Most of the replies seem to indicate that the fouling is caused by fuel, EGR, or PCV vapors from the intake manifold, which makes sense to me. That would indicate that most of the "crud" should be on the downstream face of the throttle body butterfly or on the bore of the body just downstream...
"Note also that the idle circuit, typically part of the throttle body, contains a small primitive carburetor. Hot soak will cook the fuel trapped there"
I thought the idle circuit was just an air bypass around the throttle body with a small stepper motor-actuated valve to control the flow...
Tmoose♦
The PCV system takes air at atmospheric pressure (from the intake air duct upstream of the throttle body in my car) passes it through the crankcase, and into the intake manifold downstream of the throttle body. From there it gets burned in the cylinders. So there should not be any...
In looking at the induction system of modern fuel injected engines, it's not apparent where the contaminants come from to foul the throttle body. On my vehicle the only additional connection between the air filter and throttle body is the air supply for the PCV system (which should be flowing...
