We recommend replacing the EGR valve. It is not an expensive part and sounds like your Toyota Camry's might be plugged up with carbon. Being plugged up can cause the gasket to fail, thereby blowing exhaust out the gasket leak.
Ensure to replace the EGR vacuum modulator as well. The way to test whether the EGR valve is defective is by starting your Toyota Camry and letting it run at idle. Disconnect the EGR valve's vacuum line and apply direct vacuum using a vacuum hand pump to open the EGR valve manually. When (and if) the valve opens; if the passages are clean, you should notice a large RPM decrease. If the passages are plugged up, you will not see any RPM difference.
The sensor which you knocked off of the firewall sounds like it may have been the EGR signal sensor. Disconnect the vacuum line leading to it and check for obstructions both in the sensor and in the vacuum line. If clear, simply put back in place.
The MAF (Mass Air Flow) Sensor is not near the firewall. It is part of the intake duct and filter housing, as is the IAT (Intake Air Temperature). These sensors are not related to the EGR system.
It is very common to develop EGR problems on older model vehicles, especially the older Toyota Camry. The EGR valve is constantly subjected to high temperatures and exhaust gases directly from the exhaust manifold. Its job is to move exhaust gases from the exhaust manifold back into the combustion chamber through the intake manifold. This allows for a cooler combustion and NO (NOx) reduction.
Carbon build-up is certain. Removing carbon from the valve is possible however considering the cost of a new valve, not cost/time efficient. The EGR valve modulator (looks similar to the EGR valve and located next to the valve, but does not have any passage to intake manifold) can not be cleaned and must be replaced. The modulator simply senses exhaust back-pressure, and when adequate allows the EGR valve to open. The EGR valve modulator is not responsible for delivering exhaust gas to the intake it manifold, it only tells the EGR valve when and if it should open. The EGR valve sensor tells the computer when the valve is open or closed. The EGR signal sensor is responsible for throwing error codes if the computer expects the valve to be open at any given time and the EGR valve is not.
