So, I've had my wife's old X sitting for a while that I need to sell. But, it's had the SES light on for codes P0171 and P0174 (and short-term fuel trims in the 15-18% range) for a while that's been kind of tough to diagnose. So, I thought I'd share what I've found and seek a little advice on how to eradicate this problem once and for all.
First, I ran a smoke test and quickly found a hole in the intake boot (the one that attaches to the front of the throttle body). Re-ran the smoke test after boot replacement, and all good there. I take the car out and drive it, boom - the light comes back on after ~3 miles. Pull the codes, same thing - lean on both banks. All the while, the car starts and runs great.
I figure no vacuum leaks, so now I have to look at the fuel side. I keep reading all this stuff about soft failing fuel pumps (original 140k miles), fuel pressure regulator in the filter (about 40k miles old), etc. I test the fuel pressure - about 45 psi hey on engine off, 51-52 psi engine running. Holds pressure after key off. Seems OK.
Still stumped, I drive it again with live data showing. My theory is that if the fuel system is failing, it would have more trouble under load than at idle, and the car runs fine under load. So, while watching STFT, I can see that it goes back down to around zero while under load. That's not a fuel problem but a vacuum leak, which I shouldn't have!
I do another smoke test, and, sure enough, I see smoke. However, I can't really tell where it's coming from. It looks like it's under the intake manifold towards the front or front-middle. Nothing from where the DISA plugs into the manifold, but maybe the body of the DISA?
And, just so I don't think I'm losing my mind, I do another smoke test on a cold engine. No smoke.
So, a few pointers to summarize for others experiencing this problem:
1) If your STFT's are only high at idle and go down under load, your fuel pump/reg is probably OK
2) You may have a vacuum leak that's only present when the engine is hot - do your smoke test on a hot engine and don't rule out the possibility of multiple leaks!
Now, my questions:
1) What are the possible seals that can cause a vacuum leak - DISA, throttle body gasket, CCV, any others?
2) Which of these can be changed without pulling the whole manifold?
3) Or, is it easier to just pull the intake manifold and do all the seals?
Here's a video of the smoke test:
https://youtu.be/CxcS9X7VYVg