Need help replacing PCV Valve (p0171 & p0174)
02-20-23, 03:56 PM
#1
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Need help replacing PCV Valve (p0171 & p0174)
Somehow I keep getting both these errors and replaced one sensor, cleaned maf sensor, no hose leaks that Insee and somehow t code always happens. I have an inspection in 4 days due and wanted a quick solution. My mechanic found no issues with my car too. I had him replace the bad sensor and swap timing belt since it was due. I am confused, no help on the other end and been having this as an ongoing issue but I am not sure how much more money I need to put in this car.
My car is an 2004 Lexus ES330 for reference.
My car is an 2004 Lexus ES330 for reference.
Last edited by bodack; 02-20-23 at 04:01 PM.
02-20-23, 05:21 PM
#2
Pole Position
Hello,
Please check Engine Live Data and share Long Term Fuel Trims for both heads.
Lean mixture can be caused by a variety of problems, you will have to check them each one-by-one.
The most common reason is the Vacuum Leak, and considering that you have a Lean code for both banks, I am inclined to beieve that it is the case here as well. To check it properly, you will need a smoke machine, but a quick and dirty way would be to start up the car and block the intake, the engine should cut out almost instantly, if it hesitates or you hear distinct hissing sound, there is a vacuum leak somewhere.
While I am not recommending this method, but one way to localize the leak would be to spray the engine with something flammable, like a brake cleaner, and listen for a response from the engine. It can be just as effective as it is dangerous, so try on your own risk.
Other indirect way to check for a vacuum leak is to look at Engine Live Data, check the MAF sensor readout, it should be about 3-4g/sec, if it is lower, either the sensor is going bad, or there is a vacuum leak causing less air to go through the air filter as a main restriction. I would suggest verifying that the intake is tight before checking the MAF readout, such that you can diagnose the MAF sensor along with the leaks.
Next would be to check the A/F sensor operation, here you can find a thread on how to do it, but in short, you need to look at Engine Live Data, check the Upstream A/F sensor readouts, they should be close to 3.2-ish volts, while not stuck at that value. While looking at the readouts, quickly floor and release the accelerator pedal, you should see the A/F sensor respond almost immediately going to the Rich side, if there is a delay or no response at all, you will need a new sensor.
Contrary to popular opinion, PCV valve itself doesn't cause a vacuum leak, it's rather the hoses and other seals that give troubles when they start getting rigid and brittle from heat and engine oil. The PCV valve is there only to prevent the intake pressure from blowing back into the system at higher RPM, when there is no vacuum in the intake, it's especially important in turbocharged cars where more often than not, there is pressure in the intake instead of vacuum. All that is to say that even though you checked it, you shouldn't disregard the PCV system as a potential source of vacuum leak, just not exactly in the places you may think.
Hope this helps and best of luck!
Please check Engine Live Data and share Long Term Fuel Trims for both heads.
Lean mixture can be caused by a variety of problems, you will have to check them each one-by-one.
The most common reason is the Vacuum Leak, and considering that you have a Lean code for both banks, I am inclined to beieve that it is the case here as well. To check it properly, you will need a smoke machine, but a quick and dirty way would be to start up the car and block the intake, the engine should cut out almost instantly, if it hesitates or you hear distinct hissing sound, there is a vacuum leak somewhere.
While I am not recommending this method, but one way to localize the leak would be to spray the engine with something flammable, like a brake cleaner, and listen for a response from the engine. It can be just as effective as it is dangerous, so try on your own risk.
Other indirect way to check for a vacuum leak is to look at Engine Live Data, check the MAF sensor readout, it should be about 3-4g/sec, if it is lower, either the sensor is going bad, or there is a vacuum leak causing less air to go through the air filter as a main restriction. I would suggest verifying that the intake is tight before checking the MAF readout, such that you can diagnose the MAF sensor along with the leaks.
Next would be to check the A/F sensor operation, here you can find a thread on how to do it, but in short, you need to look at Engine Live Data, check the Upstream A/F sensor readouts, they should be close to 3.2-ish volts, while not stuck at that value. While looking at the readouts, quickly floor and release the accelerator pedal, you should see the A/F sensor respond almost immediately going to the Rich side, if there is a delay or no response at all, you will need a new sensor.
Contrary to popular opinion, PCV valve itself doesn't cause a vacuum leak, it's rather the hoses and other seals that give troubles when they start getting rigid and brittle from heat and engine oil. The PCV valve is there only to prevent the intake pressure from blowing back into the system at higher RPM, when there is no vacuum in the intake, it's especially important in turbocharged cars where more often than not, there is pressure in the intake instead of vacuum. All that is to say that even though you checked it, you shouldn't disregard the PCV system as a potential source of vacuum leak, just not exactly in the places you may think.
Hope this helps and best of luck!
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