The EGR system is activated when the EGR actuator diaphragm is energized with air from the EGR energizer solenoid as shown below. The EGR system is typically activated under light throttle operating conditions and deactivated when the engine is placed under load.

When the EGR system is activated, exhaust gasses are diverted into the intake charge and consequently the EGR assembly and intake manifold is subject to high temperature (from the hot exhaust gas). In addition, carbon particles that would normally exit through the exhaust system now pass through the EGR system and into the intake manifold.

Unlike some other brands of diesel vehicles that employ EGR, the Toyota implementation in the 1HD-FTE is very robust. That said, there are situations where the EGR control system can malfunction.

The EGR system is deactivated when the vent solenoid is opened to release pressure from the EGR actuator. The EGR vent solenoid is a robust valve, however there are occasions when the vent cap can become blocked with dust. When this occurs, the EGR system does not deactivate properly and exhaust gas continues to be recirculated when the engine is under load.

Active EGR when the engine is under load creates a problem with a high level of particulate emissions through the exhaust system. There will be visible signs of black exhaust emissions when under load - but this may not be noticeable by the driver.

Continued operation when the EGR is seized open will eventually block the catalytic converter resulting in very high exhaust pressure between the exhaust manifold and catalytic converter. At first, this may not be noticed by the driver because of the lack of any significant symptoms - other than lack lustre performance. Eventually, the catalytic converter will become so blocked that the exhaust gas pressure will force the spring loaded coupling to the turbocharger. When this happens, rest assured that the driver will hear it. Look for evidence of soot deposits around the exhaust coupling and bell housing.

If caught early, the fix is relatively easy - simply clean the vent cap.

If longer, then removal of the EGR system will be required in order to free the carbon seized EGR mechanism.

It may also be necessary to remove and clean the intake manifold however the jury is still out whether or not damage can be caused to other engine components when the carbon build-up inside the manifold begins to drop off and travel through the engine's combustion chambers.

If the catalytic converter is blocked, then all of the above must be performed in addition to cleaning/replacement of the catalytic converter. Ideally this should be performed by a trained and qualified automotive engineer.

To keep this problem from occurring, the vent cap and hose to the EGR actuator should be removed and cleaned regularly. Unfortunately, when the vehicle frequently operates in dusty/muddy conditions, this procedure may have to be undertaken regularly.

Some vehicle owners who have had this occur a number of times have taken further steps and modified the EGR system to keep this problem from occurring again.

One way to stop the valve from opening is to block the actuation circuit from being activated by the EGR activation solenoid. This typically entails inserting an object into the activation hose etc to create an air tight seal - somewhere between the tee piece to the vent solenoid and the activation solenoid.

This should never be performed on any vehicle that will travel in areas where a fully functioning EGR system is required by the relevant road traffic authority.