had the intake off to replace the fuel injectors and noticed that the intake runners and plenum were coated with a hard black coating like carbon build up. Any ideas what can cause this.

 

If you're talking about inside the tubes, that's normal. Between the PCV and EGR system, gases are being recirculated back thru the intake.

 

yep,
Greg is absolutely right on...

this is normal but annoying since it does lead to some issues down the road..

The carbon crud cakes build up in the small air passages and wet the inside of everything with the cooked on oil film thats full of hydrocarbons and tends to foul the air charge the more it collects. Between this and old engine oil it can even cause a fail in HC levels in a smog test.

Once understood and ID'd, its easy enough to pull the plenum annually and douche it out with carb or TB cleaner, do the TB, idle air passages (minus the IAC motor) and runners if you can loosen then to let the goop run out and not into the intake base plate where the solvents will end up in a cylinder with all that crud...

It makes a difference ! The EGR ports get clogged up, EGR valves sticks and leaks, and the combined dose of gummy gasses accellerates sludge and engine oil comtamination.
There is every reason to spend an afternoon with the top end apart cleaning this and its not that bad of a job. Your idle gets better, the egr works better and the engine runs cleaner. Basically whatever toxins your internal combustion engine are no longer spewing into the atmosphere are now being stored in the engine stuck to parts and in the oil. I supposed the EPA would rather have us place all that crud in the lawn or the sewer system than the air...

Just remember not to hose the IAC motor down and soak it with carb cleaner, and DO try to find a small bottle brush to reach thru the runners and other small passages to help scrub off the stuck on stuff. You can even make a brush of sorts by twisting some stiff wire with a rag section in the end....use the cordless drill to grab the wire and twist while you hold the other end still. Now you have a twisted wire (stiff and strong) with a rag made in the end that can be inserted into runners and egr ports.