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Discussion Starter #1
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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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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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.
 

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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?
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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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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Discussion Starter #8
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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Discussion Starter #9
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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Discussion Starter #11
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 wasn’t able to find anything. I replaced a couple of my vac lines anyways because they were brittle and hard as plastic. I also replaced my pcv valve. Engine codes still persists and recently I've also gotten a random misfire code (P0300). That might just be my spark plugs though since I'm at 122k and i was supposed to replace them as a 120k service.
 

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I did a smoke test and wasn’t able to find anything. I replaced a couple of my vac lines anyways because they were brittle and hard as plastic. I also replaced my pcv valve. Engine codes still persists and recently I've also gotten a random misfire code (P0300). That might just be my spark plugs though since I'm at 122k and i was supposed to replace them as a 120k service.
Okay next step. With the engine running. Take some carburetor cleaner spray. Spray all the hoses and then the gaskets on the intake. If the idle changes. That is the leaking area.
 

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Discussion Starter #13
I did a smoke test and wasn’t able to find anything. I replaced a couple of my vac lines anyways because they were brittle and hard as plastic. I also replaced my pcv valve. Engine codes still persists and recently I've also gotten a random misfire code (P0300). That might just be my spark plugs though since I'm at 122k and i was supposed to replace them as a 120k service.
Okay next step. With the engine running. Take some carburetor cleaner spray. Spray all the hoses and then the gaskets on the intake. If the idle changes. That is the leaking area.
I believe i found the issue, a hose clamp going from my valve breather to my intake wasn’t tightened all the way and air was escaping in after the maf but before vacuum. So far no engine codes on about 40mi. All i have left is P0300 but like i said I'm hoping new spark plugs can fix that.
 

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Discussion Starter #14
I did a smoke test and wasn’t able to find anything. I replaced a couple of my vac lines anyways because they were brittle and hard as plastic. I also replaced my pcv valve. Engine codes still persists and recently I've also gotten a random misfire code (P0300). That might just be my spark plugs though since I'm at 122k and i was supposed to replace them as a 120k service.
Okay next step. With the engine running. Take some carburetor cleaner spray. Spray all the hoses and then the gaskets on the intake. If the idle changes. That is the leaking area.
Nevermind, the code came back after around 15mi. I sprayed starting fluid all around the hoses and intake gaskets, nothing. Didn’t find any leaks. Also changed my spark plugs and wires and still have misfire code. Any more suggestions?
 

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Nevermind, the code came back after around 15mi. I sprayed starting fluid all around the hoses and intake gaskets, nothing. Didn’t find any leaks. Also changed my spark plugs and wires and still have misfire code. Any more suggestions?
It must be a hose you are missing. I would systematically inspect or replace each hose.
 
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