Running Rich

I have a '97 GMC Sierra with a 4.3L V6 engine. Lately I have been getting a MIL indicating a running rich condition on both banks. The weird thing is that when I am pulling my camper the MIL goes out. As soon as I unhook and start driving without the extra load the light comes right back on.

Any thoughts on what might be causing my rich condition? My local Auto Zone's computer printout says to suspect fuel pressure or a bad MAF with a rich condition on both banks. I replaced the fuel pump a little over a year ago, but I realize that doesn't necessarily rule that out.

Are there any tests that I can do to narrow it down?



Thanks,

Gary

What were the exact codes to begin with, also get a fuel pressure tester on it and see if the system holds pressure when shutoff, the regualators on the Vortec are noted for leaking, any other symptoms?

The codes were P0172 and P0175.

It holds pressure when turned off. I changed the fuel filter today and it had pressure when I pressed the valve to release it. I also changed the plugs, wires and thermostat (I noticed it was running cooler than normal). The MIL has not reset since I did the work today. I'm hoping that I fixed the problem. Time will tell.

I suspected that the engine running cool was maybe causing it to remain in open loop thus causing it to run rich.