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P0171 and P0174

This is a discussion on P0171 and P0174 within the General Help forums, part of the LSx Technical Help Section category; So here's my story... it's sad but true.... bout a girl that i once knew.... she took my love and ...

  1. #1
    Chief of his tribe! LSCyaL8R's Avatar
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    P0171 and P0174

    So here's my story... it's sad but true.... bout a girl that i once knew.... she took my love and ran it around.

    ok no not really.

    anyway.... I'm throwing these codes currently and i'm at my wits end trying to figure out what they might be caused by.

    I'll provide a recap of how i got to this point to help you all out.

    I took my car on a LONG trip to TN. since i'd be driving a long distance i figured hey! great time to run a couple tanks of fuel system cleaner! So i did that on the way down and no sooner did i arrive then my engine light came on.

    Got the code read and it was the air system. Didn't want to fiddle with it down there so I bought a check valve and tossed it in the hatch for installation when I got back to NY.

    I got home swapped out the check valve flashed the code and installed the drivers side spark plugs. Was going to do the passenger side the following week.

    after installing the plugs i wanted to see if the car felt any.... better... different... whatever. upon leaving my driveway, with gusto naturally, the car did feel MUCH peppier! (stock plugs with 59k miles) but when i got to the next block the car idle dropped to nearly nothing and the car almost stalled.

    I drove home and asked my father what was up he couldn't tell because the idle had stabilized so i took him for a ride. car still felt like normal power no problems there.... but again when i went to come to a stop the idle stumbled down to nothing before bouncing around and sort of stabilizing.

    We get home and POOF service engine soon. check the codes.... P0171 P0174

    I fought these codes for two years on my Mark VIII and i'm not looking forward to it with this car. On that car it was the fuel pump but that car had no idle issues and no issues i could perceive from the drivers seat at all!!!

    In the time before i learned it was the pump i had changed the TB gasket, the PCV valve, the PCV hose assembly the MAF and had been told by numerous mechanics that it was my catalytic converters or an intake manifold crack..... So i've been through this before but am hoping that someone here may have good info on what the common cause is on an LS1.

    I've ruled out the check valve(and by relation the air system) as well as the plugs since i only changed the drivers side and both banks are coming up with the code... and the check valve was also the drivers side hence my confidence in ruling it out.

    I checked my air box and MAF assembly and all seems to be in order there. I DO need to check my PCV valve to TB connection as that was a replacement hose that may have come loose... that just crossed my mind this moment... I had forgotten about that hose... hmmm....

    Assuming that isn't the problem.... what non fuel pump items should i be inspecting?

  2. #2
    Moderator Cutlass's Avatar
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    1999 Formula WS6 M6-sold
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    low fuel pressure, bad MAF, loosely installed MAF, large vacuum leak

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    Chief of his tribe! LSCyaL8R's Avatar
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    I like the way you think..... large vacuum leak certainly sounds like it fits the bill. I checked out the car again today when i got home from work and here's how things went down....

    I erased the codes last time i drove the car so they were blank before i reached the car today. now stick with me.

    Step one: Looked for cracks or obvious sources for air entry into the intake path. PCV EGR etc. everything looks ok let's start the car.

    Step two: Started the car and immediately the car jumps into its stumbling rough terrible idle. LOUD vacuum noise. hmmm everything looked good let's look around and see if we can feel any leaks or track it by sound.... hmmm everything feels... OUCH!!! damn belt... sigh... yeah everything feels good... but... hmmm the sucking noise is actually coming from the lid... hmmm lid is tight. maf is seated.... the intake tube is sealed....

    Step three: listen to car in amazement.... what is this... the idle just jumped to like 1200-1500... omg the sucking noise.... it's going away!! and it's gone... and the idle is suddenly perfect!

    Step three: a step back.... the air pump just turned off aaaaannnndddd there's that terrible idle again... and listening... here comes that sucking vacuum noise again :-\

    Step four: surprise! the engine revs to the sky... prob like 2500-3000 annnnd the noise goes away and the idle returns to normal once more.

