PO171 (too lean) case

[vincenzo masiello]

2008 Grand Prix 3800 SE3

I drove the car more than usual last December and one day while I was going uphill on a highway when a sudden loss of power occurred that was temporary, and even felt like the brakes did not work or I just lost control. No codes.

I went straight to an expert (Ivan of PHAD who is a few miles from me) who test drove it “violently” and only noticed a lean condition even though he couldn’t see a fault code.

A few days later the light and code came on (PO171).

I don’t sense any serious symptoms but may be a barely noticeable loss of power, but if I reset the code the light comes back on in no time (a few miles of driving).

I studied some videos (especially an excellent 45min one by Doug50001) and I have a cheapo scanner and a Haynes book for this car and started investigating. I have in mind a goal of diagnosing myself for the first time this kind of tricky fault while staying as disgustingly dirty as possible because I am totally broke and if Eric of SMA and his daughter can do it, I can do it 🙂
and this is what I did so far:

1. checked all the vacuum ducts and hoses that I know of (couldn’t find a vacuum diagram online for an 08 that I could understand or a sticker under the hood) and did some water spraying at the most common places (manifold gaskets, around the throttle body, purge valve, injectors, etc) and hated scanner danner and his water bottle for a little while. No obvious leak. I’m working on a diy smoke machine that will be done soon to keep looking for any vacuum leaks.

2. I replaced the PCV valve and the air filter just because I haven’t replace those for over a year (probably have nothing to do with the issue)

3. I have an intermittent (almost rare) PO446 that comes once a year or so but I never investigated thinking that it’s probably just a gas tank not closed well after a fill or just getting old.

4. I tested the MAF by recording the g/s at equally spaced rpm readings and it was a straight line relationship with normal values (1-15 g/sec). So the sensor is fine but I cleaned it anyway with the appropriate spray anyway not long ago.

5. I drove around a little and started watching the trims and O2 sensor readings after warm up and closed loop an the O2 sensors where going up and down (0.1-.9) as they should (eliminating sensors and their wiring). The fuel trims where both crazy high (teens-30’s) as soon as I remove my foot from the gas pedal but go to close to zero as soon as I tough the pedal. That is almost always a vac leak indication, right?

6. I pulled the freeze frame of the PO171:

Freeze frame information:
——————
Fuel Status = 0 byte
Engine Load = 20.392 %
Engine Coolant Temperature = 86 °F
Fuel Trim Bank 1 Short Term = 10.938 %
Fuel Trim Bank 1 Long Term = 16.406 %
Intake Manifold Pressure = 4.931 psi
Engine RPM = 1,012.5 rpm
Speed (OBD) = 0 mph
Timing Advance = 11.5 °
Intake Air Temperature = 30.2 °F
Mass Air Flow Rate = 6.37 g/s
Throttle Position(Manifold) = 6.275 %
Air Status = 0 byte
Run time since engine start = 182 s
EGR Commanded = 0 %
EGR Error = 0 %
Fuel Level (From Engine ECU) = 40.392 %
Distance travelled since codes cleared = 20.505 miles
Evap System Vapour Pressure = 14.5 Pa
Barometric pressure (from vehicle) = 14.359 psi
Catalyst Temperature (Bank 1,Sensor 1) = 572 °F
Voltage (Control Module) = 14.265 V
Engine Load(Absolute) = 16.471 %
Commanded Equivalence Ratio(lambda) = 0.977
Relative Throttle Position = 0 %
Ambient air temp = 28.4 °F
Absolute Throttle Position B = 6.275 %
Accelerator PedalPosition D = 0 %
Accelerator PedalPosition E = 0.392 %

End of report.

7. I tested the brake booster check valve (by blowing through it) and it seemed fine and unidirectional but did not like it’s grommet very much. Pinching the brake booster vacuum hose, though, brings the maximized trims down (not sure if that means that there is a major booster leak).

I have a first timer question about searching for vacuum leaks. Do propane test and smoke machine test do the same thing? what is the difference if so?
This is probably going to be my next task as soon as the endless rain/snow ends outside, and looking forward to some wisdom and guidance from the experts here.

Next, I’m studying the kinds of curves bad O2 sensors give and taking that booster hose out and putting a blocking thing in it from the booster side to see if the fuel trims go back to normal to confirm (or is that totally wrong?) and using my to be finished smoke machine to check for an exhaust leak too (doubt it though!)

Vinny

• This topic was modified 2 years, 2 months ago by vincenzo masiello.

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[vincenzo masiello]

This place looks like totally dead or some kind of deserted town in some future dystopia scifi movie or something, but I’ll add an update just in case someone may use the experience or someone may like the challenge.

I did a water spray test in all the common places that I’d expect a vacuum leak but did not see a change.

O2 sensor reading (both upstream and downstream) seem to be normal with a little bit of lean indication.

Pinching the brake booster vacuum hose lowers the short term trim to normal (eliminates the lean condition) and causes the the long term to eventually go down. Keeping the brake pedal down to the floor has the same effect of fixing the lean condition on the fuel trims. Blocking the booster’s vacuum hose, on the other hand stalls the engine!!!

Is that normal?
Does it indicate a vacuum leak inside the booster or elsewhere?
I took the check valve between the booster and its vacuum hose out and it feels like it works (sucking and/or blowing in one direction).
I held the brake pedal down for about a minute while the engine was running and then shut it off and felt that the pedal was pushing my foot up with a moderate force.

[vincenzo masiello]

I just did a smoke test by connecting my smoke machine to the throttle body and there was no smoke coming from the manifold or anywhere under the hood that i could see

next thing will be to run the engine in a can of carb cleaner to clean the nozzles

• This reply was modified 2 years, 2 months ago by vincenzo masiello.

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