Thread: PO171 case help

2008 Grand Prix 3800 SE3

I drove the car more than usual last December while I was going uphill on a highway when a sudden loss of power occurred that was temporary, no codes.

I went straight to an expert who test drove it violently and did not notice a difference but he "felt"a lean condition even though he couldn't see real evidence in the live data.

A few days later the light and code came on.

I don't sense any 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 this video P0171 Trouble Code: System Too Lean Bank 1 Diagnosis - YouTube (great guy and great video) and I have a cheapo scanner and a Haynes book for this car (and off course the internet) I started investigating
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)

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:

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.


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.

Vinny

I would smoke test it. Might find the source of the P0440 Evap code too.

Some other things to check is make sure the battery cables aren't corroded and are tight (causes all kinds of strange issues) and make sure you don't have any exhaust leaks before the upstream sensor.

Battery is fine. Electrical/electronic is what I do for a living. I even clean chassis -ive connections nuts and bolts every winter or so. See the voltage above. I'll still take a look (could be loose or something!)

I'll do the smoke test. I guess smoke works for the exhaust leaks too, right?

This place looks dead, 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.

I don't sense anything wrong/hard in the pedal or braking now and I just inspected for rust and nothing. I did feel on the highway back in december when everything started that it took more more than usual effort to brake and slow down when I was going about 75mph on cruise control and that was scary.

I was about to do a smoke test since my (pickle jar + nichrome wire + inflating machine) smoker machine turned out great. I'll post some pictures. and then I'll do an injectors cleaning while running the engine on carb cleaner instead of gas, there are cool vids about doing that

Is the grommet (rubber ring) on the valve where the hose is attached to the booster a critical component when it comes to vacuum leaks or are the only leak prone things are the hose, the valve, and the booster itself?

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

I did put a heavy duty plastic bag thing and wrapped with rubber bands that no smoke was coming out of the air entry side of the TB and connected my smoke to the little tube that connect the top of the engine to the air duct (before I disconnected it to seal the TB). And another repeat, I connected the smoke to the hose that goes to the brake cylinder.

I ordered a fuel gauge and soldered an accurate 5mSEC X 100 times little circuit to test the injectors one by one (did not arrive in the mail yet).

Right now, I found some vids on YT that show how to disconnect the hoses on the purge valve solenoid and will go see if it is good...that is an easy test.

There is something that doesn't make sense to me (the know nothing) that, after several days of internet searching, could not find an answer to:
When cruising,
O2 sensor is dead (close to a horizontal line at 0.1-0.2 volt) and the fuel trims are maxed
as soon as I touch the gas pedal
O2 starts bouncing normally (voltage and frequency) and trims fall to normal (a little slower on the LTFT off course)
until I tap the gas pedal again
and so on

Is that normal? or am I having some kind of short circuit or bad ground somewhere?

update: the purge valve solenoid hose was very easy to disconnect (push kind, i.e. you just push the little white plastic piece towards the hose, and pull the hose), and both sides of the connection had no vacuum or anything and that is good (right?) because that valve is normally closed.

The weird thing (at least for me) is still, both idle and cruise, both O2 sensors are near the floor of the coordinates and both trims are maxed until I tap the gas pedal!

I didn't get your tip about the volts and millivolts, are you referring to accuracy? As far as I know volts, millivolts, kilovolts, microvolts, dB's, mdB's ...etc are all the same as long as you use enough digits.

I have a very important question that I failed to find a good answer for on the internet, does the PCM only cares about the average of the O2 sensor voltage? or does it read different parameters in the waveform (like max, min, frequency, phase shift or delay relative to e.g. the crank sensor pulses)?

I put together a little 555 circuit to generate a square wave to feed to the computer while the 2 heater wires stay connected (made a special 6-conductor wire for this) and will see how the fuel trims react.

This is the most complicated place to upload an image to. I give up.

Anyhow, I made the duty ratio of the waveform to be 10%-90% and independent of the frequency. The frequency is calculated taking 6 cylinders to cause 3 pulses, so a 600-3000 rpm which is 10-50 rev/sec or 30-150 Hz.

If the pcm only needs the average O2 sensor voltage, then there is no need for the square wave and a variable voltage between .2v (200mv) to .8v (800mv) should suffice to aid in the diagnosis of an aging upstream sensor (original factory, 12 years, 190 kMiles). If the test with my little square wave fails, then I'll have to add a microcontroller to the circuit so that the frequency (and may be other parameters) change with the rpm by getting another wire from another sensor or the pcm connector

it is not even possible to delete or edit posts!!!

Final update:
As soon as I hooked up a benchtop oscilloscope to the sensor (kept heater two wires connected between car and sensor but tapped them for another scope channel), I saw that the pulse coming out of the sensor was perfect even before hooking up my sensor simulator.
And since these forums don't really have any members' interactions at all like a few years ago, I gave up and went to my local diagnostician. I explained everything I have done to him and in five minutes he gave me his conclusion that he charged me $80 for (an hour of work because he was either too mean or too honest). His diagnosis was that the brake booster needed to be replaced. He did not even replace it for me and told me that he could not get two it for two weeks (later called with an estimate of $380 in addition to the $80).

I kept driving the car and tried replacing the booster check valve grommet (the valve it self was fine, I checked it with a balloon) and it did not make a difference. I called around for an earlier day and found a guy who would do it for $200 labor but the dealer does not sell this part any more and other stores sell it for double the online price.

I started getting a consistent P0137 (sensor low voltage) in addition to the P0171 after I messed with the sensor connector and later I had to take a week off from work because the car started hesitating every time I hat the gas pedal and almost totally lose power at extreme uphill highways when transmission temp goes up to 240's.

The shop's appointment is on Thursday and it will be a gamble to drive it to them 40 miles away.