009: Ignition timing adjusted to 5 BTDC by Mechanic Z. Smog check, passed, also performed by Mechanic Z. Timing subsequently verified by Mechanic X at 5 BTDC.

  • Between the March, 2007 smog check and the April, 2009 smog check there were no adjustments made to the engine. Just me doing basic maintenance like changing the oil, checking the fluids and air filter, etc. I do not have my 2007 smog certificate handy to check the results from then. However, I checked with the Bureau of Automotive Repair about tolerances for timing. I was told the timing is required to be within 3 degrees of the specification to pass the smog test. Assuming that was the case back in 2007, my timing then had to be 5 degrees +/- 3 degrees BTDC to pass. No idea what would have led to a measurement of 20 BTDC two years later with no engine work during the intervening period.

  • From January, 2006 through now there have been no major symptoms of problems. There was the occasional cold-weather engine run-on. I think this is due to a choke plate that tends to stay open then snaps shut after the engine has warmed, instead of closing gradually. It has been doing this since I got the car. This should not be a problem in measuring timing, as the emissions tests also measure the associated RPM to use the correct RPM for the tests. Essentially the behavior of the car has been normal during this entire period. Fuel efficiency has been normal (30-35 miles/gallon); no unusual noises; and no lack of power relative to average. Regarding pre-ignition, there has not been any unusual tendency to ping. Ever since I bought the car in 1994, it has been possible to cause it ping by trying to accelerate too hard, especially up hills. The car is not very powerful, so I just do not push it too hard when accelerating. On the other hand, I do regularly drive through mountains, providing frequent opportunities to make the car ping, but I simply avoid making it do that. The car’s tendency to ping has not been unusual during the last three years relative to any other normal period.

Regarding other messages and questions…

Pre-igniting like crazy: no.

Amount of advance while revving the engine; whether vacuum advance inoperative: don’t know; have not had a mechanic check.

NOx: oxides of nitrogen are not tested in my area.

Rough idle: nothing particularly unusual.

That timing makes more sense.

To answer the original question, I don’t see a whole heck of a lot of difference. IMHO the readings are within normal variations even if the timing were not adjusted. But my opinion doesn’t count, only your state’s.

Yes it does appear the 1988 Nova has a mechanical and vacuum advance distributor so the next time you have this car to the mechanic, have s/him check out the mechanical advance. On a 21 year old car it could need lubrication or be just frozen solid. This will hurt your fuel economy as well as causing overheating.

Just a heads up.

Some of this makes a bit more sense now, except for the huge discrepancy in ignition timing.
Timing that is advanced too far or an inoperative EGR system can cause pinging.

Since this is a carbureted vehicle I see no problem with a little smog reading variance because a carburetor is not near as precise a system as fuel injection is.
Even atmospheric conditions such as temperature, humdidity, and barometric pressure can influence them to some degree

EGR is examined as part of the emissions test, under the “Emission Control Systems visual Inspection/Functional Check Results”.

For both the first and second smog tests – the one at 20 degrees BTDC and the one at 5 BTDC, the results were:

ECS: EGR Visual
Result: Pass

ECS: EGR Functional
Result: Pass

What are the odds that this was a wild goose chase? From what I am reading here, timing 15 degrees in advance should produce drastic, observable change (to an observant driver) in the performance, fuel efficiency, and sound of the car. I think it would also show up in the emissions tests. But it did not.

I can monitor the timing in the future. I would be spending extra time and money for more frequent testing than the suggested 40 months / 30000 miles and to have mechanics check various hypotheses about why the timing would drift 15 degrees by itself.

Or… do I not do the above because the timing was actually fine in the first place (say, 5 to 7 degrees BTDC) and the smog test technician simply measured the timing incorrectly or recorded it incorrectly in the first place? What is more plausible?

I tend to agree with you that maybe someone made an error in all of this.
If the ignition timing was really advanced 15 degrees too much then several things should occur.

One is that pre-ignition would be horrible. Since you said you do a lot of mountain driving this should have brought out the worst in it and to be honest, the engine should have probably been trashed long ago.
Two is that an increase in timing advance will increase the idle speed; and in this case, probably quite a bit. Once this happens it is often impossible to adjust the carburetor idle speed down enough to compensate for it.
This would also lead to engine run-on when it’s shut off; or dieseling as it’s called.

The reason I suggested an EGR fault earlier is that I was under the impression from some of your posts that testing oxides of nitrogen was not done in your area and since the EGR is a big part of controlling NOX that it was a non-issue.

No idea what is going on with the timing business but I do agree with you that this could have been a wild goose chase.
There is no way the timing should drift much at all unless there was a timing belt change or the distributor was dinked around with.

The last timing belt change was January, 2006, at same time as the last ignition timing adjustment. No adjustments since the 2007 smog check that passed.

I am going to run some of these thoughts by the mechanic who did the smog tests and get his perspective on whether the 20 BTDC measurement is realistic. This does not seem to add up on a technical basis, nor on a common-sense basis.

Regarding the pre-ignition, would 15 degrees excess advance tend to cause pre-ignition during idle? For example, pre-ignition with the engine idling at 980 or 2500 rpm while the smog technician was working next to the open engine compartment? If so, wouldn’t he have had a strange and memorable experience trying to inspect my car if its timing were actually 15 degrees too far in advance?