Using a 8mm (5/16”) drill bit as a hand-held ream, clean out the port. If you’re installing the update kit, now is when you’ll drill the manifold using the bit provided; be sure to follow the cautions included with the kit so as not to damage the manifold.

The next step is a thorough cleaning, flushing all the ports and intake runners from both directions. If you’re doing the update, now is also the time to install the EGR pipe.

Finish up by inspecting the EGR port in the manifold base and cleaning as required. This is best done with the EGR valve removed.

The aforementioned update kit is fully explained in Honda service bulletin #05-026, which also details the 8-year, 80,000-mile warranty extension for the EGR system. Many of these cars have been updated, but you’ll still see plenty of them with well over 80,000 miles that will be experiencing this problem, as well as some vehicles that aren’t covered in the bulletin. Nonetheless, the bulletin is a good resource to have.

CAUSES OF A P0401 CODE
While port restrictions are the most common cause of a P0401 code, that’s not the only reason you’ll see it. With the older vacuum-controlled EGR valves, there have been issues with the vacuum-switching valves being intermittent. Testing is straight-forward. With the engine running, be sure vacuum is available at the inlet side of the valve, then hook up the gauge to the hose leading to the EGR valve.

With the electrical connecter unplugged from the switching valve, introduce battery voltage to the black/yellow wire and apply ground to the red wire at the valve side of the electrical connector. If all is well, you’ll show vacuum on the gauge. As I said, these valves have a reputation for being intermittent, so repeat the test in quick succession looking for slow or no response on the gauge.

The electronic EGR valves are quite reliable but certainly haven’t been trouble-free; the problems we’re seeing with them are more about them sticking and not doing what they’re told. The good news is they’re not hard to diagnose when you know what you’re looking at. Let’s look at the connector on a 2002 model. We have five wires that handle valve operation and reporting; the valve is power-side controlled with a duty cycle signal from the ECU through the pink wire that gets its ground from the black wire.

On the reporting side, the yellow and blue is the 5V reference signal from the ECU for the position sensor, the green and black is ECU ground, with the white and black sending the signal back to the ECU, letting it know how the EGR is responding to direction. An increase in operational duty cycle should result in a smooth increase in reporting voltage with no dropouts. Many scan tools will let you look at and log command and reporting data on a road test. Otherwise, with good access, it’s not difficult to back-probe the connector and get the same info on a multi-channel graphing meter.

We can also use what we know about the wiring to check for blocked passages and the basic operation of the valve. With the electrical connector disconnected, the valve will open by putting battery power to the terminal that the pink wire is connected to and grounding the black wire terminal. If the ports are clear and the valve is working, the engine should quickly get rough or stall, and smooth out when the valve is closed.

When diagnosing a driveability problem, it’s important to keep in mind if you have a problem on light acceleration at low RPM, when the EGR is commanded on, suspect the EGR system. Or, an intermittent rough idle may be the result of the valve not completely closing. Otherwise, the EGR system shouldn’t be suspected.

If I can leave you with one solid tip for diagnosing and repairing EGR problems on the Honda line of vehicles, it’s to check your service information system for service bulletins. Honda does a great job of making this information available to the aftermarket, and it’s just foolish not to take advantage of it. If you’re not going online, checking for bulletins and making full use of services like iATN, import-car.com and OEM info like that available from Honda (www.techinfo.honda.com), you’re just leaving too much good information untapped. The way things are going, you won’t be able to afford doing that much longer.