The 'defiant' car I will be picking on in this article is a 1999 Mazda Protege. This car had set a Code PO402 for excessive EGR flow, when in fact the problem was low flow.”
Since the inception of onboard diagnostics II (OBD-II), I have dealt with a large number of EGR flow codes on Asian vehicles. The code definitions seemed to make sense - until recently. If the code said "low flow," then there was low flow. Of course, there always has to be that one new system that comes along and defies logic.
The "defiant" car I will be picking on in this article is a 1999 Mazda ProtegÈ. This car had set a Code P0402 for excessive EGR flow, when in fact the problem was low flow. I will explain later in the article.
Let's start at the beginning. Mazda's 1995-1998 models regularly set P0400 EGR flow codes on the Protege, 626 and Millenia. On these early OBD-II cars, Mazda monitored EGR flow with an EGR boost sensor. Usually a boost sensor was found on a turbocharged engine. If you have ever driven a 1.5 Litre Protege, you know it is not turbocharged.
Mazda's "boost sensor" is just a fancy name for a manifold air pressure (MAP) sensor. In conjunction with the boost sensor, Mazda uses a boost sensor solenoid to turn manifold vacuum on and off to the boost sensor, which allows it to be used either as a MAP sensor or a BARO sensor. The boost sensor has no vacuum signal to it the majority of the time and is used for BARO input. When the engine control module (ECM) decides to test for EGR flow, it energizes the boost sensor solenoid. This sends a manifold vacuum signal to the boost sensor and now the ECM can use it to monitor changes in manifold vacuum. The ECM rapidly opens and closes the EGR valve during a flow test and looks for changes in manifold vacuum. If it doesn't like what it sees, it sets a Code P0400.
An EGR flow test is usually run during closed throttle deceleration when manifold vacuum is high and steady. The common cause of P0400 codes on these early models is a plugged port in the intake plenum, which restricts the flow of exhaust gases and doesn't create the correct changes in manifold vacuum during a flow test. To fix, remove the throttle body and clean the ports in the intake plenum. There are two ports behind the throttle body where the EGR gases are dumped into the intake. One thing that can fool you during testing is that you can kill the engine at idle if you open the EGR valve. On most engines, this means that you have sufficient flow. But on these engines, one port can become plugged and the other can flow enough that it will kill the engine at idle when you open the valve. So don't use that as a test for good EGR flow.
Back to the 1999 ProtegÈ and Code P0402 for excessive EGR flow ... "excessive" and "EGR flow" just don't seem to go together. Did Mazda come up with a self-cleaning EGR system? EGR ports get plugged and restrict flow, right? Even though the code says excessive flow, it's still the same problem.
Mazda changed the way it monitors EGR flow, starting with the 1999 models. There is still an EGR boost sensor and a boost sensor solenoid - the difference is in the pressure that the boost sensor is actually monitoring. Instead of the boost sensor monitoring intake manifold vacuum directly, this system measures the pressure in the port between the EGR valve and the intake manifold.
If the ECM doesn't like the pressure in that port, it will set a code. On these systems there is a port coming off the bottom of the EGR valve where the boost sensor hose gets its signal. Due to the location of the sensing port, when EGR ports become plugged, the ECM detects too much pressure in the sensing port and it thinks that there is excessive EGR flow. This sets a Code P0402. In this case, the fix is the same for excessive flow as it was for low flow on the early models: clean the ports in the intake plenium behind the throttle body.
This is the jist of a P0400 code.