Codes P0171 & P0174 - Lexus IS Forum
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post #1 of 10 (permalink) Old 05-31-2019, 12:58 PM Thread Starter
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Codes P0171 & P0174

I have codes P0171 and P0174 and have had them for about a month now. When i first got the code i thought it might have been a dirty maf sensor so i cleaned out my air filter as well as my maf with electronic cleaner, CEL was still there. I then decided that my maf must be bad so i ordered a nee maf and installed it, CEL remained. I then checked all my vacuum lines to makes sure there wasn’t a leak anywhere, nothing was cracked and all hoses were in good condition. Does anyone have any idea what else could be causing this?
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post #2 of 10 (permalink) Old 05-31-2019, 02:02 PM
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1 - Fuel pressure.

2 - Vacuum leak

3 - Look at the data from the MAF that the computer sees.

If you look the codes up on the manual, they'll show a trouble-shooting procedure. I'd get a can of carb cleaner and slowly spray around all the vacuum lines and intake manifold. If you hear the engine change RPM's when you spray a specific location, that's probably where the air leak is.

I'd probably experiment with pinching off vacuum lines too.
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post #3 of 10 (permalink) Old 05-31-2019, 02:32 PM
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I have codes P0171 and P0174 and have had them for about a month now. When i first got the code i thought it might have been a dirty maf sensor so i cleaned out my air filter as well as my maf with electronic cleaner, CEL was still there. I then decided that my maf must be bad so i ordered a nee maf and installed it, CEL remained. I then checked all my vacuum lines to makes sure there wasn’t a leak anywhere, nothing was cracked and all hoses were in good condition. Does anyone have any idea what else could be causing this?
There are vacuum lines under th intake that you cannot see easy. If it was the Fuel pressure or Maf. You would have more codes.

Careful & Consistent
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post #4 of 10 (permalink) Old 05-31-2019, 04:21 PM Thread Starter
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There are vacuum lines under th intake that you cannot see easy. If it was the Fuel pressure or Maf. You would have more codes.
Do you mean under the intake manifold? Should i remove the intake manifold in order to check these vacuum lines?
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post #5 of 10 (permalink) Old 05-31-2019, 04:44 PM
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Originally Posted by 01 SolarYellow View Post
There are vacuum lines under th intake that you cannot see easy. If it was the Fuel pressure or Maf. You would have more codes.
Do you mean under the intake manifold? Should i remove the intake manifold in order to check these vacuum lines?
I'd try a smoke test before I went thru effort of pulling the manifold. Check YouTube, lots of tutorials on DIYing a cheap smoke generator for this purpose.
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post #6 of 10 (permalink) Old 05-31-2019, 07:23 PM
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If it was the Fuel pressure or Maf. You would have more codes.
Like? If you disconnect the MAF, you'll get temp sensor codes but if the MAF part is inaccurate but working, it might not code at all.

I agree that it could be inside the manifold lines. The vacuum operated flap on the back of the intake is suspect too. I saw a post where a different Lexus was leaking vacuum at the shaft that went into the intake manifold.

My fuel pump was worn out, didn't have consistent pressure but it set no codes because it was working just barely enough that the ECU could compensate. The IS uses a fixed 55ish PSI fuel pressure. If the pressure is low, it'll run lean. The ECU has no way to detect that the pressure is low. There's no fuel pressure sensor on one of these cars.
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post #7 of 10 (permalink) Old 06-01-2019, 06:14 AM
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Do you mean under the intake manifold? Should i remove the intake manifold in order to check these vacuum lines?
Yes, But a smoke test would be easy and should confirm the issue.

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post #8 of 10 (permalink) Old 06-02-2019, 06:48 PM Thread Starter
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If it was the Fuel pressure or Maf. You would have more codes.
Like? If you disconnect the MAF, you'll get temp sensor codes but if the MAF part is inaccurate but working, it might not code at all.

I agree that it could be inside the manifold lines. The vacuum operated flap on the back of the intake is suspect too. I saw a post where a different Lexus was leaking vacuum at the shaft that went into the intake manifold.

My fuel pump was worn out, didn't have consistent pressure but it set no codes because it was working just barely enough that the ECU could compensate. The IS uses a fixed 55ish PSI fuel pressure. If the pressure is low, it'll run lean. The ECU has no way to detect that the pressure is low. There's no fuel pressure sensor on one of these cars.
I ordered a new AEM E85 340lph fuel pump a couple days ago as i plan to boost the car this summer. I’ll install that and see if the codes go away. I also ran some Fuel system cleaner in the tank and took it out for some 0-60 pulls to see if maybe the injectors were mucked up (since god knows when the last time the previous owner has done that). If that doesn’t fix anything then I’ll just replace all the vacuum hoses as the hoses are cheap and since my car is almost 20 years old the rubber on all of them is probably getting pretty dried out anyways.
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post #9 of 10 (permalink) Old 06-08-2019, 03:33 PM Thread Starter
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Do you mean under the intake manifold? Should i remove the intake manifold in order to check these vacuum lines?
Yes, But a smoke test would be easy and should confirm the issue.
I did a smoke test and the system held pressure perfectly,I saw no visible leaks and my smoke machine didn't let out any pressure at all. I will be installing my fuel pump tomorrow, hopefully that fixes it.
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post #10 of 10 (permalink) Old 06-08-2019, 11:00 PM
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Had those codes replaced pcv and it got rid of it
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