I have a '96 Tahoe with just over 300,000 miles. Lately, it's been running rich giving P0300, P0172, and P0175 codes. I've checked for vacuum leaks and replaced the MAF sensor and the O2 sensors with no success. I pulled the throttle body off and noticed the inside of the upper intake manifold was carboned up and smelled like fuel. Is this normal or should it be clean inside? Could it be a failed fuel pressure regulator or a leaking injector? Thanks in advance.