2000 Pontiac Grand Prix PO171 Code

malibutroublemalibutrouble Member Posts: 19
edited June 2017 in Pontiac
I bought this 2000 Pontiac Grand Prix about a year ago with 200k miles on it. I knew it was going to need work. So far, Ive done little minor things to it. Replaced a rotted sway bar, fuel filter, a broken driver side power window, and given it an oil change to name a few. I have a new EGR valve to put in it but, Im stuck on a PO171 trouble code right now. It wasnt until I had it taken to a mechanic that I started having big issues with the car. I tried to clean the throttle body about a month and a half ago and barley bumped a fuel line when the fuel line snapped. I had put some JB Weld type putty around the line to do me just until I got the car to the shop. That worked. Drove it up to the shop with no problems. I explained to them what had happened and told them the line with putty around it would be the one needing replaced. I got the car back, got it home, noticed white smoke flowing out from under the hood, lifted the hood, and seen that the return line had been replaced and not the sending line as I had talked to them about. After an understanding phone conversation with the mechanic, we chalked it up to a miscommunication issue and they agreed to fix their broken part for free. It was just an aftermarket line and the guy who worked on it missed the clip that locks the bell over the line when the line "engaged". The puttied line (even after all the trouble with the return line) still had putty on it and was still working decent. It began to leak very tiny amounts and I decided to fix that line myself. I believe it was another aftermarket line that I bought, got it installed by clamping it down to the other side of the line. It works fantastic now. No leaks. That was roughly a month ago and Ive had no problems out of it since. Again, Ive had the car for almost a year now and Ive put a good 20k miles on it. Never, since the day I bought it, has the CEL come on until two days ago when I was driving it. Plugged it up to my ODB and got a PO171 code. System too lean. After talking around with a few skilled mechanics, Ive heard that the repair amount for a shop to do it is a little out of my price range and since Im not the worst person to ever work on a car, Ive decided to do the repairs myself and save a little money. After doing a little research, I found that the main two issues with a PO171 code is either a vacuum leak or a faulty MAF sensor. The MAF sensor seems to be doing its job so, Ive moved on to a vacuum leak. I installed a new thermostat in it a couple weeks ago and noticed a plug that was hissing while the car was running and unplugged it. The car stalled out immediately after unplugging it. Looking at the position of the plug and why it was hissing, I would say it was some sort of "air valve". This plug is located on the throttle body assembly, just below the air control valve. It has two tubes coming from the boot, one to the fuel pressure regulator (I believe) and one going horizontally straight across the engine into another little box (Im not sure what that box is but, it has a sensor on it). Judging by the line coming out of the other end of the box, I would say its something to do with the EVAP system as the line coming out has a green twist top in the middle that says "EVAP" in small letters on it. After inspecting for a vacuum leak, I noticed that the boot that connects to the throttle body assembly is split from the top to the bottom and fits in very loosely. Where one line connects into the "EVAP black box", its bent and looks as if it may have a small crack or two in it at that bend.

My question is: What is that plug? What is that box called exactly? What does that entire system of tubes do? Could those cracks and splits be whats causing my PO171 code?

To add, these are my ODB's results after 5 minuets of idle:


CALC LOAD (%)- 1.57
ECT- 194
STFT B1 (%)- 20.31
LTFT B1 (5)- 16.41
MAP (INHG)- 11.22
ENG RPM- 691
MAF (LB/MIN)- 0.42
TPS (%)- 0.00

Ive heard that the STFT and LTFT results are off and may indicate a vacuum leak. Im not sure what CALC LOAD (%) should be at or why people keep asking me if thats what I really got like they are surprised either. And Im not sure wy FUEL SYS 1 says CL but, FUEL SYS 2 says CL-FAULT either.

The car starts just fine with no hesitation, Ive tried not to drive it with that trouble code stored because Ive heard that too much driving on it could hurt the engine and I want to get the most out of this engine before it completely breaks down.

Any help would be much appreciated at this point! Thanks!