    Step five: read the new codes and go hmmm.... P0300 misfire detected eh? well i'd assume so considering the stumbling and lean running you were up to.

    This leaves me thinking only two possibilities exist here. Either the IAC or the EGR. What doesn't make sense to me is that this only happened after changing the plugs and AIR check valve on the drivers side of the car. But since that sucking sound is obviously being controlled by the ECU and since i didn't throw a lean code this time (even after driving several miles once the car warmed up) I can't think of anything else that it could be.....

    If it were the plug wires it wouldn't get better with warming the car up for only 3 minutes (the length of time it takes for the AIR pump to turn off) also if the wires were the issue then when i erased the new code it would have likely returned when i drove the car around.

    Thoughts suggestions? I think i'll start by trying to clean the IAC... if it's cleanable?

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    Chief of his tribe! LSCyaL8R's Avatar
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    Did some reading today. the symptoms i described above seem to go well with the idea that the EGR is to blame. I'll get to trying to clean it and see if that helps.

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    Well... here's me wishing I wore a diaper before opening this thread...

    I've been popping these two codes occasionally for the past 2-3 weeks now, but I'm not getting any symptoms you describe. Rarely, about 5 min after starting out from a cold engine, if I'm sitting at a light my idle starts to choke up and drop low... only stalled out once (long before any codes came up.) I really didn't concern myself with it since every car I've ever owned has stalled out like that once or twice a year when getting older (current z28 has 107k on stock.)

    That's what brought me to the forum tonight and straight to this thread...

    Doing a quick internet search on the p0171 & 0174 codes was leading me to think that the issue was simply the o2 sensor going bad... Then I got my hands on a mechanics code reader. I'll have to write down what he had, I think it was a Solaris brand? I just glanced at the name. As I looked up it's service recommendations in the on-board code library, it mentioned the fuel filter being a possibility.

    Mod will have to decide if this belongs in a different thread (same codes, but different symptoms.) But I was looking to find out some info on:

    a) fuel filter = Is a stock replacement fine, or will I see any benefit from a performance filter on a stock engine (internal mods needing full fuel system work is a long way off yet.)

    b) o2 sensor = What am I looking for here? I see 1, 2, 3, 4 wire and plug ins. I'm not sure which one I would need to buy for where.

    c) (more in line with this thread,) Is either one of these going to help or is it just tilting at windmills? Cutlass mentioned MAF, but I haven't changed out my lid yet (new one is waiting to get installed,) so that shouldn't be an issue.

  6. #6
    Chief of his tribe! LSCyaL8R's Avatar
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    i replaced my fuel filter 1 year ago. probably less than 10k on it. stock replacement is fine. considering i could HEAR the air being sucked into the engine and also considering that the amount of air seems to have been controlled at some point by the ECU i feel confident that either the IAC or EGR ( i think EGR) is moving slowly hence why it takes less than 2 minutes for the car to return to a normal stable idle. I will be cleaning them and reporting back what happens.

  7. #7
    Moderator Cutlass's Avatar
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    Quote Originally Posted by LSCyaL8R View Post
    I like the way you think..... large vacuum leak certainly sounds like it fits the bill. I checked out the car again today when i got home from work and here's how things went down....

    I erased the codes last time i drove the car so they were blank before i reached the car today. now stick with me.

    Step one: Looked for cracks or obvious sources for air entry into the intake path. PCV EGR etc. everything looks ok let's start the car.

    Step two: Started the car and immediately the car jumps into its stumbling rough terrible idle. LOUD vacuum noise. hmmm everything looked good let's look around and see if we can feel any leaks or track it by sound.... hmmm everything feels... OUCH!!! damn belt... sigh... yeah everything feels good... but... hmmm the sucking noise is actually coming from the lid... hmmm lid is tight. maf is seated.... the intake tube is sealed....

    Step three: listen to car in amazement.... what is this... the idle just jumped to like 1200-1500... omg the sucking noise.... it's going away!! and it's gone... and the idle is suddenly perfect!

