Everything You Wanted To Know About Passing Smog

The usuall checks for an MR2 involve the following;

Functional - EGR valve opens (and idle speed drops) when vacuum is applied to the valve, EGR plumbing provides vacuum to the valve between 3-4K RPM, timing is set to the factory setting (this is frequently not checked), idle speed. 

Gas measurement - Acceptable levels of the measured gases change by type of car, it's age and where the car is registered. I have given levels below that my '85 MR2 had to pass. 

Gas tests are run at idle speed and at a cruising speed. Some 49 state cars are only checked at idle. Idle speed is defined as 600-1200 RPM and cruising speed is usually between 2-3K RPM. 

HC, hydrocarbons, basically unburned and partially burned fuel. Usually must be under 200 PPM (parts per million).

CO, carbon monoxide, a product of combustion, poisonous in high concentrations. Pre-catalyst CO level varies directly with air/fuel mixture and is commonly used to adjust the idle mixture. Proper pre-catalyst levels for cars vary from 2% for a 70's era car to .5% for modern lean-burn cars. Required post-catalyst levels are usually under 0.5%. 

CO2, carbon dioxide, also a byproduct of combustion, used to verify that you are not diluting or leaking your exhaust before it exits the tailpipe. A common trick in the early emission days was to rig cars with air injection pumps to push clean outside air into the exhaust system and thus dilute the exhaust to the point of passing. Since CO2 is produced during combustion and by the catalyst reducing CO and HC, true exhaust will have a much higher level of CO2 than air. A MINIMUM limit is set on measured CO2, if you produce less than the limit, you fail. Typical is 8%. 

There are many things that will affect your emissions, and there are ways to compensate for some of them. Make sure the car is running right (ignition, plugs, rings, etc) before bothering with any of the things below. Almost all of the information pertains to passing a car has been modified for performance. 

Catalyst - You have to have one. It is unlikely that you will pass HC and CO emission levels withouth a functional catalyst if you have made any modifications to the car. A newer catalyst will perform better than old ones (over 50K miles on them) thus giving you some extra margin on how much HC/CO the engine can produce. Using certain types of race gas, aviation fuel or any lead containing gas will reduce the effectiveness of the catalyst, sometimes permanently. A larger catalyst will reduce emissions as well as provide less backpressure. There is a limit to going oversize though. The catalyst must run at a very high internal temperature to work. If you have a large enough catalyst, you may not be able to keep it hot enough at idle to work well. In California, aftermarket catalysts have to be certified. Practically, if the catalyst works well enough for you to pass the test you can use anything you want. There is no easy way for the inspector to tell if the catalyst you have is certified or not. 

The only way to avoid a catalyst is to use a thermal reactor (such as the Honda CVCC motors used) which involves a new exhaust manifold with special plumbing in it. Most thermal reactors are more restrictive than a catalyst anyway. 

A MkI NA motor can use a MkI SC catalyst and exhaust sytem with no impact on emissions. You will have to swap O2 sensors since some cars use a 2 wire sensor while later models and the SC car use a 4 wire O2 sensor with a heater element. The exhaust manifolds are NOT compatible due to different EGR systems. 

Ignition timing - Retarding ignition timing will usually reduce emmisions somewhat. Howevever, once you start nearing 0 degrees the idle quality will go down so much that you will produce more emmisions due to misfires. Also, if the idle timing is checked, you may fail for having it set wrong. MkI motors have three timing marks on the crank pully. TDC (0 degrees), 10 and 15 degrees. When the timing is adjusted with the electronic jumper in place (as per the factory manual) the timing is set to the 10 mark. After the jumper is removed, the computer moves the timing up the 15 mark. You can retard the timing back to the 10 mark or even further back without it appearing that the timing is off (unless the test person is familiar enough with this motor to know which mark should be used). In any case, the testers are frequently too slack to even check and simply assume it's set right. 

Idle speed - The allowance for idle speed of the car is quite high. This helps quite a bit in reducing emissions. If the car has high overlap cams, the idle will be quite rough at the stock idle speed of 800 RPM and the HC and CO levels will be significantly higher. As the engine speed increases the idle becomes smoother and emissions fall. Setting the idle speed to 1200 RPM will usually produce a significant decrease in HC and CO levels, as well as as smoother idle and allow you to lean the mixture out more if you have installed larger sized injectors. This is covered below under the air mass sensor section. 

Air Mass Sensor - The air mass sensor is used by the computer to adjust fuel delivery at start up, idle and during sudden throttle changes. There are two adjustments to the sensor, both of which are normally sealed. Idle mixture is controlled by a screw that looks like the idle speed screw. On the side of the sensor there will be a port that looks similar to the idle speed adjustment port of the throttle body, except that a metal plug seals the top. Remove the metal plug by drilling a small hole in it and extracting it with a screw or punch. Turning the screw out will lean the idle mixture and turning it in will richen the idle mixture. 

