Lean codes

KAY0274

MEMBER
2006 CADILLAC ESCALADE
143,000 MILES

You
I have a 2006 Cadillac Escalade. The check engine light keeps coming on with the two lean codes p171 and p174. I took it to a Cadillac dealer. $4000 dollars later they found the MAF sensor loose and reseated it. They tested it and found both long term fuel trims back to 25%. They swapped the sensor, no change. They unplugged the sensor and purged the valve. Long term fuel trims to 5% and 8%. Smoke tested intake system. No leak. Pressurized the intake sytem, no leaks. Fueled the car for long term fuel trims and it is at 15% for both banks. They heard a whistle on a cold start around 1800 rpms. They replaced the intake manifold and gaskets. They performed a fuel injection system device and reset fuel trims. After a road test the codes returned. They smoke tested the exhaust. They found the the oxygen sensors go rich in both banks. The sensors function normally. They tested the fuel and found fuel trims base line at 8 to 9%. They drained the fuel system and refilled with fresh fuel. Cleared codes and reset fuel trims. Road tested again and found long term fuel trim at 21% on highway, 16% on idle. They reprogrammed the PCM again. No change. They installed a known good bank one sensor and an oxygen sensor, no change. Swapped a PCM and no change. Performed an injector balance and all tests good. Performed an exhaust pressure test and found both side manifolds are leaking. At this point they had the car for over a week and the expense was more than I can handle. Can the codes really be coming from the exhaust manifold and should this have been so hard to diagnose?

3 Replies

SCGRANTURISMO

MECHANIC
4,897 POSTS

Hello,

Believe it not, yes, these codes can be a nightmare to figure out. I wrestled with this exact same thing on my 1997 Ford Mustang GT for two weeks. I had bank one was going straight to -20% short term fuel trim and bank two would go straight to +20% fuel trim as soon as the pcm went to closed loop. I swapped O2 sensors in and out, I checked the EVAP system, vacuum hoses, wiring, finally I got bank one figured out after a couple of days. I had a badly frayed wire to the O2 sensor signal return wire. Bank two took a solid week to figure out. Here's why. I was using an unlit propane torch to check for any vacuum/intake leaks. Torch passes over were the leak is, propane get introduced into the system and engine revs up. Leak found. Sounds simple, right. I had a leak where my intake manifold gasket had failed on cylinder #8. But the gasket failed on the inside of the mating surface, i.E. In the engines valley. It was covered by the intake manifold, had a coolant tube covering it and was pretty much inaccessable to me. Nothing happened when I ran the torch on the outside of the intake manifold. It was only after exhausting all other possible diagnoses that I reluctantly removed my intake manifold and found the problem.
So what I am trying to say is, and know it is not what you want to hear, but sometimes it is a real pain to get these things tracked down. I hope me sharing my misery with you has somewhat assuaged yours.

Thanks,
Alex
2CarPros