Upper Intake Manifold Question

[myguy62]

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.

 

[myguy62]

A quick update. My son and I checked the fuel pressure today. With the vehicle idling, I had a steady 54 psi (60-66 psi normal) and when my son turned the vehicle off, the pressure increased momentarily to 62 psi and then began to fall off ( I believe the bleed off is normal for the 220 throttle body injection system). I also read (Chilton manual) that the cam position sensor sends a signal to the ECM to indicate when the No. 1 cylinder is on a compression stroke and if the sensor fails, all the injectors will default to delivering fuel at the same time. My question is this: Could that be the reason I am not getting the proper fuel pressure (60-66 psi)? Could all the injectors be delivering fuel at the same time and in turn, causing the rich condition on both Bank 1 and Bank 2 (Codes P0172 and P0175)? I am in the process of changing out the OEM poppet style injectors to the upgraded MFI style injectors.