For about 2 months now I have had codes P0171 and P0174 pop up while driving. Thursday this week I finally had some time to take a look at it. I figured that the easiest thing was to change the fuel filter. When I went to bleed off the pressure it barely let out a tablespoon of fuel. When I puled it off I checked to see how restricted it was and it was plugged pretty good. Typical black crap came out. I installed the new filter and turned the key on and checked for leaks and was good. I relived the pressure and now I get about a cup of fuel out. I drove it for about 800 miles this weekend and all went good until I got to my destination and the stupid CEL came on again. Same codes as previously given. So I looked at some of the possible causes and it listed Leak after the MAF sensor, worn plugs (mine are new), faulty O2 sensor and of course the fuel system restriction. I checked for leaks on the intake after the MAF and found none. Plugs are new, less than 3K miles on them and a new fuel filter. While looking at the air intake I seen an orange seal on the filter and said to myself, wonder when that was changed last. It was 8 years old and had 53K miles on it. I pulled it and it was of course dirty. I don't have a new one so I blew it out like your not supposed to and put it back until I can get one on Tuesday as it is a holiday weekend. So with all that I still get the code.
When at an idle it runs rough sometimes and has died a few times at a full stop. Other than that it runs great. I get usually 28-29MPG with it on road trips even with the light on and that is driving 79MPH most the time. But the check engine light bugs me. I clear it and it pops back up again of course. I don't have a fuel pressure test set but think I need to buy one. The big question after the book I wrote is what should the fuel pressure be at? And what are the chances of losing 2 O2 sensors at the same time. The car has 141K on it so won't surprise me if the pump is weak and the way the fuel filter looked it could have been the cause. Is there a readable point off the OBD2 port to read the fuel pressure? If so I may need to upgrade my scan tool. Any thoughts would be great.

 

Has ANY maintenance been done recently ???…If your scan tool can look at long and short term fuel trims let us know and we can get your car diagnosed !!…get yourself a fuel pressure gauge because it may help in a diagnosis too and it’s just a good tool to have…lean codes can be vacuum leak, MAF sensor, clogged fuel injectors, weak fuel pump…fuel pump pressure key on engine off should be 55-62 psi !!

 

You need to connect a gauge to your fuelrail like this to be able to read the pressure.
If your scanner is able to view live front O2 sensor data the voltages shall switch between approx. 0.1-0.9 volt when your engine is in idle as shown
in the last screendump from my scanner

 

As mentioned above, you need to gather more data. But its a good bet that if the fuel filter had black goo in it that you passed some up to the injectors.

 

I am ordering a fuel pressure test set. I have been putting it off for years but they seem to be pretty inexpensive now days. Were oelarse shows the pressure gauge is where I bleed the pressure off to change the filter. I can graph on some points on my scanner but not many on the C5 for some reason. Works much better on my F250. It will take a couple days to get the test set and will continue at that point. Thanks for the info so far. enjoy the last day of a long weekend

 

For now if you can just look at fuel trims that would be a start…let us know what they read at idle…if high (above 10%) increase the RPM to 3000 and hold it there for a few minutes…if long term fuel trims decrease you have a vacuum leak…easy peasy !!..if not a vacuum issue we can see what happens after the fuel filter is changed.

 

I had 1 bank running lean, turned out to be a vacuum leak.

 

^^^ minor one on the air intake, other was tfhe intake manifold.
Dont overtorque them

 

I had a lean condition same as you on my 2001 coupe. I had a vacuum leak......it was the small pcv hose on the passenger side of the throttle body, or just behind the throttle body, had a small hole in it. Easy fix.

 

Second this. I haven't had this problem on my Vette, but I have a Jaguar that throws those two codes about once a year. It's always a vacuum leak. The Brits can't build a piece of rubber to save their lives.