If your car is able to run with the EGR removed and not blocked off the passages are blocked. It should stumble or stall. Removed it should stall or not start.

The s Series dohc will run with the EGR removed w the ports open. It is quite unstable but will run if you give it gas. Definitely does not want to idle. To be honest, I always have someone in the driver's seat that shuts it down before it might stall, so dunno. Makes sense that it would. But runs long enough to blow the gunk out of the one pipe.

I believed I've fixed the problem with the insufficient flow code. This time I sprayed the cleaner down the ports with the engine hot and let it sit overnight. I drove it 50 miles today and there was no code or pending code and the monitors all ran. Thanks for all the help.

Didn't clog the valve?
I'm not complaining....
Good times....

it didn't clog the valve and even initially before any cleaning there was not much carbon in the valve.

Do these cars have a DPFE sensor or are they just looking for vacuum drop during the monitor? The pcm is calculating egr flow one way or the other.
All my L61s do not have EGR, the L81s have EGR.

These engines are fairly simple and just have a MAP sensor.

What is actually inside the head of the egr valve? Thought about position sending, but I never put any thought towards the nature of the feedback to the PCM.

Gonna poke around a bit.

Hi, derf. Amazingly I also had the dreaded 401 code on my 02 sl2 dohc for the longest period of time. Finally I got busy and removed the exhaust manifold and air management pipes. Cleaned everything. The port at the end of the head was all plugged up. Kept putting carb cleaner and gm top engine cleaner in the port. Took a piece of weed eater line and chucked it in my cordless drill along with an old Speedo cable inside and kept ramming and spinning until I broke through. Pulled the get and cleaned the lower pipe and replaced the get valve with a Delco valve. I also purchased a Tomco EGR clean screen. The crud cannot get past the stainless screen to the valve, it works great. In almost 180k I have never had the code return not have I had to pull the egr. Wondered if you have ever used the Tomco clean screen? Thanks for all your advice and help. I have read through lots of your posts. All good! Nick

Sorry. That should be egr not get! Tablet correction! Not good.