Those Pesky Lean Condition Codes

When a vehicle rolls in with its malfunction indicator lamp (MIL) illuminated, do you experience excitement or dread? I always get a little giddy. Why? The thrill of the unknown and the process of diagnosing always seems "fun" for me. Granted, the path is often quick and relatively easy; however, it can also become a labor-intensive task that requires time, research and patience.

This particular vehicle gave us two codes: PO171 and PO174. It is important to note that neither one of the codes were current; they only appeared under "Memory Codes." In a situation like this, I look at freeze frame data, and we had the following data snapshot from when the code set: vehicle speed, 38 mph; coolant temperature, 189 F; long-term fuel trim for both engine banks were at 14 percent and 17 percent; engine rpm, 1803.

What do these numbers mean? The most important data - the fuel trim numbers - mean that the computer is compensating for a lean condition. In addition, due to the V-6 engine configuration, the data shows a lean condition on both banks. In my experience, this usually means the engine has low fuel pressure, a dirty mass airflow (MAF) sensor or a large vacuum leak. If the fuel trim numbers were high on one bank and not the other, there would be a lean condition on just one side of the engine, eliminating the MAF and fuel pressure as possible causes. Because of the high fuel trim numbers from the freeze frame data page, I studied the live data stream page and focused on the parameters associated with a lean condition. These fuel trim numbers were much lower, but still on the "add fuel" side of 0. You may notice that HO2S22 is at 0 millivolts; this is because there is only one downstream sensor in use, which is HO2S12.

Next, I decided to grab my can of MAF sensor cleaner and give my sensor a bath. After doing so, my fuel trim numbers did not improve. However, I suspect that many vehicles are running around with dirty MAF sensors, and can benefit from cleaning.

I decided to pull up the diagnostic information for a P0171 to aid in my diagnosis. Although I had cleared the code and the light had not returned in a few days of driving, I did not feel comfortable sending my customer away without finding a plausible root cause for the lean condition.

My next test was to hook up a fuel pressure tester to check for fuel pressure. This vehicle has a mechanical, returnless system with a key on, engine off pressure listed as 55-65 psi. My gauge showed 70 psi, so I moved on to checking for an injector fault by scrutinizing injector data. As expected, the injector data did not show an injector fault, so I moved on to testing the injector flow rate. With my fuel pressure gauge still hooked up, this was a simple task to energize the fuel pump for each injector and energize each one with my tester. Each one had an even drop indicating balanced flow.

Next on deck, I decided to test the induction system for air leaks, so I pulled out my trusty smoke machine, pulled the vacuum hose off of its PCV valve and inserted the smoke tester's output line. I capped off the throttle body on the intake side and did the same to the exhaust pipe exiting out back, and then I activated the system. I have performed this test on different vehicles, so I am aware that it will reveal vacuum leaks small enough to evade a stethoscope's ability to hear.

Smoke pouring from the intake seals, just below the braided steel hose.

Upper intake seals that have hardened, causing vacuum leak.

After repeated tests, I could not find a leak of any significance so I decided to hold the vehicle over and test the engine cold. In the past I have found leaks on a cold engine that have not shown up on warm ones.

This vehicle would turn out to be just such a case when I checked for leaks on a cold engine, and smoke came pouring out from the mating surface where the upper plastic intake joins the lower aluminum intake. Upon disassembly, I found the rubber seals to be hard-as-a-rock. Comparing the old seals with some fresh ones revealed just how much the old ones had shrunk.

In a related tip, our customer had indicated the "Check Engine" lamp illuminated on one of our recent below-zero, winter days. I'm certain this further shrunk our seals. This made sense to me: the Freeze Frame data indicated that the code set before the engine had completely warmed up.

Installing the smoke check machine into the PCV hose.

Note to the wise: Always search technical service bulletins (TSBs) on your information system when hunting for intermittent issues. In addition, I found many threads on the International Automotive Technicians Network (iATN) regarding this issue. Most techs found that dirty MAFs and vacuum leaks from hoses and/or the intake manifolds on these engines caused their lean running codes.

Did the intake gaskets fix my Ford? Time will tell, and I am confident that the condition of the intake seals, coupled with their high failure rate, will combine to have been the root cause for this customer's concern.

Brian Manley is a vocational automotive instructor for the Cherry Creek school district in Aurora, Colo. He is an ASE master certified automobile technician and a former member of the National Automotive Technicians Education Foundation (NATEF) board of trustees. He can be reached at manley_brian@hotmail.com.