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Okay a P0171 is “system too Lean (fuel trim)”
This means that the system has added as much fuel as allowed to attempt to correct a lean condition that it sees and has been unable to.
Now this could mean many different things. You could have a vacuum leak or a bad fuel pump or a MAP sensor or a bad ECT sensor or a clogged injector (or wiring issue for an injector) or a O2 that is reporting the wrong information to the system.
The steps in the Toyota manual are to verify the output of the O2 sensor than check fuel pressure than check injectors than check the MAP sensor and engine coolant temp sensor than check the secondary ignition system and last check for fuel leaks.
I removed the plug to the IACV and than plug it back in the code goes away. Then a week later, it comes back. Did the same thing, and the code goes away. Is it the plug that is loose from the valve? Or is it a dirty valve causing intermittent CEL? The starting is fine.
Should I replace the throttle body gasket and clean the IACV at the same time? Where can I find that 5 point torx bit cheap?
I have seen several Toyotas with intake manifold gasket leaks. They mostly only leak when the engine is not fully heated up. You could try spraying some throttle body cleaner around where the manifold meets the cylinder head to see if the engine idles differently.
I usually find the problem with a smoke machine.
It could be stuck causing a lean code, but I doubt it. Unplugging and plugging it back in is not really a test to see if it is bad and if I remember right Toyota IACVs are pretty pricey. What you really need is a scope to see if the IACV is seeing a proper duty cycle and responding properly. When you unplug and re-plug the IACV you force the system to do a re-learn for idle control and often the ECM will ignore codes until the re-learn and monitor are complete. So by unplugging ang re-plugging sensors you might be “resetting” the system.
Remember a code P0171 is NOT a bad IACV code. It is a “system to lean (fuel trim maxed)” code.
Check for vacuum leaks around the engine… The intakes manifolds on the 2003+ had issues with leaking because they switched to plastic that year, but the 8th Gen (1998-2002) intakes are usually pretty reliable.
I don’t see many issues with the IACV in this model year as well…
If you have a scanner, check the upstream O2 sensor readings…. These 1.8 1ZZ-FE engines were known for oil burning and that can wreak havoc with the O2 sensor… If you ever replace an O2 sensors on these vehicles, use ONLY DENSO brand.
The other suggestions are pretty good… Check fuel pressure (there is no port, so I had to make my own tester by using old pieces of fuel rail to clip into where the fuel line attaches to the fuel rail)…
Also, FYI there is not an easily serviceable fuel filter on this vehicle, there is one in the fuel pump assembly in the tank…. Luckily, the fuel pump is easily accessible under the drivers side rear seat (just pull the rear seat up)….
I have owned three Gen 8 Corollas, and they are very simple to work on… My suggestion…
Hook a scanner up to it and read the what the engine coolant sensor is telling the engine, also check how well your B1S1 O2 sensor is working (upstream sensor)…
Check for vacuum leaks (best done with smoke)
Check fuel pressure
Check fuel injectors (may just need a cleaning)… I have seen cracked injectors on this model as well…
Check around the exhaust manifold and upstream O2 sensor location, Any air is getting in/leaking prior to the upstream O2 sensor can cause this code…
Was there any work done recently to the vehicle or anything unusual before this code was thrown? Do you have to add oil on a regular basis to this car? Since you have the VE (Value Edition), Hopefully you do not have a three speed automatic in this, they run the engine at higher RPMS and are more prone to the oil consumption problem than the 4 speed autos or 5 speed manuals….
Cheers,
-Karl
Unfortunately, it’s a 3 speed. I do have to add oil to it every now and then but not very much. I am going to take it to a shop and see what they say and go from there. I don’t have the equipment nor the time right now to do much diagnostic tests.
I’ve changed my MAF sensor and it fixed my high idle problem. My STFT is near 0 but my LTFT is 30 and on rev at 2k RPM it increases. I have checked for any vacuum leaks and its all good. My guess is my fuel filter is ready for a change or the fuel pump assembly is ready for a change.
[quote=”joh17″ post=86527]I’ve changed my MAF sensor and it fixed my high idle problem. My STFT is near 0 but my LTFT is 30 and on rev at 2k RPM it increases. I have checked for any vacuum leaks and its all good. My guess is my fuel filter is ready for a change or the fuel pump assembly is ready for a change.[/quote]
What year is your Corolla?
Since you say you have a MAF it has to be a 2000 year model or newer. I very rarely find that the fuel filters needs changed… The fuel filters are in the tank. I would definitely do a fuel pressure test before I replaced a fuel pump on a Corolla.
If yours is a 2003 (Gen 9) or newer, the pattern failure that caused a P0171 was the intake manifold gasket…
-Karl
It’s 1999, 1999 or older fuel filter is in engine bay driver side bottom of firewall. And it has no maf so lean code has to be set by fuel delivery issue or some other sensor.
