I've been struggling with fixing a P0171 (too lean) condition on my 2005 Celica GT with 145k miles on it. I’ll try to be as brief as possible. I have one of those “ELM” bluetooth OBDII adapters, and use Torque. Some things I have noticed or tried:
1. If I disconnect my battery to reset everything, the engine will idle VERY poorly and have near zero power. After driving around for a couple minutes it clears up and will idle and accelerate like normal. Checking for “pending faults” with Torque shows P0300, P0301 and P0304 (cylinders 1 and 4 misfire). These codes however DO NOT end up as logged faults since they seem to go away after the first couple minutes of driving. Shortly after, P0171 shows up as a pending fault as well, then after a few more minutes it becomes a logged fault (the only logged fault) and the MIL comes on.
2. I checked the PCV value and it seemed fine (blowing from the engine side allows air to pass, blowing from the hose side allows no air to pass). I replaced the PCV valve anyway just in case. The new one also seems fine.
3. I have replaced two gaskets: Intake
manifold (Toyota 17171-22060) and valve cover (Fel-Pro VS50542R). Both were brittle (pulling or pinching them would easily cause cracks).
4. I cleaned the MAF sensor, but it seemed clean to begin with. And yes, I cleaned the sensors tucked inside the tube too. I then replaced the MAF sensor just in case (Denso 197-6030).
5. I replaced my spark plugs even though my old ones only had 60k miles on them. I used the same Iridium plugs. (Denso SK16R11).
6. I have checked my Air Filter
. It’s only 6 months old, and still looks new. Perfectly white filter element, gasket is not cracked or deformed.
7. I replaced my fuel tank cap (with another genuine Toyota cap) since the o-ring had several small cracks.
8. I have tried checking for leaks: with the engine at idle, I have sprayed brake cleaner on every hose I could find, around the air box, along the air duct, and all along the Intake
manifold and valve cover. Idle never changed.
Driving around with Torque running on my phone, I am monitoring the long-term fuel trim, short-term fuel trim and the vacuum. Here’s what I see:
- At idle: LTFT stays right around 31-33%. STFT varies smoothly between -2% and 20%. Vacuum stays right around 23-25 in/Hg.
- While driving: LTFT is mostly around 25-33%. It will occasionally drop toward 7 - 20%, but only for a second or two before going back up to 25-33%. STFT varies between -5% and 21%. Vacuum drops pretty linearly with load. If I accelerate hard, or drive up a steep hill, the vacuum drops to 2 - 5 in/Hg. When cruising at a constant speed the vacuum is around 18-20 in/Hg. Vacuum quickly responds to changes in throttle, reflecting the load on the engine.
I’m nearly out of ideas. I just filled my tank up with gas and noticed that loosening the cap doesn’t result in a noticeable “woosh” of air. There’s just a hint of pressure. Would this indicate a problem with a VSV or the charcoal canister?
Sorry for the long post. Thanks for any comments.