DTPC Smog Test: Making it Legal

by CFlo

If you DT readers and Volvophiles can clear your minds of unburned hydrocarbons for a few minutes to remember all the way back to early June, you’ll recall that I had successfully upgraded the Daily Turismo Project Car’s EFI system to a later, more common version of Bosch’s LH-Jetronic. Prior to that I had replaced the entire engine wiring harness due to starting and rough running woes, and welded up the free hanging exhaust system. All of this was in preparation for getting the car to pass California’s esoteric and Draconian emissions inspection. So what ever happened with the smog test? We’re finally ready to share after an extended recovery period. This is the conclusion of the first stage of the DTPC saga – getting it running and legal!

First, a little background on why all this pre-work was necessary. Historically, California smog testing was on a rolling 30-year exemption cutoff, and every year a new crop of old beaters were freed from the shackles of smog testing. But in 2005 the rolling party was frozen and thereafter any post-1975 model year car was required to pass emissions every two years, and at the time of sale. Under the previous law our ’83 would now be exempt (yay, 2013!) but the sad reality is that emissions limits are actually decreasing for older non-exempt vehicles, yes – as in getting tighter as the vehicle ages. In a very non-DT friendly turn of events this makes older cars less and less likely to pass every time they are tested – even if the cars themselves are maintained well and blow just as clean as they would have brand new!

No, we’re not tempted to leave the DTPC for dead in the desert.
This is a prouction shot from the independent film “51 West,” which featured the DTPC thanks to the previous owner’s involvement.

Back in the old days, gasoline smog testing in California’s high population density areas like LA, San Diego and the San Francisco Bay Area was done with the basic Two Speed Idle (TSI) method, in which the exhaust gas sniffer was inserted in the tailpipe and an inductive sensor was clamped on one of the spark plug wires. Then the engine was tested with the trans in neutral at both its natural idle speed and at 2500 rpm by simply opening the throttle a bit, still in neutral. This no-load testing required minimal equipment and capital expense for the state’s privately owned smog stations, but was pretty tenuous in terms of correlating to actual driving conditions since there was no load on the engine apart from driving the accessories and internal friction.

ASM, or “Acceleration Simulation Mode” dyno testing began around 1997 in Enhanced (i.e. high pollution, high vehicle concentration) areas. This tested emissions under more realistic conditions since the engine is loaded on a chassis dynamometer, which simulates acceleration in normal driving on the street. This test continues today for gas powered vehicles in the high population areas, but if you’ve got all wheel drive, your car is still tested with the TSI procedure. That’s right, a little known fact is that tuner favorite, high power AWD beasts like the MItsubishi Evo and Subaru WRX / STI are tested more leniently than our ’83 Volvo, everywhere in CA. A late model Land Cruiser, 325ix or Outback gets off easy as well – basically anything with full-time AWD. Furthermore, so-called “Basic” lower population areas of CA still require biennial TSI only, and in some areas it’s only required when ownership is transferred. A full explanation and map can be found here on the BAR’s website.

CA sets maximum limits for three major exhaust emissions: unburned hydrocarbons (HC) including methane and benzene, carbon monoxide gas (CO) and oxides of nitrogen (NOx). It also monitors the other two main gases that are products of combustion: carbon dioxide (CO2 or “carbon” as the media likes to call it now) and oxygen (O2).

The catalytic converter (at center) is pretty fresh – I stubbornly resisted the temptation to swap it out as a cure-all

We can get a great idea of how the particular limits have morphed over the years by looking through the extensive records we have for the DTPC – including just about every smog test report since it was four years old in 1987. These limits are allegedly specific to make, model, engine type and model year, so the limits given below for our ’83 Volvo 242 DL with its 2.3L naturally aspirated B23F engine and manual trans will probably be about the same as an equivalent naturally aspirated Volvo from a later model year (say, a 1988 245 wagon or a 1985 740 sedan). 

