Tech Tip 3
This month’s tip is a study of EGR function and the diagnosis of an EGR flow code. The particular vehicle was a 1998 Mazda Millenia, but this tip applies to many other makes and models as well. (some of the pictures shown where tests conducted on other cars as well)
Okay, vehicle shows up at your shop with the "Check Engine Light" on and code P0400, EGR Flow Malfunction.
According to the shop manual DTC P0400 will set when the difference in intake manifold pressure is not within preset value during EGR operation.
Possible causes of the problem are any components in the EGR system, MAP system or the following: MAF sensor malfunction, TP sensor malfunction or IAT sensor malfunction.
Well, what is the first test we all use to see if the EGR is functioning or not? Sure, you reach up under the valve with the engine running and pull up on it, right. When the engine stumbles you know the EGR is working. WRONG! That old standby test isn't going to work on OBDII. Yep, the engine will stumble but that won't tell you what the PCM wants to know. The PCM wants to know how much EGR actually flowed when the EGR was applied. It knows this by monitoring engine vacuum (on this system, other systems use a sensor in the exhaust to monitor changes exhaust pressure).
So the computer is looking at intake manifold pressure to determine whether the EGR valve opened far enough of flowed enough.
Okay, we can also look at intake manifold pressure to see if the EGR is flowing enough. All we have to do is hook into a good manifold vacuum source and activate the EGR and see what kind of drop in vacuum we can achieve with full EGR operation. A good rule of thumb, as was discovered though the hard work of some dedicated professionals, is 6-8 inches of vacuum drop during 2500 RPM no load testing.
So lets put this to the test!
First we hook a vacuum gauge or transducer up to a good vacuum source on the manifold. Next we note the reading at 2500 RPM with no load on the engine. (I recommend using some sort of throttle depressor/holder for accurate testing)
Fig. 1
Note we have about 22 inches of vacuum at 2500 RPM with no load.
Okay now lets apply full vacuum to the EGR Valve, or activate the EGR fully electrically and watch the engine vacuum.
Fig. 2
Looks like we are getting a drop to about 19 inches of vacuum.
Lets see, had 22 inches drops to 19 inches, that's only a drop of 3 inches.
Definitely not the 6-8 inches of vacuum we want to see.
So lets go and find out why the EGR is not flowing as much as it should. On most systems, as well as with this one, the EGR passages are in the intake manifold and are fairly small and prone to plugging up with carbon. In Figure 3 we have a picture of the EGR passages on the Mazda. These are located behind the throttle plate and and require the removal of the throttle body to access for cleaning. Before cleaning you could hardly see that these ports were there. They just looked like bumps in the casting of the manifold.
Fig. 3
EGR ports located behind throttle plate.
So lets go take a look at the vacuum gauge now that we have this all put back together and nice and clean. Again we want to make sure that we are at 2500 RPM and no load.
Fig. 4
Hey look at that, down to about 16 inches of vacuum. that is a 6 inch drop from our base reading of 22 inches.
That falls within our 6-8 inch drop guidelines. I have done this test on several vehicles, with EGR codes and without EGR codes,
and this seems to be a very accurate way of diagnosing an EGR flow malfunction code.
So that is it for this month. The key to successful diagnostics is knowing the system you are working on.
If you have any questions about this month's tip please feel free to contact us.