I own a 1996 Lexus LX450. I rebuilt the engine in its entirety in early 2020. I’ve put 6,000 miles on it with no problem. Although the engine is in good shape, all other systems have 305K miles on them. Here is my P0171 story:
- Brake booster has been making a sucking sound and sometimes the brake boost does not assist making braking difficult. This hissing/sucking has be on and off for 6+ months.
- CEL came on for P0171 about 2 months ago (it’s a weekend vehicle).
- After several tank refills, I noticed that the CEL cleared itself as a full tank began to deplete, maybe by ¼ of tank. Car always ran smooth.
- After several weeks the CEL came back shortly after full tank partial depletion, same as above. Seems there is a correlation to tank ullage.
- I purchased a new OEM gas cap and cleared the code. Then drove for another few weeks.
- CEL came back on and is on permanent.
- Yesterday, a sudden rough idle occurred after a morning start (46°F). Very erratic and acts like its going to stall.
- Definitely, at higher idle, such as pressing the throttle or turning on A/C seems to smooth the rough idle out. But rough idle is now with me.
- I can hear sort of a large sucking of air sound (kind of brake booster sound mentioned above) coming from the air plenum area.