    Step three: a step back.... the air pump just turned off aaaaannnndddd there's that terrible idle again... and listening... here comes that sucking vacuum noise again :-\

    Step four: surprise! the engine revs to the sky... prob like 2500-3000 annnnd the noise goes away and the idle returns to normal once more.

    Step five: read the new codes and go hmmm.... P0300 misfire detected eh? well i'd assume so considering the stumbling and lean running you were up to.

    This leaves me thinking only two possibilities exist here. Either the IAC or the EGR. What doesn't make sense to me is that this only happened after changing the plugs and AIR check valve on the drivers side of the car. But since that sucking sound is obviously being controlled by the ECU and since i didn't throw a lean code this time (even after driving several miles once the car warmed up) I can't think of anything else that it could be.....

    If it were the plug wires it wouldn't get better with warming the car up for only 3 minutes (the length of time it takes for the AIR pump to turn off) also if the wires were the issue then when i erased the new code it would have likely returned when i drove the car around.

    Thoughts suggestions? I think i'll start by trying to clean the IAC... if it's cleanable?
    some sucking noise through the lid at idle is normal. Mine does that. The EGR should be completely inactive at idle so it shouldn't be that unless there is something mechanically wrong with the EGR valve.
    IAC is a possiblity but I still leaning towards a vacuum leak.

  8. #8
    Moderator Cutlass's Avatar
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    Quote Originally Posted by TokerM View Post
    Well... here's me wishing I wore a diaper before opening this thread...

    I've been popping these two codes occasionally for the past 2-3 weeks now, but I'm not getting any symptoms you describe. Rarely, about 5 min after starting out from a cold engine, if I'm sitting at a light my idle starts to choke up and drop low... only stalled out once (long before any codes came up.) I really didn't concern myself with it since every car I've ever owned has stalled out like that once or twice a year when getting older (current z28 has 107k on stock.)

    That's what brought me to the forum tonight and straight to this thread...

    Doing a quick internet search on the p0171 & 0174 codes was leading me to think that the issue was simply the o2 sensor going bad... Then I got my hands on a mechanics code reader. I'll have to write down what he had, I think it was a Solaris brand? I just glanced at the name. As I looked up it's service recommendations in the on-board code library, it mentioned the fuel filter being a possibility.

    Mod will have to decide if this belongs in a different thread (same codes, but different symptoms.) But I was looking to find out some info on:

    a) fuel filter = Is a stock replacement fine, or will I see any benefit from a performance filter on a stock engine (internal mods needing full fuel system work is a long way off yet.)

    b) o2 sensor = What am I looking for here? I see 1, 2, 3, 4 wire and plug ins. I'm not sure which one I would need to buy for where.

    c) (more in line with this thread,) Is either one of these going to help or is it just tilting at windmills? Cutlass mentioned MAF, but I haven't changed out my lid yet (new one is waiting to get installed,) so that shouldn't be an issue.
    You situation is a little different then LSCyaL8R.
    Fuel filter is possible and a GOOD quality stock replacement is fine.
    If you need an O2 sensor, just go to the auto parts store and have them look up the replacement. That way you'll know its correct and will fit.
    Generally if you have both P0171 and P0174 together its not just an O2 sensor. It would have to be both O2 sensors on our cars. It more likely to be the MAF or low fuel pressure.

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    Chief of his tribe! LSCyaL8R's Avatar
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    oh trust me i know a sucking noise is normal..... this was most definitely beyond the normal sound.... couldn't hear anything over it until the ecu adjusted and it died down. I just bought a top notch bluetooth scan tool and have some excellent software on android. i'll take a peek tomorrow night. i'll have the tool by tomorrow afternoon. i'll set up a screen with iac and egr and secondary air info and duty cycles and see what's going on as it happens on warm up. then i'll check the misfire records and see what and where they are going down.

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    O U 8 1 2 Spaz's Avatar
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    maybe your air pump is going???

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    I am with Cutlass...vacuum leak. Spray around the intake and sealing surfaces with some brake cleaner and see if the engine idle changes. If it does, you found where your leak is.