[quote=”valde” post=86547]It’s 1999, 1999 or older fuel filter is in engine bay driver side bottom of firewall. And it has no maf so lean code has to be set by fuel delivery issue or some other sensor.[/quote]
I was answering the previous posters question (joh17) who says he has a MAF (not the original Poster)….
(Valde) Where are you located in the world? The 1999 Corolla (at least the ones I have seen in North America) does not have an OEM fuel filter in the engine compartment… They have a fuel sock on the fuel pump and a fuel filter cartridge on the fuel pump (which practically never needs replacing)
I have personally owned and rebuilt three Generation 8 Corollas (One 1998, 1999 and a 2000) and have done engine swaps at least ten times on different Gen 8 Corollas and there was never an OEM engine bay fuel filter…
If you have an OEM fuel filter in the engine bay on a 1999 Corolla I would love to see a picture of it…
The sensor which in turn throws the P0171 code is the upstream O2 sensor. This is caused by detecting too much O2 in the exhaust and the ECM increases the fuel trims accordingly, until LTFT gets 25% or above and the ECU sets the P0171 code. Interestingly enough, The Toyota Factory manual considers less than 25% LTFT to be within Spec. Most healthy Gen 8 Corollas I have serviced usually have an LTFT of less than 10%…
Yes, you are correct about the MAF. The 1999 does not have a MAF, they didn’t start putting MAFs on Corollas until 2000 when they switched to the VVT version of the 1.8….
Most of the time a P0171 on a 2000-2002 Corolla engine is a dirty/Bad MAF sensor… The next suspect is a vacuum leak… Gen 9 Corolla (2003-2008) P0171’s are usually caused by either the MAF again or a vacuum leak at the intake manifold (they switched to plastic intakes that year)… When a new MAF is installed and the code remains it is usually fixed by a new Intake manifold gasket. FWIW I have also seen this code caused by non OEM air intakes…
While a P0171 can be caused by a fuel delivery issue, I have not seen that to be an issue with these Corollas… The bad part about fuel pressure testing these Corollas is that there is no test port… I had to make an in-line tester using pieces of an old fuel rail to clip into the male and female ends of the fuel line where it clips into the fuel rail….
Another thing that can cause this is a bad Engine Coolant Temperature sensor. Check with scanner…
The 2000+ have an Intake Air Temperature Sensor in the MAF.
If an Intake Air Temp Sensor is off more that 10 degrees from ambient air temperature you may have issues.. check with scanner as well…
Lastly… Only use Denso O2 sensors on these cars… Bosch O2s are known for causing exactly this type of issue… The same with MAFs… I know they are expensive, but Denso is the only way to go IMHO…
-Karl
[quote=”ToyotaKarl” post=86548](Valde) Where are you located in the world? The 1999 Corolla (at least the ones I have seen in North America) does not have an OEM fuel filter in the engine compartment… They have a fuel sock on the fuel pump and a fuel filter cartridge on the fuel pump (which practically never needs replacing)
I have personally owned and rebuilt three Generation 8 Corollas (One 1998, 1999 and a 2000) and have done engine swaps at least ten times on different Gen 8 Corollas and there was never an OEM engine bay fuel filter…
If you have an OEM fuel filter in the engine bay on a 1999 Corolla I would love to see a picture of it…
[/quote]I live in Finland, Europe. European versions have an OEM fuel filter in the engine compartment. Here filter practically newer need replacement either. Here is replacement video of filter http://www.youtube.com/watch?v=Uar7z8XalGQ
I own a 2000 Corolla. I’ve replaced my MAF sensor which fixed the high idling problem. I’ve changed my intake and throttle body gasket because I was already in the process of cleaning all the gunk out. So far no vacuum leak. If I do a fuel pressure test what is the normal range for its reading?
Thank you very much.
-John
I believe the fuel pressure should be around 50 psi…
Question for you… did you reset the ECU (disconnect battery for 2 to 3 min) to reset factory fuel trims for the ECU to relearn the fuel trims after you installed the new MAF?
Karl
No I did not reset by pulling the battery. I used a scanner to reset my check engine light after I replaced my gaskets. Am I missing something here?
-John
Yes, you can reset the code, but it may take a while for the fuel trims to relearn….they will relearn, but you have to drive it a bit..
I recommend reset the ECU and drive about 20 minutes. You can even keep the scanner going and check the live data… hopefully your fuel trims will drop within acceptable ranges and not trigger the P0171… Worth a try.. if not its back to diagnosis.. with ltft around 30 I really think you could have or had vac leak… poss bad O2… fuel pressure would be my last check (because it is not easy without the proper tools). It is possible the new MAF will fix it and bring the ltft back in line…
Karl
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