Best Answers


  • malibutroublemalibutrouble Member Posts: 19

    To to be more clear, the plug right under my IAC valve is the part in question.
  • imidazol97imidazol97 Crossroads of America I70 & I75 Member Posts: 25,917

    I see there's one tube coming from the throttle body that the block of rubber fits onto. You can pull it off and look into the end of the tube but I suspect it's just a splitter so that the vacuum goes to the purge valve and the fuel pressure control valve.

    I believe that system of tubes is sold as one piece. I did some patching on mine where the soft rubber has split. I put a shrink tubing piece over the "U" as the end close to the purge valve and let it do the sealing. I'd suggest looking at a pick and pull to see if you can get replacement pieces. Many of the 3800 engines from GP to Pontiacs to Buicks to Riveras to Oldsmobiles may use the same piece.

    The block of rubber next to the throttle body might be sealed up by tape or by "caulking" the cracks with PL polyurethane sealer, caulk, adhesive.

    2014 Malibu 2LT, 2015 Cruze 2LT,

  • malibutroublemalibutrouble Member Posts: 19
    Could that piece be what threw off my P0171 code? It is split from top to bottom, the plug piece is.
  • thecardoc3thecardoc3 Member Posts: 5,463
    First change the units on your tool so that the MAF reads in the more traditional grams per second. But for now
    applying a little math, .42 lbs/min is equal to 3.1grams/sec

    A rule of thumb commonly used is that an engine should pump about 1gram of air per second, per liter of displacement at 600rpm.

    So at 600rpm your MAF reading if you changed the units should be pumping 3.8 to 4 grams of air per second.
    At 691 that number should move to approximately 4.3 grams/sec. That means the value is about 72% of what would be expected.

    Now lets have some fun with the fuel trim values.

    According to the freeze frame your system is adding 20.3% short term and 16.4% long term. Keep in mind the short term number is a snapshot of a moving value when the system is in closed loop. That could be the high, mid or even the low point of the correction. But for that moment in time your total trim correction would be the sum of the short and long term which is 36.7% correction.

    When the MAF error and the average fuel trim correction are roughly equal then false air (a vacuum leak) becomes one of the possible suspects. At the same time, while a vacuum leak is plausible it only accounts for about 2/3rds of the error from this single moment in time and it's not the only possible cause of such an error. Now if the short term capture was a low point and goes higher then that would get closer.

    BTW the Calculated Load data is a direct result of the air that is being measured entering the engine. The higher the airflow, the greater the load. If the MAF is under reporting the airflow for any reason then the calculated load will be lower than expected.
  • malibutroublemalibutrouble Member Posts: 19
    So, could it be two bad things at one time? A vacuum leak and a bad MAF sensor?
  • malibutroublemalibutrouble Member Posts: 19
    Is there any other place besides Amazon to buy the part? Ive been searching for it and I cant find anything. I paid Amazon (or GM America Parts) $30 for it and I never got it. They are saying it was delivered Thursday and it is currently Saturday...
  • imidazol97imidazol97 Crossroads of America I70 & I75 Member Posts: 25,917
    edited June 2017
    Here's an Ebay source. Shipping will take longer unless you're close to Arizona where the parts are.

    Was the part from Amazon shipped from Amazon or from a third party? Have you contacted Amazon? Was the address exactly right on the order?

    IT shouldn't be your problem to wait for the third party to decide to ship again if it came from a third party. I always like it when items are shipped by Amazon from one of their warehouses.

    I'd have them ship another while they investigate the claim for the first part not being delivered.

    My son just had FedEx deliver a clothing item and the box was not there when he got home. It's an area where it would be hard for FedEx to make a mistake on the address. USPS and UPS both leave packages at the apartment leasing office instead of at doorstep. But his is obscure and across from the rental office.

    The shipper (Brooks Brothers) shipped him a replacement while they're settling the claim and FedEx looks for it.

    Search for the part # 24506671. Myself, I'd see what they wanted for it at my local Pontiac dealer (Chev, Buick, Cadillac). Amazon shipping is 2-day (I pay for the Prime and order lots through the year so it pays for itself).

    2014 Malibu 2LT, 2015 Cruze 2LT,

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