HC  ( Hydrocarbons )

 

 Measured in parts per million (ppm) of the concentration in the exhaust gas. Usually associated with the left over Unburned Fuel during combustion.

 

Represents the amount of unburned fuel, due to incomplete combustion, exiting through the exhaust. This is a necessary evil. If it's too low, lean misfires will occur. We don't want it in the atmosphere, so try to keep it as low as possible.

An approximate relationship between the percentage of wasted fuel through incomplete combustion and the ppm of HC is about 1/200 ( 1.0% partially burned fuel produces 200 ppm HC, 10%=2000 ppm HC, 0.1%=20 ppm HC )

 

High HC readings usually indicate excessive unburned fuel caused by a lack of ignition or by incomplete combustion. Concentrations are measured in parts per million (PPM). Common causes include a faulty ignition system, vacuum leaks, and fuel mixture problems. As with any quick reference, there are other less likely causes that may not be included in the list.

Here are some of the more common causes:

            o Ignition system failures

                        -incomplete combustion due to fouled spark plugs

                        -improper timing or dwell

                        -damaged ignition wires

                        -faulty ignition secondary component

                        -faulty individual primary circuit on distributorless ignition system

                        -weak coil output due to coil or primary circuit problem

            o Excessively lean air/fuel mixture

                        -Vacuum leaks

                        - leaky intake manifold gasket

                        - worn throttle shaft

            o Excessive EGR dilution

                        - EGR valve stuck open or excessive EGR flow rate

                        - EGR modulator bleed restricted

            o Restricted or plugged fuel injector(s)

            o Closed loop control system incorrectly shifted lean

            o False input signal to ECM

                        -incorrect indication of load, coolant temp., O2 content, or throttle position

            o Exhaust leakage past exhaust valve(s)

                        - tight valve clearances

                        - burned valve or seat

            o Incorrect spark timing

                        - incorrect initial timing

                        - false input signal to ECM

                        - worn piston rings or cylinder walls

            o Insufficient cylinder compression

            o Carbon deposits on intake valves

 

 

Catalytic converter intervention and HC concentrations:

High HC readings at the tailpipe are an clear indication that there is a problem in at least one part of the system, but an HC reading that appears within "normal" ranges or is only modestly elevated is not necessarily a reliable indicator of proper or even acceptable system performance. HC readings at or near "normal" are possible, and not uncommon for a malfunctioning engine equipped with a properly functioning catalytic converter. In such circumstances, truly elevated pre-catalytic converter HC levels will be masked by the catalytic converter and the potential for an HC problem must be further evaluated in the context of other readings of abnormal gas concentrations and AFR / Lambda readings.

 

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