Need some guidance on codes P0171 and P0174 which are for system 2 lean bank left and right. Would appreciate some direction as to the most probable cause for this problem.
thank you in advance.

 

Assume a vacuum leak in the PVC plumbing until proven otherwise. That covers about 90% of the incidents.

 

Year and engine would help nail it down better.

 

Thanks for the replies!
Sorry, the truck is a 1999 F-150, 4.6 XLT 4x4.

 

Follow the PCV hose back from the PCV valve to the back of the intake, that's where the rubber elbow rots and leaks. Replace the elbow.

 

Thank you very much! I will check this out.

 

have you fitted headers or hi flow cats?

 

No, I have not, but my exhaust is basically none existent from the cat back, but this has been the case for several years and the codes just recently developed. Haven't had time as yet to check the recommended vacuum components.

 

Exhaust leaks in front of the front sensors will trip lean codes.

 

it would be necessary to have pretty much the same leak on both exhaust manifolds in order to set the same code for both banks.

Go with the odds and check the most common stuff first. Exhaust leaks would be way, way down the list.

 

Thats accutally incorrect, if you have a leak on one side the pcm will use the short term and long term fuel trims to correct the issue the 02 sensor is seeing and it will cause the other bank to go into a rich or lean state as well, but i dont believe its a leak in more of a flow issue, if you have a cat convertor that has become restrictive or pushed back into the exhaust etc it will certainly set this coding as well, i have seen this many times over the years when a custom brings a car in for a dyno tune with those codes and i check the cats and find a issue

 

Hello again, today I had a look at it and there appears to be a PVC valve on both valve covers. On the passenger side the hose is cloaked with a insulating sleeve and travels toward the fire wall where turns right the turns 90 degrees to a block on the top of the intake. Did not observe any elbow although there could be one and it is inside the insulating material. The PVC on the driver's side is simpler and its hose, also cloaked, travels forward to a visible elbow on the intake. It looked ok, no visible cracks. Am I missing some thing in my viewing?

 

Yeah, that elbow in the back. It's there, keep looking.

 

There isn't a PCV on the driver's side valve cover. It's just a connection. Air is supposed to flow from the intake tube after it gets filtered to the driver's side valve cover. Then it travels down the heads to the oil pan and back up to the PCV on the passenger side, then to the intake that is under vacuum. Keeps the gas fumes from exiting the engine. It's part of the emissions controls.

 

My son in law's 98 4.6 didn't have an elbow per se, but it's the hose that goes from the PCV valve on the passenger valve cover to the back of the intake that has to be checked out. Also check all other vacuum hoses including the brake booster and the evap purge line.