Emissions testing and diagnosis

From V6SHO
Jump to: navigation, search

Smog Tests measure the level of HC (Hydrocarbons), CO (Carbon Monoxide) and NOx (variations of Nitrous Oxide).


HC is fuel that went through the engine without being burned.

A list of common causes of HC related smog check failure:

1. Ignition Timing Advanced above specifications. Fixed on our cars, so not a problem, unless caused as a symptom of another problem that's throwing the timing off, such as a bad computer.

2. Ignition system problems. Parts like the ignition coil, ignition wires, spark plugs.

3. Vacuum leaks, Gasket leaks that cause vacuum leaks. Broken, disconnected or misrouted vacuum hoses. Vacuum component failure such as a power brake booster. This causes a very large vacuum leak.

4. Catalytic converter failure.

5. Engine damage (burned valve, low or no compression in one or more cylinders). The cylinder compression would have to be so low that the fuel won't ignite.

7. Sensor problems and or computer problems.


CO is created when gasoline is not completely burned. High CO (running rich ).

A list of common causes of CO related smog check failure:

1. A dirty air filter can cause high CO.

2. A common cause for high CO is a faulty Oxygen Sensor. This sensor tells the computer how to fine tune the air fuel ratio. Old sensors run rich.

3. Other sensors like the MAP and MAF sensors can also effect the CO level.

4. A faulty Catalytic Converter can cause high CO and Smog Check failure.

5. If your car has been running rich (high CO) you should change your oil after you get it repaired. When you have rich condition all the fuel cannot be burned and you start to saturate the motor oil with CO and HC. This in itself can also cause a rich condition.


NOx is formed inside the combustion chamber when excessive heat is present, usually because the engine is running lean.

A list of common causes of NOx related smog check failure:

1. Check the EGR system. This system is designed to reduce NOx. It consists of an EGR valve, PFE Sensor, EVR Solenoid and vacuum hoses. Its job is to reroute a small amount of exhaust gas back into the engine to help reduce combustion chamber temperature. Not all vehicles have an EGR system. If yours is one of those, pull the Shorting Plug (near the EEC Self Test Port) to reduce NOx significantly (factor of ten in my case).

2. Next thing to check is the air / fuel ratio. If the vehicle is running to lean, NOx emissions will increase.

4. Some possibilities are a restricted fuel filter, low fuel pressure, vacuum leaks, oxygen sensor, load sensor such as a MAP sensor, MAF.

5. Check the cooling system. An extra increase in water temperature will increase NOx production.

6. A defective catalytic converter can also increase NOx. The Cat reduces NOx that has already been produced.

7. I have a client who halved his NOx by cleaning all the crud out of the intake with Seafoam. The carbon buildup in the engine was raising compression and absorbing the fuel mixture so combustion was running lean.


Nick Chrimes Owner, BaySHO Performance 1680 Almaden Expressway, Suite G San Jose, CA 95125 W (408) 295-4778


SPOUT = SPark OUTput; it's the signal that EEC sends to the ignition module to control ignition event timing and dwell. If SPOUT is disconnected from the DIS module, ignition timing reverts to base timing of 10º BTDC, which is determined by the crank sensor signal to DIS. The engine will run with fixed 10º timing, and NOX emissions should be reduced, and the engine will feel awfully flat. The SPOUT shorting bar is near the CID sensor. The other plug some have referred to is the ignition timing shorting bar. This is a hold-over from EEC's early days as a sort of Hail Mary to pull back everything in the timing tables by 2 or 3º. The idea being that if you got a load of crap gas, you could pull the timing jumper and back off the timing for awhile. On the SHO, the timing shorting bar is located in the harness by the driver's side radiator end tank. Gary Morrell Colorado Springs, CO