08 GMC Sierra Fuel Trim Problem

Bowler807 · November 19, 2012

Hey guys. I have a problem with my truck and humbly ask for the groups help. It is an 08 Sierra Crew Cab 5.3 with 34,000 miles. I threw 2 codes last week. They were P0171 and P0174. The definitions are Bank 1 lean, and Bank 2 lean. Using a scan tool I was able to determine I had an under-reporting MAF. To be certain, I plotted a graph of readings at idle, 1500, 2500, 3500, and 4000 RPM. A good MAF will have a linear graph across the RPM range. This one did not. I replaced the MAF and again charted the results. The new one is perfectly linear.

I however am still throwing the same codes. The codes only appear after I have gone down a hill and have had my foot off the gas for a while. I looked at the data stream for the short term fuel trim. While driving everything looks fine. The readings are between -5% and +10% for both banks. As soon as I stop and let the truck idle, the STFT on both banks starts to climb. If I rev the engine, they return to normal for a short time. Left to itself idling, the STFT will continue to climb and reach a point setting off the check engine light.

I don’t know where to look next. If it was happening at higher RPM’s I would think un-metered air was entering the system, but that seems far fetched when only idling.

Thanks in advance for your ideas.

Bowler807 · November 21, 2012

I found the problem! I'll post here in case someone in the future has the same issues.

Everything pointed to a vacuum leak, but I couldn't locate it. There is no one in my area capable of "smoking" the engine to locate a vacuum leak, so I was forced to use the old carb cleaner spray as a stand-by. After several cans, I was ready to give up, when I sprayed an area of the intake manifold itself. The engine stopped. What I found was a factory sealed vacuum port on the rear passenger side of the intake manifold just under and almost covered by the air tube assembly. This port was cracked. I found a vacuum port cap in my basement on an old carburetor and put in on.

I re-set the codes. Everything stabilized and the truck is running great now.