Traditional Tips for Passing Emissions Tests

By David Wells, Larry Wells, & Glenn Harnden

Introduction

Many states require an annual or bi-annual inspection for emissions. If your state is one of them, then you must get your Capri to pass the inspection if you want to drive it on the street.

This is somewhat harder on a Capri than it is for other cars. This is mostly due to their age. Cars built before 1968 are usually exempt from emissions tests. European Capris were imported between 1970 and 1977, when emissions technology was still primative. Unfortunately for us, emissions standards became more strict almost every year during the 1970s, and many states require us to still meet these standards.

Your authors are somewhat less knowledgable about "Fox" Capris and Australian convertibles, so we will not specifically discuss them here, however most of the basic principles apply.

We therefore present some traditional tricks which should be able to help your Capri pass emissions.

We should note that these tricks are primarily intended for stock or near-stock engines. If you have raised your compression ratio, and/or are using a long duration camshaft, we can't promise that these tips will help you.

Types of Pollutants

Usually, emissions tests look for three types of pollutants:

Hydrocarbons (HC) This is essentially unburned fuel.
Carbon Monoxide (CO)
Oxides of Nitrogen (NOx) Oxides of Nitrogen are a byproduct of any combustion in our oxygen/nitrogen atmosphere. High compression engines typically have difficulty achieving low NOx numbers.

Four Relatively Easy Steps

Retard the spark timing a bit. From a performance standpoint, Capris respond well to advanced spark timing. This is especially true of those with the 2000 OHC engine. Unfortunately, this is not good for either HC or NOx emissions. Retarding the spark timing up to 4 degrees will usually help both. Keep in mind that this trick may hurt CO emissions.
If you have a dual diaphragm distributor, the timing should be set with all vacuum lines to the distributor removed and the engine running probably no higher than 600 rpm to make sure you don't get into the centrifugal advance. Make sure you plug both vacuum lines removed from a dual diaphragm distributor (since one of them goes to the intake manifold.)
Lean out the carburetor Most carburetors have one or more screws to adjust the mixture of fuel and air. A leaner mixture (i.e. less fuel and more air) will usually produce fewer pollutants.
- Stock Capris with 2.0 OHC 4-cylinders, 2.3 "Lima" 4-cylinders, 2.6 liter V-6s and 1974 2.8 liter V-6s have some variant of the Weber 32/36, (or the Holley Motorcraft 5200 license-built copy) . The mixture is adjusted with a screw on the side of the carb.
- 1976 and 1977 Capri IIs with 2.8 liter V-6 engines have a Motorcraft 2150 carburetor. The mixture is adjusted with the two screws in the front of the carb base. Each screw adjusts one barrel of the carb. Make sure that you turn the screws equal amounts.
Set the idle mixture with engine fully warmed up. When adjusting for "best idle" (see below) turn the screw(s) in very small increments (about 1/16 of a turn at a time), and allow about 15 sec for each change to affect idle speed, on either carb.
If you have an idle limiter cap on the idle mixture screw make sure it is set to max lean.
If you don't have a limiter, set the carb to nominal curb idle (Ford spec), adjust the mixture for best idle (usually the highest rpm), if "best idle" increases rpm more than 50 rpm above previous setting, reset rpm to nominal again and repeat until "best idle" is within 50 rpm of nominal, then lean the mixture (turn screw(s) clockwise) until the idle speed drops 50-100 rpm. (Important: This idle speed is NOT the "raised" idle speed from #3 below, it is the idle speed specified by Ford.)
Raise the idle speed slightly. If you know what the allowable (by the state) upper limit is for your engine shoot for about 50 rpm below that with engine fully warmed up and distributor adjusted properly.
Change the oil. We're not 100 percent sure why this works, but it does. Some have speculated that with cleaner oil, the oil that blows by the piston rings will burn more cleanly. Dave Wells' current mechanic claims it was an old trick of his, however Dave had discovered it himself some years earlier, somewhat accidentally, with his 1973 2000 OHC. He failed New Jersey's inspection, changed his oil, and then passed! (go figure!) Changing the oil seems to benefit hydrocarbon emissions the most.

Should These Methods Fail.....

Adjust the Valves. If you have already failed the smog test because of high HC, try lashing all your valves .002"-.004" looser, e.g., if the shop manual calls for .014", use a .016" or .018" feeler gage. This will, in effect, slightly reduce valve overlap. Reduced overlap reduces exhaust residuals in the combustion chamber, which results in a more complete burn, and hence, lower HC emissions.

In the unlikely event that you have failed because of high NOx, try lashing all your valves .001"-.002" tighter. In the example above, use a .013" or .012" feeler gage instead of .014". This will, in effect, slightly increase valve overlap. Increased overlap increases exhaust residuals in the combustion chamber. (internal EGR) These residuals dilute the air/fuel mixture which slows down the burn and lowers combustion temperatures, and hence, lower NOx emissions. Tighter lash will also slightly reduce effective compression and combustion temperature.

In case you are wondering, changing valve lash ALWAYS reduces one exhaust constituent (HC or NOx) and ALWAYS increases the other.

IT IS NOT RECOMMENDED THAT THE LASH BE REDUCED BY MORE THAN .002".

CAUTION! Tight valve lash reduces the time the valves have to transfer heat to the cylinder head and can result in exhaust valve burning under extreme conditions, therefore, THIS METHOD SHOULD NOT BE USED IF YOU ARE PRONE TO FLOGGING YOUR ENGINE MERCILESSLY.

Other Tricks

Change the Spark Plugs Obviously, the fuel/air mixture will not burn properly if your spark plugs are fouled. New spark plugs will burn more cleanly than fouled ones, and new spark plugs are cheap, however, if your spark plugs are fouled, you might have other more significant problems.
Fuel Additives Some people have advocated adding things to your fuel, such as denatured alcohol, which will increase the oxygen content of the fuel. We have never tried this trick, and we do not advocate it except as a last resort. Avoid using alcohol in your fuel if possible.
- Alcohols are notoriously incompatible with engine oil.
- Alcohols also have been known to attack rubber hoses and gaskets rather severely in these older cars.
- Alcohols are also hygroscopic, so over the long term they will promote corrosion. One tankfull of gasohol may not to do any harm, so long as it's burned off as quickly as possible. Filling the tank with gasohol before storing the car for the winter would be a bad plan, though!
- Alcohols have been known to stir up the crud in the bottom of your gas tank.

Further Assistance

The authors regularly monitor the Capri-List mailing list, so join us, and contact us there. Putting "Emission Question" in the subject line will help us spot your question.