PO171 and PO174
 

I have a 2002 1500HD with a 6.0 with 133000 miles on it, a couple of days ago it started running really bad but no check engine light came on, I was able to drive it back home and hooked the scanner up and it through these 2 codes, I cleaned the MAS air flow sensor and replaced the air filter and it ran fine so I cleared the codes and drove it a few miles then it developed a slight hesitation off of idle, I have checked for vacuum leaks and have not found any, checked the fuel pressure and its only running 25 to 30 pounds with vacuum line unhooked at the regulator, I changed the regulator and it still reads the same, what should the fuel pressure be and is there a certain way your supposed to check it, thanks in advance,

Re: PO171 and PO174
 

Have you check for intake manifold leaks? I have also heard an exhaust leak before the first O2 sensor(s) can cause that lean condition issue.

Re: PO171 and PO174
 

Key on, engine not running, fuel pressure should be 55-60 lbs
Turn key off and fuel pressure should not drop by more than 5lbs...
25 to 30 running is way too low....its a wonder it runs at all...

Sounds like a bad fuel [pump or bad fuel pressure sensor....my money is on the fuel pump...

Re: PO171 and PO174
 

Ok I found out the fuel pressure tester I was using wasn't reading right, my pressure is 55 not running, I did change the fuel filter and it has been running great, thanks for the help.

Re: PO171 and PO174
 

Mine did same thing turned out to be intake gaskets