first time to remove throttle body and plenum. saw this black film all around the inside of these parts and the runners. wondering what is it before putting them back in. aren't they supposed to be squeaky clean? thanks for info.

 

Yes, clean it.

 

what would be the best solvent to clean these parts? film seems to be black oily type.

 

also what causes this film to accumulate inside? hope the causes don't present major problems. the car runs great. i am replacing the egr at this moment why plenum is out. since i bought this car 4 years ago, i replaced the air filters with k&n filters. by the way, i got '89 iroc 5.0 liters tpi 5 speed. thanks again.

 

It's carbon and it's perfectly normal. On higher milage engines the carbon will also have a little oil in it. The carbon gets in there by a process called reversion. If you want to clean the plenum and runners really well, remove them from the car and soak them in mineral spirits. Hot soapy water and a scrub brush works too...but is's more work

 

Actually its from your EGR, If you look into your plenum you'll see 2 holes right behind you TB. The EGR gasses are what leaves the carbon behind. Dont even worry about cleaning it, cause 20 miles or so afterward it will be back. Its perfectly normal.

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Convertible, L98, Mini-ram, ZZ9 cam, Comp Cams 1.6/1.52 steel roller rockers, Trick Flow 23* heads, K&N's with chopped airboxes, March underdive pulleys, Edelbrock ceramic coated TES, Flowmaster cat-back exhaust, Dynomax high flow converter, Accel box & coil, Fastchips custom chip, MAC sub frame connecters, Lakewood LCA & Adj. Panhard rod, SLP 2500 stall, 3.23 rear gears, white face gagues, and NOS!