But the State of CA is cagey about how the division is made exactly – so we can’t say for certain which similar vehicles are covered by these same limits without seeing historical test reports for various other years and models. We’re not even sure if the 240 Turbo models are lumped together with the NA cars – the state won’t divulge that info either. But it’s probably safe to assume that a 1983 Cadillac Eldorado with a 6.0L V8 had slightly different tailpipe limits than our 242 did, and hopefully still does in 2013.

Plotted below are the maximum allowable limits (not the recorded values for the car) for HC, CO, and NOx to illustrate how they’ve changed over time from the period between Aug 22, 1987 to May 15, 2013.

So it’s pretty clear that the HC limits have changed a lot since 1987, but let’s look a bit closer. What we’re calling the “low speed” limit is the limit for idle in the TSI test, and 15 mph in the ASM test. This limit started at 100 ppm, crept up to 175, then finally settled at the current level of 135 ppm in 2001. As this limit was rising the state was implementing the ASM dyno testing in “enhanced” high population areas, which went into effect in 1997. The “high speed” limit started at 220 ppm and then dropped in two steps to 108 ppm. 

When comparing TSI vs. ASM testing, we can see that high and low speed limits have flip-flopped. This is because unburned HC emissions are higher at low or no-load (like an idling or cruising engine) where combustion can be incomplete, and they actually decrease with load, tapering off as the engine starts working harder and burning fuel more completely. Loading an engine on a dynamometer as the ASM test does will reduce HC emissions as load is increased, so the limits at the higher load condition are now lower. With no-load TSI testing, there are more HC emissions at high engine speed simply because the engine is burning more fuel per unit time, and there’s less time for combustion to happen in every power stroke.

Overall, the HC limit at low engine speed has increased by 35%, and the limit at high engine speed has been reduced by 51% since 1987.

Looking at carbon dioxide (CO) limits over the same time period, we can see that they’ve flip-flopped as well. Just like HC emissions, CO output will actually drop as an engine goes from no-load to part-load conditions, so we see that our ASM test criteria have the high speed / high load limits now lower than at the low speed / low load condition. What is now the low speed limit was only reduced by 3% over all those years, but the current high speed limit started out at 1.20%. It is now down to 0.77% max allowable, for a stringent 36% reduction.

For further reading, a very thorough report on emissions vs. engine load from the UC Berkeley Department of Mechanical Engineering can be found here.

Oxides of nitrogen (NOx) was not even monitored prior to 1999 in the DTPC’s test records, but after the introduction of ASM dyno testing the NOx limits were brought in, then reduced, then reduced again. Over just a four year period, the 15 mph NOx limit was tightened by 53%, and the 25 mph limit fell by a comparable 49%.

Typical LA smog. Image from pdphoto.org

I have to pause here before I start sounding too much like a climate change skeptic or conspiracy theorist and say there are some very real benefits to strict emissions standards. Especially in ultra dense urban areas like LA or the San Francisco bay area, keeping old vehicles running clean provides health benefits, and greenhouse gas reduction (per vehicle). 

NOx is one of the main components of actual smog (= smoke + fog); it reacts with volatile organic compounds and sunlight to form the characteristic brownish haze that often hangs over the LA basin. Excessive NOx in the atmosphere not only decreases visibility but poses serious health risks, so reducing its presence is obviously a good thing for us humans and the environment we live in (it just makes owning an older car in LA a bit more work!). 

Another roundabout benefit of ever more stringent emissions standards is to the economy; automotive industry and used car markets get a recharge of customers when older vehicles are scrapped. Sad for the little guy and the enthusiast, but true.

With the data analysis above, a freshly upgraded car, and several horror stories from smart friends and colleagues who have fought long and bloody battles to get their beloved older cars through smog legitimately, we turned to the helpful local Volvo pros at Hi Performance Auto Service for some assistance.

Ian says: “We now have 4 types of smog stations in California. Test & Repair, Test Only, STAR Test & Repair, and STAR Test Only. The DMV decides where you have to go (the DTPC has been directed to a STAR smog station). The STAR stations are held to a higher standard and can lose their STAR status if they don’t comply. Apparently they can also charge more for the test (and most are charging for re-tests) and they won’t try to help get you passed by pre-heating the cat. Strictly by the book.”

