I don't know about this car but I do know that the build up is common on all port fuel injection systems. When I get a car in with idle problems the first thing I do is clean the throttle body. Ford has a sticker on their throttle body's that says "Special coating, DO NOT clean". If they are dirty, I have the choice of "Clean" or "Replace". I clean them. It has helped many,many times. The problem is caused by engine blow-by traveling up the vent hose to the air cleaner side of the intake system. You can have the same problem on the CIS systems at both the air sensor plate in the CIS and at the throttle body down under the CIS. There is nothing wrong with the system, it is just a design flaw (my opion) in most gas engines.

Paul

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84 500 SEL (307,xxx miles)