The other adjustment requires removal of the plastic sensor cover. Carefully use a knife to cut out the silicone sealer around the outer edge of the cover and then pry it up with a small screwdriver. You will see an arm that has a circular spring attached to it and a wiper that runs along a black arc. The black arc is a resistive compound and the wiper arm has a wire attached to it that provides a varying signal to the computer as the arm moves. Also note that when the engine is off, the arm opens a small set of contacts at the end of the arc. This switch tells the computer that there is no airflow in the intake (ie a start-up condition). 

If you start the car and watch the arm movement, you will see that it's movement corresponds to the airflow in the intake. If you change the adjustment screw while at idle, you will notice the arm moves slightly as you change the screw. The screw allows some of the air passing through the sensor to bypass the measuring vane that is connected to the wiper arm. This will allow you to change the idle mixture. Make sure that you do not adjust the idle mixture such that the start-up contacts close during idle. 

The spring connencted to the arm can also be adjusted to vary the amount of back pressure it exerts. Changing this will change the mixture across the entire range of the sensor. Make a mark on the spring wheel before you start. If you let the spring unwind or change the tension too much you could make the car undrivable. The spring wheel has teeth on the outside like a gear and there is a metal prong that holds it in position. Hold the wheel firmly, pull up on the prong and rotate the wheel one notch at a time to adjust the pressure. I would not recommend going more than three steps in either direction. Adding spring pressure make the car run leaner, removing pressure makes the car run richer. 

For low emissions, run the car as lean as possible while maintining a smooth idle. If the car is misfiring due to running too lean, the HC output will go up as you dump unburned fuel from a misfire into the exhaust. 

Measuring idle mixture - The best way to tune a motor to pass emissions is to use a professional HC/CO meter. A decent used one runs 400-600 dollars. The cheap 100 dollar CO units most places sell are quite inaccurate. They are usefull, but you must let them stabilize for about 30 minutes and can only measure accurately for about 60-120 seconds before you have to recalibrate them. Their absolute accuracy, especially as you go under 1% sucks. They are better than nothing, but only for checking if you are in the ballpark. The best way of getting the mixture right without a meter is to use a very accurate RPM meter. Keep leaning out the mixture untill the idle drops about 50 RPM under the best idle you can normally get. 

I have also found that watching the movement of the motor while leaning out the mixture can be used to tell when you have gone too lean and are getting misfires. When there are no or few misfires, the engine will have very little side to side motion (side to side meaning intake to exhaust direction). Every misfires causes the motor to rock noticably front to back and there is a very obvious increase in this as the idle mixture leans out too much. 

If the car has high overlap cams it will be tough to accurately measure RPM or watch the rocking motion. Your best bet is to adjust the cams for less overlap (and smoother idle) or increase the idle speed as high as you can get and still stay in the allowed test range.

O2 sensor - The high RPM emmissions test will cause the car to go into closed loop mode and run off the O2 sensor. Since fuel delivery is adjusted by the O2 sensor any changes you have made to the fuel system should be compensated for automatically. Your cams can cause problems here if the overlap is exceptionally high. The only option is to change cam timing (see below). 

Injector size change - Many people change injectors to a larger size to richen the mixture. In closed loop mode the O2 sensor provides feedback and automatic compensation. In open loop mode the mixture is set off of a map in the computer depending on RPM, throttle postion and temperature. Although the bigger injectors may cause the car to run acceptably at wide open or partial throttle, most modifications make small or no changes to the idle airflow. This causes the idle mixture to be very rich. The most common problem with large injectors on a stock computer set-up is not being able to lean out the idle mixture enough.

As the air flow sensor air bypass screw is set to a lean position, the wiper arm may open the start-up switch causing the engine to run even richer. If this happens, increase the idle speed (which will move the wiper and open the switch) and keep leaning out the adjustment screw. If the screw is already all the way out or no longer making a difference, your only other option is to increase the wiper arm spring tension. 

Cam timing - Adding hotter cams (more lift and overlap) will cause HC and CO output to rise as the intake valves open earlier the exhausts close later. This causes some of the incoming unburned mixture to get out into the exhaust and also causes an irregular idle.

To make adjustable cam timing gears remove the cam gears and have a machine shop cut arc shaped slots where the cam locating pin sits. The gears are easy to remove (don't drop the timing belt down the cover !) and I would recommend not cutting more than 5 degrees in either direction as a few degrees of cam timing can make a big difference. Re-install the gears and position the pins in the middle of the slots. The cam end bolts provide enough friction against the cam gears so the locating pins do not have to be against either end of the slot. You can now also change the cam timing by just loosening the bolt and rotating the engine while watching the pin. 