More info on the STAR program can be found on the BAR website here.

One added benefit of AB 2289, the senate bill that introduced the STAR program, is that most 2000 and newer model year vehicles won’t be subjected to tailpipe emissions testing any longer; instead the smog technicians will use an OBDII scanner to check for emissions related error codes. This allows more cost-effective testing for the individual privately owned smog stations, who now won’t need to have expensive 5-gas analyzers and dynos unless they test older vehicles through the STAR program.

Ready for the first round of testing

Enough background…and on to the task at hand. Since the DTPC had recently been treated to a new engine wiring harness to fix its no-start and rough running issues, plus the upgrade to LH2.2 fuel injection, it seemed reasonable to take the car into the closest STAR station for the smog test without doing any further work except for an oil and filter change, since unburned fuel probably made it into the oil during its rough running days. Unfortunately this plan backfired; results are in the table below. The numbers in red are above the allowable limits and therefore caused a Fail at 15 mph, meaning a big fat Fail for the whole test although everything else was within spec.

Test DateSpeed (mph) RPM  %CO2  %O2  HC (PPM)  CO (%)  NO (PPM)  Result 



















The DTPC was about 7% over the limit on HC, and a whopping 62% over on NOx. Crap.

High HC and NOx at low speed likely meant combustion temperatures were too high, there were vacuum leaks, and/or a very lean fuel mixture. The DTPC being an ’83 model year does not have exhaust gas recirculation like later Volvo engines, so we’re at a disadvantage in the NOx department since EGR’s main function is to reduce oxides of nitrogen. However there are ways to tone down its presence, without resorting to more aftertreatment. I was really hoping that the catalytic converter didn’t need replacement already; they aren’t cheap, this car has a fairly new one installed, and a CA law enacted in 2009 requires use of a CARB-approved catalyst (around $200 plus installation).

Before going any further, the car got a thorough checkup. Ian at Hi Performance Auto suggested replacing the following parts that he deemed to be likely to make the car fail the smog test:

Fancypants Snap-on 5 gas analyzer
  • Intake manifold gasket (verified vacuum leak)
  • Injector o-rings (verified vacuum leak)
  • Inlet snorkus tube (visibly cracked and allowing unmetered air to enter after the MAF)
  • In-tank fuel pump (verified dead last year, by applying power directly)
  • Assorted vacuum hoses (crumbling)
  • Positive Crankcase Ventilation (PCV) valve / flame trap (clogged)
  • Fuel filter (as a cheap precaution)

Once the car was reassembled and running, we took it back to Ian and did a drive test with a muchodinero Snap-on handheld 5-gas analyzer. It’s got a sniffer unit that gets inserted into the tailpipe, then the car can be driven around and emissions monitored at various speeds and loads. We tried to replicate the smog dyno’s conditions as best as possible; I put the car in 2nd gear, then simultaneously gave it about 75% throttle and left-foot braking to reach 15 mph under part-load. This worked again at 25 mph, just less braking required. Ian monitored the analyzer and it showed readings that were well under the limits (the image to right shows 11 ppm HC and 143 ppm NO, but this was at idle speed with the trans in neutral).

Shortly after the test, the brand new replacement in-tank fuel pump died an early death and was sent to Valhalla in flames, to dine with the gods since it was killed gloriously in battle. When the in-tank pump died it restricted fuel flow to the main pump and rendered the car undrivable. This of course coincided with a 30 year old power steering hose rupturing and spewing hydraulic oil all over the ignition components. 

Back in the Hi Performance Volvo shop, Ian upgraded the fuel pump to the later model 240 style which is a more robust design and a bit higher flowing. We thought perhaps marginal in-tank pump performance may have been a factor in the high emissions by reducing fuel flow and leaning out the mixture, which is why it was replaced the first time. Another not-so-minor point: the car also generously got a known-good “loaner” AMM (aka MAF sensor), a brand new Bosch part that lists for almost $400.

