How To Get A Car To Pass Emission Testing

A frequent cause of heartache for technicians, and car owners alike, is the failed E-test. Truth be told, just about any properly running vehicle will pass emission testing. This post will not be a laundry list of cheats. Rather, this post will be a comprehensive list of repair information, to aid in actually repairing the vehicle so it is in accordance with emission standards.

This post is meant to serve as a guide for all. If you are a technician who struggles with E-test failures, or a car owner looking to try to repair the car yourself, or just a car owner who wants to double check a shop’s diagnosis, this post should be helpful, I hope.

First and foremost, make sure your catalytic converter (cat) is hot before you go to emission testing. A cat is just a honeycomb of metal that becomes so hot it burns any left over fuel the motor was unable to burn. In order to incinerate the left-overs, the cat must become extremely hot.

The best way to ensure your cat is hot, is to cruise your car down the highway for 15 to 20 minutes before you go to test the vehicle. The exhaust temperature is the hottest during light throttle cruise. Running the car down the highway will heat soak the cat, to ensure it will “lite off” during testing.

Here are a few emission rules of thumb:

Nox is a result of high combustion chamber temperatures. Common causes are: a lean condition, a vacuum leak, too much ignition timing, improper crankshaft/ camshaft timing, a faulty cooling system, a failed O2 sensor, a failed MAF (mass air flow) sensor, a failed coolant temperature sensor, a high compression motor, a failed EGR (exhaust gas recirculation) system, a failed catalytic converter or a failed secondary air system.

HC is from unburned or partially burnt fuel, and is generally a sign of misfire. Can also be that the engine is just way too rich. Common causes of HC are: an old or faulty ignition system, a rich misfire, a lean misfire, a failed O2 sensor, a failed MAF sensor, a failed coolant temperature sensor, improper crankshaft/ camshaft timing, Improper ignition timing, a failed catalytic converter, or a failed secondary air system. A fuel leak near the tail pipe can also cause HC failure. The machines that suck the exhaust in and test it will also suck in the raw fuel and cause a HC failure.

CO occurs when a car is too rich. Common causes include a failed O2 sensor, a failed MAF sensor, a failed coolant temperature sensor, a fuel pressure regulator leaking fuel into the vacuum port, too little ignition timing, a failed catalytic converter, or a failed secondary air system.

You’ve probably noticed a pattern here. Honestly, there are a handful of systems which keep the car running properly, and the emissions low. As long as all the systems are working properly, you should be fine, the trick is determining which systems have failed.

You have to look at all three to get an idea of what is going on. For example, if NOX and HC are both high, CO are low, and the idle is too high, I’d check for a vacuum leak causing lean misfire. To really hone in on the problem, you will also need a proper scan tool to watch data streams, and a 5 gas analyzer.
automotive repair scan tool
This is a relatively inexpensive scan tool made by autolink. It pulls codes as well as reads and displays your basic drive-ability data. I would recommend this to anyone who wants to repair any modern car (1996 or newer). I have one, and i use it all day long.

5 Gas Analyzer Specifications at 2000 RPM extended hold, in neutral or park.
HC = 100 PPM or less
CO = Less than 0.3 %
CO2 = Over 13.5 %
O2 = Less than 1 %
NOX = 100 – 700
Lambda, will vary, depending on if you are under load.

Check your easy stuff first. Plug in a scan tool and run through your basic data, engine coolant temperature, MAF grams per second, baro pressure, intake air temperature, etc. and make sure nothing is out to lunch. For example, is the coolant temperature stuck at 0 degrees Celsius, causing the vehicle to permanently stay in cold start enrichment?

Make sure your vehicle is in fuel control. Modern cars have two fuel control modes, closed loop and open loop. When the car is in open loop, it is simply running a pre-made map, and makes little to no changes to the amount of fuel it delivers. When a vehicle is in closed loop, it is closely monitoring the engine and adjusting fuel and timing to create the proper air fuel ratio, and a good clean burn.

Watch your front oxygen sensor/s. The sensors should switch back and forth between high and low values. Many oxygen sensors are one volt systems, and will therefore switch between .02 and .98 volts. Other systems are five volt, some are 1.5 volt. The main thing to look for is rapid switching (every second or so) between low and high values. If the oxygen sensor is switching, then the ECU (engine control unit) is adjusting fuel. If an oxygen sensor is switching slowly, or sticking at a value, then there is a gross failure in the fuel control system (MAF, engine coolant temperature sensor, Oxygen sensor, etc.)

