The EGR in a 1994 Honda Accord with a VTEC engine has two functions to cool cylinder head temperatures so the ignition timing can be advanced and to reduce oxides of nitrogen, or NO2, from forming. Oxides of nitrogen are a product of combustion produced under high temperatures in the range of 2,500 degrees or greater.
Method of Operation
The EGR picks up spent exhaust gas from the exhaust manifold and, with the help of the computer and its related components, introduces the gas into the intake manifold. The exhaust gas dilutes the fuel and air intake charge as its drawn into the cylinder, where it reduces the temperatures in the combustion chamber.
Timing of EGR Operation
The EGR never operates at or near an idle as it would cause a rough idle or stalling. Cylinder head temperatures are low at an idle and the ignition timing is retarded, as well. As the rpm rises above an idle to part throttle, the computer begins to meter the amount of exhaust gas into the cylinders as necessary to control the combustion temperatures. The higher the rpm, the higher the EGR flow. Under sudden increased load situations, the computer will shut off EGR flow momentarily.
Symptoms of a Blocked EGR
With no EGR flow, the cylinder head temperatures will rise dramatically due to the advanced nature of the ignition timing intended to work with proper EGR flow. Oxides of nitrogen will begin to form and enter the atmosphere. The increased temperatures will cause serious detonation within the cylinders that will be picked up by the knock sensor.
Computer Response to Low EGR Flow
The computer will receive the signal from the knock sensor and immediately retard the spark and increase the fuel flow to reduce the combustion chamber temperatures. This will stop the detonation with a tradeoff in fuel economy and power.
The engines power will be seriously compromised at high cruise rpm and fuel economy will drop due to the retarded spark timing. The computer will recognize this failure and set an EGR flow code and illuminate the check engine light on the dash.
