gas analysis - failing inspection?

Here is some info from Bosch.

Gas Analysis

Bosch Technologies Group


Three Reasons for Gas Analysis

* Identify engine performance and mechanical problems
* Test the running efficiency of the engine
* Test for exhaust emissions against state and federal standards

Combustion Chemistry and Gas Analysis.

* The combustion process releases the energy stored in the fuel
* Fuels consist basically of carbon(C) and hydrogen(H) referred to as hydrocarbons (HC).
* Under ideal conditions all of the HCs would be converted to H2O, CO2 and heat.



Combustion Chemistry and Gas Analysis.

* Unfortunately this complete combustion is rarely if ever achieved
* Short burn times, continuously changing mixture ratios and air temperatures, and combustion chamber quenching produce conditions which give rise to the formation of pollutants

Pollutants

* Incomplete combustion produces carbon monoxide (CO)
* Unburned fuel produces hydrocarbons (HC)
* High combustion temperatures produce oxides of nitrogen (NOX)

Other Exhaust Gases

* Carbon dioxide (CO2) is a product of complete combustion. Therefore the higher the level of CO2 the more efficient the engine is running.
* Oxygen (O2) level in the exhaust indicates the fuel-air mixture.

High O2=lean

Low O2=Rich

Gas Analysis as a Diagnostic Tool



Gas analysis

* Is a quick and accurate way to determine the running conditions of an engine
* By observing the 5 gas readings and understanding their relationship to each other you can diagnose the 3 major engine areas:

Fuel delivery

Ignition

Engine Mechanics


High HC Only


Hydrocarbons are unburned fuel. This can be caused by:

Ignition Timing (base and advance)

Vacuum Leaks (hoses, brake booster, manifold, evap. etc.)

Ignition (misfires, coil breakdown, etc.)

Mechanical (low compression, bad cam, etc.)

Catalytic Converter Failures


High CO Only


* Carbon Monoxide is a result of incorrect fuel/air mixtures. Causes include:
* For Carburetors (float level, power circuit, choke circuit)
* For Injection (injectors, pressure regulator, temp sensor, MAP/MAF sensor)
* For Both Systems (air filter, PCV, Evap, O2 sensor, air injection



High Nox Only


* NOX is caused by high combustion chamber temperatures. Check:
* EGR (valve, controls, solenoids, passages, vacuum hoses)
* Ignition Timing ( base and advance)
* Engine Temperature (cooling system, fan restricted exhaust, Thermostat)
* Vacuum Leaks (hoses, booster, evap, etc)
* Mechanical (carbon deposits, converter, etc)

High CO and HC


* Normally, when HC and CO are high it is the result of a CO failure driving the HC high.
* Correct for high CO first (Excessive fuel, restricted air, Evap., etc.)
* If HC is still high, test as high HC (ignition, timing, mechanical, etc.)



High CO and Nox

* These problems are normally incompatible. If they occur on the same test they will most likely occur at different points on the drive cycle
* Correct CO failure first
* Retest and locate NOX failure in drive cycle and correct as NOX failure only



HC and NoX

* High HC and NOX normally occur as the result of a lean misfire. Check for:
* Lean run (vacuum leaks, mixture problems, fuel delivery, etc.)
* Mechanical (oil burning can cause high combustion chamber temperatures)



CO, HC and NOX

* This is a complete breakdown of the emission systems or multiple failures. Check:
* Catalytic Converter (missing, damaged)
* PCM (closed loop, wiring, etc.)
* Drive cycle ( if failures occur at different points diagnose each failure separately)