EGR Insufficient Flow Troubleshooting
 

I've been getting the 401 code recently on my 97 SW2 and suspect the intake passage is partially clogged. I ran the engine with the EGR removed and the engine speed changed very little. Am I remembering correctly that it should be hitting the rev limiter when running with the port open? Thanks.

Corrected for massive typos

No, it will not go to the rev limiter, but it will likely be unstable, since removing the EGR causes a massive vacuum leak at the intake manifold. The ECM tries to adjust for it but can't. So the idle gets unstable. Usually the valve is removed for cleaning and for soaking the input pipe to loosen up carbon deposits making their way towards the EGR. Don't want to run too long with EGR off, because without that exhaust flow being sent back to the intake, cooling of the combustion temperature is not taking place. I usually soak with choke cleaner and let it set for 10 or 15 minutes, and then cover that pipe with a rag, and hold it there while someone else starts the car and pulses the accelerator. This should blast carbon residue onto the rag. Then the EGR itself gets a thorough cleaning. If you don't clean out the input pipe to it, you clean the valve but the carbon in the pipe makes its way down to the EGR and clogs it again. This is an ongoing problem once you start burning oil significantly. It is what it is. The alternative is to continually replace EGR valves. I know in the past others have had difficulty with off-brand EGR valves. The issue seems to be that the expected voltages for the EGR valve at certain positions are produced by the EGR valve and so the ECM surmised that the valve is not in the correct position and throws a code for EGR performance.

Thanks. I will try cleaning everything again.

I took off the valve again and it wasn't dirty at all so I swapped valves with my wife's 98. The 401 immediately came back so I guess the exhaust passage is clogged. I think the SOHC has a pipe but on the DOHC it is part of the head. I tried to snake it with a thin wire rope but it wouldn't make the turn in the passage. I may unbolt the exhaust manifold next.

I believe the vehicle must complete 1 drive cycle after the issue is resolved on order to clear this code. If you didn't take it for said drive cycle w either valve on there, you have not given the PCM the opportunity to possibly clear the code.

So it's possible you've already addressed the problem and you just need a problem free drive cycle to clear the code from the PCM. Even if you manually clear the code, all the monitors go back to "not ready" and need a drive cycle to run all the monitor tests.

I cleared the codes before driving and after 5 miles I got the 401 and a pending 401.

Dang.

Have you tried the suspect EGR in your wife's vehicle?

I swapped the valves between cars and the problem stayed with my car.

So that rules out the EGR valve itself as the problem in addition to the current being supplied and the voltages being sensed for different positions.

With the valve removed:

You should be able to easily pass air from the port sourcing the exhaust to the exhaust manifold.
You should also be able to pass air to the intake manifold via whatever path it takes. I therefore suspect the path between the EGR valve output and the intake manifold is partially blocked with carbon. I believe you stated it was a passageway in the head. I don't have an sohc but I can look it up in the service manuals. Tomorrow. I also search for videos describing how to clean that passage on an sohc.

I see I introduced some confusion regarding the engine. I have the DOHC which has the passages integral with the head. I was able to blow compressed air into both ports.That makes sense what you say about the intake port being partially blocked. I did try to snake it but I couldn't get around the bend near the valve. Thanks for the help and suggestions.