If the car is equipped with a MAF, check to make sure it is clean. A dirty MAF can cause a lean condition. If you have an old style MAF, such as this
mass airflow sensor
you can actually open them up and tune them. They are simply a flapper door that swings open as the engine draws more air in. The main downfall of these early MAF’s is that the spring wears out. As the spring wears out, the door will open further than it should, and the car will run rich. Watch your five gas analyzer and adjust it one click at a time. If you don’t have a five gas, you can always adjust a few clicks and then run it back through emission testing, it’s just far more time consuming. Generally speaking, you will need to tighten the spring two or three clicks per 100,000 miles.

If you have a distributor on the car, set your timing. Ignition timing that is too far advanced will result in high NOX. Ignition timing that is too far retarded may result in high HC or CO.

While you are at it, also inspect your spark plugs to ensure they are new. Always, make sure you have the proper spark plug in your vehicle. If you are unsure exactly what plugs should be in your car, email scotty@clvr.tv, and Scotty will tell you the exact plugs you should be using.

You can sometimes find a faulty ignition system by soaking the engine with water. The water will make it easier for the ignition system to arc to ground rather than going through the spark plug. If your car has coil on plug ignition, you can stress test the coils by making them jump a gap before the spark plug.

Ingnition TesterThis is a tester you can put in line in your ignition system. By making the spark jump a gap, you can stress the ignition system and find a failing system. Very handy.

A good old fashioned top end de-carb is always a good idea before emission testing a vehicle. If you can get a full injector purge, that is best. Simply adding BG’s 44k to the gas tank and sucking a bottle of de-carb through a vacuum line also works well. The carbon in a motor becomes hotter than the normal parts of the motor, which can lead to detonation, misfire, or high NOX due to elevated combustion chamber temperatures.

You can also test your cat to verify it is working properly. First, you can install a back pressure tester in a pre-cat oxygen sensor bung. Rev the motor while in park or neutral and ensure the back pressure is low enough. If the back pressure is too high, you car is plugged, and requires replacement. You can also watch the front and rear O2 sensors to get an idea if the cat is working.

Further more, watch your five gas analyzer to verify the cat is working. Watch the five gas as the car warms up. Once the cold start enrichment has leveled off, drive the vehicle and watch the emissions. Before the cat is hot enough to light off, you will see all the emissions the car produces. At a certain point, if the cat is working, you will see the emissions drop as soon as the cat lights off.

Check your EGR system (if equipped), which recirculates exhaust gas to lower combustion chamber temperatures, and mainly lower NOX. Most EGR valves are opened by engine vacuum and a solenoid. With the engine running, apply vacuum to the EGR valve. If the valve works and the ports are clear, the engine should stumble and die. If the engine remains running, you have a faulty EGR system. If the Valve holds vacuum, but the engine doesn’t die, the ports are most likely clogged. If the EGR valve doesn’t hold vacuum, the valve is most likely bad. Certainly, the valve can be bad AND you have clogged ports.

Similarly, check the vacuum supply going into the EGR solenoid. if the solenoid has proper engine vacuum going into it, check the power and ground at the solenoid. Also, supply power and ground to the solenoid with the engine running. If the solenoid works, it will open and send vacuum over to the EGR valve.

Also check your secondary air system, if equipped. Secondary air system pumps air into pre-cat exhaust to help the cat light off. If your secondary air pump is electric, power it up and check to make sure it flows air. If your air pump is mechanical, let the engine idle and check to see if the air pump works. Mechanical air pumps often also utilize a solenoid which opens when the secondary air is required. Power up this solenoid and ensure it is working.

Check your cooling system. A faulty cooling system can cause pre-detonation, and high nox. Let your car cool down, and make sure the coolant is full, and as recommended here replace your radiator cap every spring. Make sure your cooling fans come on. If you have access to a scan tool, watch your coolant temperature as you drive around, surface streets, traffic as well as on the highway.

As a last ditch effort, you can also check and ensure the crankshaft/ camshaft timing is correct. I have heard rumor of a new timing belt bringing a NOX failure back in line, once, but that would be fairly rare.

That is about it. If there is anything else you need help with regarding E-test failures, email Scotty@clvr.tv and Scotty will help in any way possible.

See also: Check engine light causes.

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