I have a 2008 Uplander with 3.9L engine. I have a couple of codes coming on, they come back after a short drive after resetting them, usually P0172 and P1174 together, sometimes one or the other.

I started by cleaning MAF sensor with no change. I then tested fuel pressure at the rail, I got 60 psi with key on but engine off, and 67 psi with engine running at an idle. This seems a little high, the correct operating range I found was 59-62 psi. I was going to test the fuel pressure regulator but this engine doesn't have a vacuum controlled regulator on the rail. There is an electronic component on the end of the rail but I don't know if it's a pressure regulator, some research suggests it might be internal in the fuel tank or built into the pump.

Is 67psi at an idle too high? Does this engine have a regulator I can test?

Next steps I was thinking about Ohm testing the injectors. Also wondering if there is a way to test the EVAP purge valve/canister.

Thanks for the help!

 

the pressure does seem high...I believe the ppump pressure is pulse width modulated by the pcm .
flex fuel vehicle? is yes, check the flex fuel %. if more than 10% and you are not using flex fuel...reset the flex fuel sensor. if the code returns I would take a guess and replace both precat o2 sensors.

 

Thanks for the quick reply. The vehicle is flex fuel, I do use E85 when the price makes sense to run it, but haven't used it for the last few tanks. My scan tool didn't show me alcohol %, but I borrowed a friend's scan tool that would read this, and it was at 16%. This tool didn't allow me to reset this value. 16% seems realistic, as I could have had some E85 mixed in still.

I forgot to mention that I had already replaced the precat o2 sensor with no change.

I'm thinking about Ohm testing injectors next. Other ideas where to go next?

Thanks again

 

reset the oh content...find a way. if the sensor readings are askew...it will run rich on both banks.

 

I'll try resetting the alcohol content next week, have to find a shop to do it for me. 16% seems realistic so I'm not sure that is my problem, so I thought I would go ahead and test the injectors today.

The 6 injectors test anywhere from 12.4 to 12.7 ohms. Is this close enough or would 0.3 ohm difference potentially be a problem? I can't find a range for what they should be.

Also, this 3900 engine only has 1 pre-cat O2 sensor, so the P0172 code doesn't help me narrow down between front and rear bank.

 

maf cleaning never works. if the reset doesn't work replace it.

if the injectors ohm readings are all close to the same; electrically they are probably fine...doesn't mean they are flowing the same. GM injectors are pretty good...rarely have I seen a problem with them. without a injector leak down tester or the gm specialty active fuel injector tester...all you can do is pull the rail and check for leaks or bad spray pattern....something I would do last.