P0171 degrades into rough idle over several months (1 Viewer)

Joined
Feb 28, 2018
Messages
38
Location
California
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.
Overall, there is a definite degradation occurring from the first CEL P0171 to months later becoming a rough idle. Is this degradation, rough idle, boost loss, sucking sound, pin-pointing the problem?
 
Joined
May 6, 2011
Messages
1,001
Location
Davis County, UT
I would say so. Do a thorough visual inspection of your vacuum hoses and egr, and especially your large rubber hose between the maf and the throttle body. If it's old, it's likely cracked on the bottom
 
Joined
Mar 29, 2019
Messages
148
Location
91107
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.
Overall, there is a definite degradation occurring from the first CEL P0171 to months later becoming a rough idle. Is this degradation, rough idle, boost loss, sucking sound, pin-pointing the problem?
Yes, either pinch or plug hose going to booster to verify.
 
Joined
Feb 28, 2018
Messages
38
Location
California
Yes, either pinch or plug hose going to booster to verify.
Starter fluid sprayed over the air plenum caused jump in RPM. Discovered that my EGR Valve stud and nut were missing (gone) and the gasket had swung down. Clamped it tight and truck runs perfect. Stud/nut ordered an on the way. Thanks for your insights.
 

alia176

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Joined
Aug 21, 2003
Messages
13,336
Location
Tijeras, NM
Starter fluid sprayed over the air plenum caused jump in RPM. Discovered that my EGR Valve stud and nut were missing (gone) and the gasket had swung down. Clamped it tight and truck runs perfect. Stud/nut ordered an on the way. Thanks for your insights.

I need to do this trick as I have the same code that pops up now and then.
 
Joined
Feb 28, 2018
Messages
38
Location
California
Good luck. It worked in my situation and led to the root cause. Although this is the case I really enjoyed the diagnostic routine shown in the attached YouTube.
 

alia176

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Joined
Aug 21, 2003
Messages
13,336
Location
Tijeras, NM
I watched that video and couple of others by the same dude and they're excellent. I learned quite a bit about short and long term fuel trim as that was a mystery to me. I feel more equipped to chase my P0171 code.

Now, to find a decent scope for the money....
 

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