Diagnosing Insufficient EGR Flow

1999 Mercury Cougar 2.5L V6
OBD-II trouble code P0401

The check engine light came on and gave me code P0401 for insufficient EGR flow. I should have gone straight into diagnostics mode instead of shop-and-swap. I replaced the DPFE sensor because I had one fail on my old 1995 Windstar and figured this one had failed too. I also replaced the EGR valve because that's the first thing back-yard mechanics replace when there's an EGR code. I knew better, but was lazy. Both those parts still had some life left in them. Oh well. With my luck, they probably would have failed eventually. But that didn't take care of the trouble code. So I needed to get to work and properly diagnose the problem.

I connected my OBD-II scanner and set it to capture the DPFE sensor voltage continuously. The flow sensor was reading about 1 volt for no flow. So far, so good.

Next, I needed to apply vacuum to the EGR valve's vacuum port. All you need to fully open the valve is 5 inches Hg. I revved the engine up and the voltage didn't go over 1.5 V. Holding the EGR valve fully open at 2000 RPM, it only read 1.2 V. Fully closed is 1 V; full EGR flow is 5 V. I wasn't getting much flow.

I needed to narrow down where the restriction was. So I removed the EGR valve, plugged it's intake port and ran the engine. Exhaust came out of the tube, as expected. Revving the engine got the DPFE sensor to read up to 5 V at wide open throttle. Bonus! Max EGR flow reads 5 V. The new DPFE sensor was working as it should.

Next, I needed to see if the EGR valve itself was much of a restriction. This step was just to exhaustively diagnose the flow restriction for each part. I attached the old EGR valve to the tube, without bolting it to the intake manifold. Revving the engine with 5 in Hg applied to the EGR vacuum port gave me over 4 V. If I was being scientific about it, I would have recorded the exact results and test conditions.

EGR flow sensor - check.
EGR valve - check.
Intake manifold ports - need to check.

Now it appears that the most likely location for the flow restriction is in the intake manifold. I reassembled everything except the vacuum hose to the EGR valve. While revving the engine with 5 inches vacuum on the EGR valve port, it didn't get over 1.5 V on the DPFE sensor. Bingo. My intake manifold EGR ports were plugged.

I took off the upper intake, which I didn't really have to do. Of course, I didn't know that at the time. The job can be done by removing the throttle assembly. But by taking off the manifold, I could spray it off and blow it out without getting garbage in the intake.

By the way, spraying carb cleaner and mass airflow cleaner in the EGR port didn't work. Blowing compressed air into the EGR port didn't clean it out either. It seemed like there wasn't a restriction because the compressed air could blow through, but if you look at the picture below, one of the three ports was completely blocked off. The other two were mostly blocked. If this was a heart, and the EGR ports were heart arteries, the patient would have been dead.

Gray arrows showing clogged EGR passages

I peeled off the gasket and cleaned out the EGR ports. Unfortunately, I forgot to take a picture of it all cleaned out. A screwdriver was all the technology I needed to remove the carbon build-up.

Carbon build-up in EGR passages

After re-assembling it, I repeated the EGR flow test. I watched the DPFE sensor readout while applying 5 in Hg vacuum to the EGR valve vacuum port and revving the engine. I got over 3 V, so cleaning out the ports definitely made a huge difference. Again, sorry I didn't get the exact measurements and test conditions. It was enough to show me that it was working.

Simply driving for 10 miles didn't turn the light off, so I resorted to resetting the check engine light with my OBD-II scanner. I was hoping the code would go away on its own so I didn't have to wait a day before getting the emissions checked. Amazingly, after driving another 10 miles or so, the readiness monitors were all ready for emissions testing. I was thinking the evaporative emissions system wouldn't be ready for a day. The light didn't come back on, and it passed emissions. Yay! It barely passed safety though. The emissions guy advised me on the left rear caliper. It wasn't braking as much as it should have been. I have a feeling I know what's coming soon on this blog.