    Your codes are for BOTH banks, which is almost always indicative of a vacuum leak.

    I had these codes on my 3800-II and the intake was leaking at the gasket.

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    Chief of his tribe! LSCyaL8R's Avatar
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    yes but the lean codes only came up while driving a cold engine.... starting it cold, with no codes, and letting it idle till codes come up results in P0300. random misfire. and the "vacuum leak" that i can hear.... goes away entirely.... twice!!! and after about 3 minutes of running... no knocks pings rough idle anything....

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    Moderator Cutlass's Avatar
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    You probably have run the car longer than a few minutes to reset the lean codes.

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    Chief of his tribe! LSCyaL8R's Avatar
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    or i used the little hand held reader to reset them

    yes i know it hasn't gone through all it's checks yet and the next time i start it well... aside from tonight to move it.... will be tomorrow night. and tomorrow i'll have the new scan tool and i'll be able to watch with greater detail what's going on in there.

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    Chief of his tribe! LSCyaL8R's Avatar
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    Update for those paying attention. I was just got my new scan tool in the mail today.... i had been looking forward to getting it to see just what was going on.... might not need it...

    So i needed to move the car last night. I started it... once again having had erased the codes the last time i drove it.

    Started right up. fell into a perfect idle... never stumbled or anything. no vacuum noise... nothing... sounds like something was stuck to me. must have loosened it up last time and all is well now. i'll keep my eyes on it and let you all know if anything new develops.

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    O U 8 1 2 Spaz's Avatar
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    Quote Originally Posted by LSCyaL8R View Post
    Update for those paying attention. I was just got my new scan tool in the mail today.... i had been looking forward to getting it to see just what was going on.... might not need it...

    So i needed to move the car last night. I started it... once again having had erased the codes the last time i drove it.

    Started right up. fell into a perfect idle... never stumbled or anything. no vacuum noise... nothing... sounds like something was stuck to me. must have loosened it up last time and all is well now. i'll keep my eyes on it and let you all know if anything new develops.
    shit like that worries me man... if it happens once it will happen again... next time might be worse than the first...

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    ^^Agree. usually "problems" just don't disappear on their own...I mean these are LS1's

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    Chief of his tribe! LSCyaL8R's Avatar
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    well it still adds fuel to my belief that the EGR is to blame.... getting sticky... stuck open... makes idle suck.... sllllooowwwwlllly closes and all is right in the world again.

    No puns were intended in the writing of that post....

    or were they!?!?!

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    Moderator Cutlass's Avatar
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    Have you considered deleting the EGR and AIR systems? Might simplify things a bit.

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    Chief of his tribe! LSCyaL8R's Avatar
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    heh heh while it may simplify things a bit mechanically it will complicate things a lot inspection wise.... gotta love NY for that. although my mechanic told me as long as he can plug in the state OBD reader and have nothing come up he doesn't care what i do to the car. so i guess as long as he is in business i'm all good in that sense. i'd actually love to do the delete in some ways. less weight.... less things to go wrong.... but i'm not sure if i'm getting rid of my cats.... so the EGR and AIR help with those don't they....

    On another note i finally took the car out for a nice drive new scanner all hooked up. First off... I LOVE THIS THING. I had an awesome screen showing me my manifold pressure, MAF flow, throttle opening, IAT, and even Horsepower!!! but i need to calibrate that screen... it was quite a bit off... showed peaks of around 400hp.... but that could be me accelerating while on a downward slope since it uses the weight of the vehicle and the OBD MPH readout to determine that.... i think....

    anyway.... yes it rocks but also i got the lean codes back. still can't hear anything sucking air under the hood. i'm going to have to smoke it out one of these days and see what happens.

    on another note. how much timing advance do these cars run typically? my car seems to be running between 20-40 degrees while driving. i didn't log any data so i can't peek at what it does at WOT.... Also can't get any readings from the knock sensors... methinks that the included PID's for them must be wrong. Although it's entirely possible it's user error on the new software. lol

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