black layer in intake manifold and TB

[buc]

What would cause a black, rough layer to accumulate in the intake manifold (and intake area of the head, except for a stripe down each side)? From the layman's perspective it would appear that some exhaust is coming back into the intake manifold. I'll clean it out, but I need to know what may be causing it, or if it was causedby a one time event.

LATEST NEWS:
I have been getting a cyl 8 misfire and have eliminated plugs, wires, coil, and injector as causes. I switched each one of those independently, cleared the codes, and it always came back as cyl 8 misfire. The only choice left was air/vacuum leak. So I took the intake manifold off. The "gasket" looks only OK. At #8 it may? look flatter than the others, but no profound leak path. Incidentally, a couple of weeks ago I found the intake manifold bolts to be quite insufficiently torqued. They have been torqued for some time now.

I have had a sporadic #1 ox sensor code (since a few days after changing spark plugs). The book said to watch for spark plug wire placement affecting rt wheel ABS sensor and ox sensor readings. I re-routed the spark plug wires a bit and have not had an ox sensor code since. ABS is still not working.

BACKGROUND
I have had a host of problems that seemed to peak (for one problem) and begin for the others when the fuel pump went out. The #1 ox sensor tripped a code and was replaced. Was running rough, the injectors were so fouled (but I didn't know it immediately), many were stuck open, pouring fuel into the cylinders and out of both tailpipes and onto the ground. As I coming back the last block to work, the car burped when I was trying to start it. Within seconds, as I was driving, slight amounts of smoke were coming out from under the hood. I arrived at work with seconds to find only a small flickering under hood at the coolant tank, but the hood insulation had been singed over most of its surface. Had injectors cleaned, it ran fine for a couple of days. Then it got hard to start and threw ox sensor #1 code and cyl misfire codes and evap codes. I fooled with the vacuum lines routing of spark plug wires and it ran and started fine. A couple of days later it ran rough again, now only throwing the present #8 misfire code.

(Could the burp have caused the black rough layer in the intake?)

The black layer was present in the throttle body with hard to start condition, so I cleaned it and the car immediately was easy to start.

After a couple of days, it became harder to start. Threw a cyl 8 misfire code, etc. That brings us to the present.

So, my 1st question is, what could cause the black layer to build up in the intake manifold, and would it have been from a one time event or from something being wrong? The 2nd one is, could that cause a cyl#8 misfire, or if not, what else should I consider beyond what I've already done above for cyl 8?


buc

[buc]

I forgot to give the details: 1998 Aurora (non-Autobahn) 157,000 miles

buc

[sllee]

The black layer is caused by the crankcase vapors pulled in through the PCV valve over a period of years-part of the emisions control system(normal operation)
You'll probably have to replace the coil for cyl #8 to corrcet the misfire problem.Check for fuel contamination (cut the fuel filter open). I had a large amount of metal filings in my system (source still unknown)Good Luck