After getting the car back, running much smoother and happier I might add, it seemed like a good time to strap this thing back onto the smog dyno and see what happened.

On the dyno rollers during the ASM test (tailpipe sniffer wasn’t in place yet)

Test DateSpeed (mph) RPM  %CO2  %O2  HC (PPM)  CO (%)  NO (PPM)  Result 



















% change








% change








OK, so eight days and a ton of work later, we’re looking much better but still not all the way there. Hydrocarbon output at 15 mph is now under the limit, but not by much. NOx at 15 mph however is now higher than it was the first time. I’ve also charted the percent changes in each of the exhaust gasses; you can see that all of our work did have very measurable effects as everything except NOx at 15 mph was significantly reduced. Because HC was now under control, we can rule out a lean mixture as the likely cause of high NOx and focus on two remaining hypotheses:

1) High combustion temperatures due to localized detonation, likely carbon buildup in the cylinders causing “hot spots.”

2) Ignition timing too far advanced – as spark is retarded NOx will drop, so perhaps our timing put the ignition event too early before the power stroke.

To address the first point, after the second test failure I did a “Seafoam” treatment in an attempt to remove carbon buildup from the ports, valves, and cylinders. I can’t say for sure if this had any positive effects but it sure is fun since you let the running engine suck the stuff in, which then burns incompletely and creates a helluva big black exhaust cloud that wafts slowly down the street into your annoying neighbor’s face.

After smoking out the street, I checked ignition timing with an inductive timing light. It was right in the middle of the allowed range per Volvo’s OE spec, so I loosened the distributor clamp bolt and rotated it until the flywheel pulley timing mark showed 10° before top dead center – as retarded as is allowed by Volvo and CA. Normally we would have to agree with Kirk Lazarus / Robert Downey Jr. in saying “never go full retard,” but in this case instead of ending an acting career it brought our NOx emissions under control. Retardation is a good thing for reducing NOx; even Simple Jack can understand that.

Back to the smog dyno one last time, and we get results!

Test DateSpeed (mph) RPM  %CO2  %O2  HC (PPM)  CO (%)  NO (PPM)  Result 



















% change








% change








The NOx levels at both test speeds are well under control now, and the other controlled emissions (HC and CO) are actually even further reduced. The DTPC is finally clean and legal, and was rewarded with a shiny new 2014 registration tag for all our effort.

I’ve heard and in no way have any firsthand experience that placing a small BB or ball bearing in the vacuum advance line to the ignition control computer will disable vacuum advance and keep the ignition timing in its retarded state regardless of engine load. Furthermore, I’ve been told that this type of temporary action could significantly cut down NOx emissions if all else fails, at the expense of power of course. Not that I have any firsthand experience with this…

Looking resplendent in the San Diego sunlight, at the Big SoCal Euro show last weekend.

Thanks again to the guys at Hi Performance Auto Service for the guidance. The DTPC has been getting exercised regularly for the past few months, now that there’s no reason to fear the terrible shrieks of the Nazgûl Smogwraiths constantly at our backs.

Total From Last Time


Initial tune-up parts: air filter, oil filter, oil


Major service: intake manifold gasket, PCV, intake tube, vacuum hoses, fuel pump & filter


Smog test, including free re-tests within 30 days 


Total so far 


If you’re nerdier than us and want to do some even further reading, check out these links (edit: removed the Wikipedia link since it is full of unsubstantiated facts and opinions, and added a few new ones):

California Bureau of Automotive Repair: Smog Check FAQs

The entire friggin CA Smog Test Procedures Manual

CARB’s page on the health effects of air pollution

CARB’s list of smog-legal aftermarket upgrade parts

AirNow’s live map of air quality across the USA

South Coast AQMD’s homepage

Have your own smog sob story (or SAAB story)? Email us here: tips@dailyturismo.com