emission understanding

[shogun]

helps to diagnose faults
http://www.autoshop101.com/forms/h56.pdf

[M-what]

helps to diagnose faults
http://www.autoshop101.com/forms/h56.pdf

That is a great deal of useful information. However I am searching for little more info on how emission at idle and emission at speed varies.

My 90 535im has been failing the emission for last 2 times due to high HC during idle. At speed, everything is fine and it passes. I did a full tune up, adjusted the valves, cleaned the ICV and TB, and it helped alot, but still was too high. The HC reading came down from 900pm down to 380pm. It needs to be down below 200pm to pass.
Before I start replacing expensive parts (injectors, oxygen sensor, etc) any insight will be appreciated.

[shogun]

Here is some info from Bosch.

5 Gas Analysis

Bosch Technologies Group

2003
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)