PCM Codes P0171 and P0174
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  • 1 Post By Submariner409
Northstar Engines and System Technical Discussion Discussion, PCM Codes P0171 and P0174 in Cadillac Engine Technical Discussion; I have been reading a little about these codes. I currently have them both as current codes.. My car is ...
  1. #1
    postman2000's Avatar
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    PCM Codes P0171 and P0174

    I have been reading a little about these codes. I currently have them both as current codes.. My car is a 2000 Deville..Does anybody know what other problems beside the plenum gasket would throw those two codes together..Is the bottom of the plenum able to be inspected with a mirror or is it real tight in there? Its been raining for a couple days here..have'nt been able to look at it yet..lol.. Thanks guys..

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    edb150 is offline Cadillac Owners Member
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    Re: PCM Codes P0171 and P0174

    Check the coupler between the the plastic intake plennum and the throttle body its prone to leaking . Spray a small amt of carb cleaner around the mating surface and underneath it to check for leaks . Do the same at the intake gasket. Check the plastic pcv tube where it goes thru the intake plennum behind the power steering pump.

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    Re: PCM Codes P0171 and P0174

    No I don't think you can see it. Follow edb150's advice and flex the rubber plenum a little with your finger to open the rip (if there is one) while spraying. Those two codes almost always lead to a ripped plenum.

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    Re: PCM Codes P0171 and P0174

    Thanks for your help guys..I tried to look under the plenum with a mirror today and i coudnt see much..A few local car guys swear that its the mass air flow sensor..Im skeptical of that though because i unplugged the MAF sensor and noticed a considerable difference in the idle..And it then threw a P0102 code..What Rpm do these cars normally idle at?

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    Re: PCM Codes P0171 and P0174

    Quote Originally Posted by postman2000 View Post
    Thanks for your help guys..I tried to look under the plenum with a mirror today and i coudnt see much..A few local car guys swear that its the mass air flow sensor..Im skeptical of that though because i unplugged the MAF sensor and noticed a considerable difference in the idle..And it then threw a P0102 code..What Rpm do these cars normally idle at?
    when my plenum was cracked and i was getting the ses codes and ruff idle and crappy running
    i wen tto auto zone to have them pull my codes this was the 3rd day of ownership had no idea i could pull my own codes
    the guys there swore it was the MAF so they sold it to me let me use some tools i put it in they came out tried to clear the codes they where still there

    then they wanted to insist it was maybe a bad MAF so then they replaced that one with another one and same problem
    when i finally insisted ok im going to take it to a mechanic please just give ma refund and you can have this bad
    they said we can't because now it is used we can't sell it as new
    dam bastards well finaly after arguing and making a big stink i was ready to call the cops into it just to have on my side a police report in case i decided to go to court over the crap
    then they finaly said ok we will just refund it
    lol

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    Re: PCM Codes P0171 and P0174

    Quote Originally Posted by postman2000 View Post
    Thanks for your help guys..I tried to look under the plenum with a mirror today and i coudnt see much..A few local car guys swear that its the mass air flow sensor..Im skeptical of that though because i unplugged the MAF sensor and noticed a considerable difference in the idle..And it then threw a P0102 code..What Rpm do these cars normally idle at?
    I would all but guarantee and put money on the plenum. Idle speed is 650 at operating temperature.

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    Re: PCM Codes P0171 and P0174

    Has anyone on the forum ever had both of these codes and it not be the plenum? Also should the idle speed be 650 also in gear with foot on the brake? Is the ripped plenum not visible sometimes until disassembly? I was messing with the plenum and didnt see a rip, maybe its under the hose clamp..Thank you guys for all your advice..

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    Re: PCM Codes P0171 and P0174

    Has anyone on the forum ever had both of these codes and it not be the plenum?
    Not sure


    Also should the idle speed be 650 also in gear with foot on the brake?
    Once up to operating temperature the PCM will keep it at 650 at all times. In gear, in neutral, A/C on, A/C off.


    Is the ripped plenum not visible sometimes until disassembly?
    It usually rips on the bottom and is rarely visible until disassembled. See post #2 & #3 to test it.

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    Re: PCM Codes P0171 and P0174

    Thanks Ranger..Im gonna try to check it out if it ever stops raining here...

  11. #10
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    Re: PCM Codes P0171 and P0174

    Click on my username, get to my profile and open the two 3-page picture albums. There are many pictures and diagrams of the intake manifold, throttlebody, plenum (connector), MAF, and other ideas.
    Mickeyblu likes this.

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