2009 Aveo P0171 Troubleshooting

[PalouseAveo]

For the past month, my 2009 Aveo has been throwing a P0171 code on and off for 1-2 days at a time. I finally got an OBD reader but didn’t have time to mess with it much until today. The signs point to a vacuum leak, but I’d like to get some input before I go too much further.

The past few days I have noticed high RPM excursions at startup and when feathering the gas/clutch at low speed (e.g. pulling up to a fuel pump). Two days ago, I heard what almost reminded me of a turbo spooling sound when I would gently press the gas pedal after coasting at around 25mph. This happened all afternoon, I haven’t been able to replicate it since though.

The P0171 code first started a few days after my mechanic replaced several parts that may (or may not) be related to the problem. The manifold converter and oil cooler were replaced at the same time. The throttle body and MAP sensor were replaced about a week later (P2135 code).

I replaced the air intake hose last week; it had the standard Aveo tape job and was a likely suspect for the leak.

Today I popped the hood to try and locate the vacuum leak, only to find the throttle body heater inlet pipe was squirting all over the engine bay. Luckily the closest dealership had the hose in stock and I was able to replace it.
This afternoon, I finally had time to sit down and do some logging with the OBD reader and Torque Pro software. I really don’t know what I’m doing, but I found some videos and a website that got me down the road a bit. I also read several posts on this forum.

https://www.aa1car.com/library/what_is_fuel_trim.htm
https://www.youtube.com/watch?v=YBE6-zTtTwc

I tried my best to do a smoke test with a cigar, but my lung capacity probably isn’t as good as a smoke machine. I also tried to check the purge valve. Here is a list of what I did today with a graph I made from the log data. I also have many other parameters from the log data saved.

Step 1 - Scan all codes

• P0171. System too lean

Step 2 - Look at Freeze Frame from when the code was triggered (read 1/10/2020 – 2:20 PM)

• Engine RPM: 901.25
• Vehicle speed: 0 MPH
• Time since engine start: 358 sec
• Number of warm-ups since DTC last cleared: 5
• Distance traveled since DTC last cleared: 234.26 miles
• Short term fuel % trim - Bank 1 -1.56%
• Long term fuel % trim - Bank 1 28.12%
• Intake manifold absolute pressure: 9.45 inHg

Step 3 – Clear the code, go for a drive, log data
Step 4 – Graph out data relevant to DTC

• Fuel trim and speed on left x-axis. RPM on right x-axis.

Step 5 – Do a smoke test

• Disconnect the line that runs from the outlet of the purge valve (GM 12597567)
• Make sure to take off the coil pack cover to look for PCV valve for leaks (NA for this year?).
• Blow smoke into the hose that was disconnected from the purge valve.

Step 6 – If no smoke is seen, check the purge valve (GM 12597567).

• The purge valve should be normally closed. So, if power is disconnected and air is still moving, there is a leak.
• Unplug the electrical connection to the purge valve
• Unplug the inlet to the purge valve (opposite the hose disconnected in step 5).
• Start the car, the purge valve may or may not make a hissing noise.
• Put your finger over the inlet to the purge valve and watch the live fuel trim data. If the purge vale is the source of the vacuum leak, the short term fuel trim percentage should steadily drop. When you take your finger off the inlet, the trim with climb back up.

Step 7 - ?

[Deano]

Based on your detailed description, my first thoughts are that i'd be looking for the vacuum leak around the throttle body. It could be a worn or misaligned throttle body gasket at the intake manifold, or the air intake tube wasn't properly seated on the throttle body, or PCV hose improperly fitted on the throttle body, or PCV hose improperly fitted to the valve cover. That's where i'd be looking first. I have found a water sprayer bottle as a handy help for locating vacuum leaks. When the water spray hits the source of the vacuum leak, you'll notice a change in idle.

Deano

[PalouseAveo]

Thanks Deano. Don't know what I'd do without you. I'll keep at it and post back here if/when I get it figured out.