Long time listener, first time caller. First Gen in Portland Oregon

Hey y'all. 2002 2.7 4x4 AT Standard Cab, ~200,000 miles. Portland Oregon. Peeling clearcoat, faded fenders, and a utility shell that doesn't quite fit right.

Been lurking for years, and I've definitely benefited from all of the build threads, technical advice, and so on, so thank you all!

I'm nearly done chasing down a P0171 code, and I'm confident that I've got it nailed down. There have been several helpful writeups on this issue, but if I solve it I feel like I owe it to the community to do a complete write up that summarizes not only what the fix was, but the process of diagnosing it that I went through. It can be so many things, and I've been obsessed with avoiding throwing parts at the problem. It's been quite a journey. Hopefully I'll be able to make it a little easier for people down the road. Stay tuned, and hopefully my next post will be me yodeling my triumph from the rooftops.

-Ras

 

So what have you tried so far

 

Welcome to TW!

 

Welcome aboard Ras

 

Thanks everyone, I appreciate it!

Well, I started with the basics: P0171 means that the computer believes that the engine is running lean. Generally, if your sensors are good, this means that there is unmetered air getting into your system, or not enough fuel getting into your system. Often, this is a vacuum leak. Because the fuel system intimidated me, I began with the air part. In my case, it has turned out to be several different issues. Somewhere near the beginning of this, I purchased a OBD II Scan tool with live data to help me pinpoint the issue. I'll do a longer more in depth guide later; this is just what I did. It was a learning process for me, and I'd definitely go about it a little differently next time. Also, I have yet to recieve the final piece to the puzzle, so I may not be done yet.

I began by visually inspecting all my vacuum hoses, including pinching off the brake booster. I replaced them all a few years ago, so they looked pretty good. However, just to be sure, on a cold engine, i sprayed carb cleaner around any points where a leak could occur and listened for a change in idle. No reaction.

I don't want to just throw parts around, so I used my scan tool to check the data from my 02 sensor to see if it was behaving as it should. It seemed to be within range.

After looking around the forums, I saw many people had experienced a cracked exhaust manifold, so I took a air-bed inflating pump and attached it to my tailpipe. Then I sprayed soapy water around the exhaust system and watched for bubbles. I wasn't able to see it, but I heard a hiss under the manifold's heat shield. My exhaust manifold was cracked. This was sucking air into the exhaust stream, causing the upstream 02 sensor (also referred to as an A/F or air to fuel ratio sensor) to report too much oxygen in the exhaust. When this happens, the computer responds by attempting to dump more fuel into the combustion chamber. On the scan tool, this can be seen as the fuel trim.

If the exhaust leak had been my only problem, even though the computer thought I was running lean, I was actually running rich, because the extra air was coming into the exhaust after the combustion had already happened. However, that was not the case.

Once I replaced my exhaust manifold, my fuel trims improved at speed, but were still much too high at idle; the code didn't go away. Normally, this would point to a vacuum leak at/after the throttle (because of pressure difference), but as I'd just inspected and found none, I checked elsewhere.

Side note: Earlier on, I had checked the PCV Valve hose, but I never checked the actual valve. Once I took it out, I could clearly see that the spring inside wasn't seated properly, and it was probably letting air into the crankcase when it shouldn't have. Not a likely culprit, but I'm pretty sure it contributed to the end issue.

After this, the check engine light went off when I was driving down the freeway! But then it came back on when I parked and idled for a bit. Progress, but my fuel trims at idle were still much too high. At this point, I checked my MAF Mass Airflow sensor data to see if it was in spec. It was in spec on startup, but over the warmup period it reported lower and lower airflow numbers until it was lower than spec by about a quarter, where it leveled off. I cleaned it with MAF cleaner and a paintbrush, but it wasn't really dirty to begin with. While it was out, I used a multimeter and checked its resistance according to the Hanes manual. It was in spec.

After this, I decided to look closer at the throttle body. It was filthy! I ended up removing it and cleaning it. As I did, I read about it to understand each part and what they did, and my focus settled on the Idle Air Control valve. After taking it off of the throttle body, it was extremely dirty. I performed the tests in the Hanes manual, and unsurprisingly it failed. I believe that the PCV being stuck open blew oil into the chamber over time and ended up burning out the motor so that it couldn't close. At speed, this wasn't as much of an issue due to the amount of airflow, but at idle it made unmetered air flow into the engine.

I've got a OEM replacement IAC valve on the way. Should be here on the 5th. While it's en route, I'm going to clean my intake manifold because it's filthy. Because the fuel filter is under the intake manifold, I'll be replacing that while it's off. I'm sure that there's more to it than I've said, so feedback is welcome. I'll be sure to keep you all updated.

thanks,
-Ras

 

Welcome to TW!