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Hello. I hope someone can shed some light on this. I have a 02 Corolla with 257k on it with a p0171 code. The car runs good with no misfires. The issue is that I have checked all the usual things that cause this code. No vacuum leaks that I can find, no exhuast leaks, MAF is clean and was changed about 20k miles ago, fuel pressure is at spec, injectors changed about 30k miles ago, even though it probably didn't need to be changed I went ahead and replaced the intake manifold gasket anyway. The upstream O2 was changed about 40k miles ago. PVC valve and hoses just changed.
LTFT stays at 45 at idle then drops to around 37 at a steady 2k RPM, then goes right back to 45 quickly at idle. STFT bounces from -3.8 to 4 and stays about the same at idle and around 2k RPM.
Under load LTFT drops from 45 to about 27 at a very light throttle then if i give more throttle it goes back up to 43 or 45. STFT bounces all over.
The upstream O2 does switch from 0.0 to 0.8 at idle and will hit all zeros with a throttle snap.
I am pretty stumped with this. I have had a few techs tell me the engine is just worn out and this is why is has that code. I did have a compression test done. 1,3 and 4 were around 155 and number 2 was around 100. So any insight to this would be very helpful.
The engine is worn out I checked the compressions on a Toyota MR2 last year the compression pressures were about 210psi on 3 cylinders 195psi on the remaining cylinder.
Normal specification is lowest cylinder should be no more than 10% or 1 bar (14 to 15 psi) below the highest. A 50 psi difference is huge especially as the other cylinders although OK for high miler are already on the low side.
I would do either a wet compresion test or a cylinder leakdown test to determine if it is a valve or piston sealing problem, a valve problem might be worth fixing but piston problem is more involved.
" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)
It may be a good idea to take a fuel sample and test it for excessive ethanol content. E85 in a non-flex vehicle will cause this condition, and can be a real head scratcher.
"Without data, you are just another person with an opinion."
I would agree that the compression issue is indeed concerning though. I probably wouldn't worry about fuel trims until the mechanical problem was corrected.
"Without data, you are just another person with an opinion."
Andy.MacFadyen wrote: The engine is worn out I checked the compressions on a Toyota MR2 last year the compression pressures were about 210psi on 3 cylinders 195psi on the remaining cylinder.
Normal specification is lowest cylinder should be no more than 10% or 1 bar (14 to 15 psi) below the highest. A 50 psi difference is huge especially as the other cylinders although OK for high miler are already on the low side.
I would do either a wet compresion test or a cylinder leakdown test to determine if it is a valve or piston sealing problem, a valve problem might be worth fixing but piston problem is more involved.
Agree with Andy! Difference in compression is too high. Leakdown test if you can...
What engine is this? Just curious.
SniperDan wrote: It may be a good idea to take a fuel sample and test it for excessive ethanol content. E85 in a non-flex vehicle will cause this condition, and can be a real head scratcher.
Good idea here.
Andy is right on about the mechanical side, too. 100 PSI is barely enough to support any kind of combustion.
Thanks everyone for the info. While I will agree that the engine has some miles and is well worn, it just runs way to good to be "that" worn. There is no misfires, and it idles very smooth. You can't even feel it run at a stoplight. Of course the compression test does prove one cyclinder is weak.
From everything I have read or talked to a few techs about, it is possible it could be the opposite. Low compression would cause a rich running engine.
The engine is the 1.8 1ZZ motor that has the metal intake manifold that uses the metal intake gasket.
I will look into the leakdown test. It may just turn out this low compression could be the issue. I am just not exactly sold that the one cyclinder is causing the issue.
Just for fun earlier, I rechecked the LTFT and this time it was at 44.5 at idle with no load and revving it up to 1500 to 2500 RPM it dropped to 23.4 to 25.5. Let off the pedal and instantly went back to 44.5 and just stays there.
My thinking is on the compression stroke a leaking exhaust valve pushes unburned fuel and air out through the exhaust port, and the oxygen sensor sees this as weak mixture so ramps the fuel trims.
" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)
Putting it that way it makes sense. I am going to end up selling the car soon. I dont have the money to mess with it. I just thought maybe it was just something simple I kept overlooking. Thanks for your time and replies.
So today i decided to look at the fuel trims again and noticed that if I have the A/C on the STFT goes negitive to -10 but if I turn it off it goes positive to around 10 to 12. LTFT stays at 44.5 the whole time. Just found that to be odd. I wouldnt think that fuel trim should be affected by the A/C going on and off. Any thoughts on that? Is it at all possible the IAC is flaking out or maybe the pintle is stuck a little? Would that have any effect on having the lean code?
Update: I decided to take the car to a guy who specializes in engine repair and trouble codes. He ran lots of tests and used wave patterns and long story short he found 2 of the fuel injectors were pretty much clogged. He also recommended a piston top soak. He cleaned all 4 injectors and did the soak and the lean code is gone. LTFT is around 3% now and the car runs like a top.
