UPDATE: I think I found the problem. Traced to crappy aftermarket intake, put there by the previous owner.
Cliffs:
Sensor wires frayed, grounding on metal intake tube
Repaired sensor wiring
Beat tube with sledge until it cleared sensor
Reinstalled, no codes yet.
Full story:
So, yesterday I pulled my EGR temp sensor off to clean it...and noticed that the wires coming out of the top were frayed down to bare wire. The insulation had all been worn away. Weird, I thought. I knew the intake tube was close to it, but didn't think it was touching. So I cleaned the sensor (it wasn't that bad, a little soot / carbon), put short lengths of heat shrink tube around the exposed wires to re-insulate them, a little electrical tape over that, and finally a larger section of HST around that to keep everything in place. I also did a little JB welding on some cracking areas of my intake by the hose connections. I replaced some of the easy to get to vacuum lines under the hood as well, as they were old and brittle and some were loose - including the one to my fuel pressure regulator. And finally, I did the alternator wiring upgrade, which I'd been meaning to do for a while now anyway.
I went to reinstall the intake today after the JB had cured, and noticed that the bottom of the tube was all dented and dinged...right where the EGR temp sensor mount is
and the paint had worn away to bare metal. Wait, what? Metal? I assumed the tube was plastic!
It seems that the two exposed wires on the top of the temp sensor were grounding to each other on the aluminum intake pipe, which I think is what was giving me my EGR problems. I think the reason this took so long to show up is that when I got my car, it originally had a cone filter on the end of the aftermarket intake. The filter was sort of bobbing around on the end where the stock air box goes, and not affixed to anything. I should mention that there are two rubber couplers that hold the intake tube to a sawed-off portion of the factory intake, which allow movement.
I didn't like the hot-air intake cone filter, so I reinstalled the stock airbox (retaining the aftermarket intake tubing) and bolted everything up securely. This was about a month ago. Well, when I did this, it put the metal intake tube RIGHT UP against the wires on the EGR temp sensor, and KEPT it there, forcing it against the wires - unlike the cone filter, which was able to move around and didn't force contact with the sensor. Vibrations from driving wore away at the insulation until it was gone, and then it started grounding out and giving me problems. I kind of feel like an idiot that I didn't check / notice that in the first place.
So I beat the intake tube into submission with a 4 lb sledge until there was a nice dimple in it.
It's sort of ugly from underneath, but it clears the sensor nicely now. I cleared the codes and fired the car up, and no EGR code yet. Hopefully that's the last of my EGR problems.