This modification will allow you to adjust the the timing of each cam as well as their overlap. A very small difference in position can make a large difference in where the power band is. For emissions, start at the stock setting and slightly advance the exhaust. Moving the cams apart decreases overlap and makes the idle smoother and thus less HC and CO. Note that changing the exhaust cam timing changes your ignition timing since the exhaust cam drives the distributer ! You will need to adjust the ignition timing after moving the exhaust cam. 

If the idle is still not smooth, retard the intake cam some. You will notice that changes in intake cam timing make much more difference in power than the exhaust. Advancing the intake moves the powerband lower and retarding the intake moves it higher. More overlap makes more power, but the motor becomes more "peaky" and looses some torque in the low end and midrange. 

Throttle dashpot adjustment - The throttle dashpot is a small device by the throttle body that holds the throttle open just a bit while there is a high vacuum in the intake manifold (under deceleration). The purpose of this is to allow extra air into the motor and provide a nice smooth (and lean) transition from a high RPM back to idle. Any time you adjust the idle speed or any part of the throttle body you should check that the dashpot is adjusted within the specifications in the service manual. On early model MkI cars there is a hose that runs to the back of the dashpot. 

To test the dashpot open the throttle and then pinch the hose closed. Release the throttle and check that it does not close fully against the throttle stop untill you release the hose. On later model MkI cars there is no hose and you must remove the cap and filter on the back of the dashpot and then plug the vent hole with your finger.

The dashpot is adjusted by loosening the locknut on the body and then rotating the dashpot in or out. The car should run at about 1800 RPM when the dashpot is extended and drop back to idle in about 5 seconds after you release the dashpot pressure. If the dahspot is not functioning, you may fail CO or HC idle tests in areas that perform deceleration (high speed to idle transition while gas is being sampled) tests. 

A recent smog test for several MR2s. These cars were completely stock except for the HKS pully and had a new no-name brand catalyst installed on the SC car. 
 

California Emissions Testing

Test Parameters				IDLE HC ppm		IDLE CO %		2500rpm HC ppm		2500rpm CO %
Date	Car ID #	Year	Type	Measured	Allowed	Measured	Allowed	Measured	Allowed	Measured	Allowed
11/08/96	1	1988	5SP,49	28	120	0.00	1.00	18	140	0.00	1.00
10/21/96	2	1992	5SP,Turbo	6	120	0.00	1.00	4	140	0.00	1.00

Tricks for when you get stuck. 

So, you've done all of the above and the car still won't pass. What now ? 

Methanol and Ethanol both mix with gas and burn much cleaner. Some rubber components may be destroyed by them, so extended use is not recommended. Run the fuel tank down to almost empty. Add 1 or 2 gallons of Ethanol or Methanol. If the car idles or runs rough, add some gasoline untill the idle is smooth again. Have the car tested. After the test fill the tank with gasoline and drive it to remove as much of the alcohol as possible. Most large hardware stores will have Methanol or Denatured Ethanol. Do NOT use other types of alcohols such as Isopropyl (rubbing alcohol). 

The CO2 level required is usually a good bit lower than the engine produces, which means you can dilute the exhaust stream some with clean outside air. This involves adding a small exhaust port with a one way valve, air pump or compressed air bottle attached to it and leaking some clean air into the stream. Best bet is to add it after the catalyst, as the hot exhaust gas may have enough fuel in it to cause backfires when more O2 is added from the fresh air. 

Don't do this...
You can sometimes find someone who will produce a valid certificate for you for 100-200 dollars (what some people call a monetary waiver...). Don't expect any of the large chains to do something like this. If you follow the above tuning tips, you hopefully won't have to resort to one of these. 

We want INFORMATION !

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Smog test report

State:
County (if smog requirements differ by area): 
Smog tests required [Y or N]:
Physical inspection of smog devices [Y or N]:
Frequency of tests:

Year of vehicle:
MkI or MkII:
Milage:
Modifications (if any):

Tailpipe measurement standards
CO (Carbon Monoxide):
HC (Hydrocarbons):
CO2 (Carbon Dioxide):
O2 (Oxygen):
NOx (Oxides of Nitrogen):

Actual levels measured during test.
CO (Carbon Monoxide):
HC (Hydrocarbons):
CO2 (Carbon Dioxide):
O2 (Oxygen):
NOx (Oxides of Nitrogen):

Please state units of measurement if known (%, PPM, mg/l, etc).

Engine speeds tests are preformed at:
EGR functional test [Y/N]:
Ignition timing check [Y/N]:
Any other tests performed:
Test performed by state or independent facility:

Comments:

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