P0171

Questions: I have a fuel trim issue that a dealer and a family mechanic can not seem to come up with a solution. if you have a moment maybe you can steer me in the right direction.

2010 equinox 2.4l ecotec engine

had a p0171 and could not find a vacuum leak. replaced the following. maf, purge valve, purge valve hose, brake booster vacuum hoses, intake manifold, valve cover gasket, exhaust manifold gasket, dip stick tube and dip stick, gas cap.

I am getting good ST and LT fuel trims at idle, however at speed on the highway the LT is 14= and the ST is all over the place. Im getting horrible gas mileage for a 4cyl around 17 mpg avg.

with all that said the system tests all pass and the p0171 is long gone. it accelerates great and idle perfectly.

fuel injector lazy or dirty?

po171 is it a real lean or a false lean is the O2 lying or telling the truth ???

I see these posts that don't get any attention one thing we might need is a little more info on codes symptoms any measurments you might have made and of course year make and model just trying to help

I do have new o2 sensors.
So an update.
I bought a used fuel rail off a 2012 equinox. Installed it and the fuel trims dropped. However I now have a rich condition. The part number from 2010 to 2012 fuel injectors are different.
With that said I bought brand new fuel rail and injectors oem for 2010 equinox.
Installed it and now my fuel trims are right back up in the high teens same as the original fuel injectors.
I went back to the 2012 injectors that are mildly rich.

Let's check O2 circuit integrity see if there is a bias voltage on O2 when you unplug it that usually says your wiring is good back to the computer then we can move forward next thing I would do is hook up scanner and look at switch rate should oscillate evenly from 2-900 mv also are we sure of no vac leaks

Suggesting a logical aproach I'd have a look at fuel pump operation, fuel pressure and fuel volume as a starting point......

The o2 sensors are reading well. With the upstream oscillating as it should and the downstream barely moving .75 ish.
There are not many vacuum lines on the ecotec. Pcv is internal. I replaced everything including evap line. Brake vacuum hoses. The dip stick tube and dip stick. The gas cap. The intake manifold. The exhaust manifold gasket and the valve cover gasket. I was sure to clean everything good and make sure all passageways were clear.
Assuming the o2 s are working properly. Why would the 2010 fuel injectors give me a lean condition +18 and the 2012 fuel injector give me a rich condition. -13 to -18 area. But good numbers driving.
The number for the lean condition don’t improve when I bring up the rpms or when driving on the highway for example. If anything they get worse with wild swings.
The 2010 fuel injectors are only for that year. 2011 forward are a different part number. I wonder if the 2010 has an updated part number? I can’t find a service bulletin regarding it.

The scanner I use shows fuel pressure in low 50’s. I don’t have any advanced tools for volume measurement or resistance testing.
There are no symptoms of an issue , runs and drives as it should however I can see the fuel trim numbers and when I do the enhanced scan option I see the pending rich condition. No check engine light.
If I reinstall the 2010 fuel injectors then the lean condition comes back.
There has been no misfire or any other drive ability problem. Passes all systems tests.

jscordo wrote: The scanner I use shows fuel pressure in low 50’s. I don’t have any advanced tools for volume measurement or resistance testing.
There are no symptoms of an issue , runs and drives as it should however I can see the fuel trim numbers and when I do the enhanced scan option I see the pending rich condition. No check engine light.
If I reinstall the 2010 fuel injectors then the lean condition comes back.
There has been no misfire or any other drive ability problem. Passes all systems tests.

A reduced fuel volume initally will be detected by the O2 sensors. The pcm will try to correct this issue , increasing the fuel trim.

As long as the reduced fuel volume is not that excessive the pcm will be able to adjust for it and you will not experience any driveability problems, just higher than normal fuel trim number.

Once the fuel trim numbers surpass a specific treshold the pcm will flag codes and the cel will come on. An even more severe lack of fuel will drive the pcm's correction abilities beyond its limits and driveability problems will occur.

Did you check for alcohol content in the fuel with your scanner?

I did not check alcohol content of fuel. I don't have that advanced of a scanner.
with all that said, if I have a lean condition with one set of injectors and rich condition with the newer model set can it still be a fuel volume issue?

When your trims switch from positive to negative by changing the rail and injectors, are you looking at ST, LT, or total trim? I’m only asking because you didn’t mention it. If LT is positive, the ST will be negative after a fix until LTFT comes back down. Your scanner should be able to read the alcohol content data pid if it’s a flex fuel vehicle

the flex fuel equinox was a year or two after 2010. they have two choices of fuel injectors in those years.
the fuel trims are LT and ST combined. though the ST is all over the place so its hard to get a consistent number. its high but not high enough to set the check engine light. when looking down the runners to the valves, they look pretty darn clean. not alot of built up considering its Direct injection.
any obscure things to check for a vacuum leak?

regarding fuel trims switching the 2010 to 2012. the lean condition on the 2010 rail is far worse than the rich condition caused by the 2012 rail.. my gas mileage has gone from 300 miles a tank full to 400 with the 2012 rail

I wonder if the 2010 injectors leak. Hose it down with water while it’s running and see what happens

i was thinking the same thing so i bought a branch new oem 2010 fuel rail with 4 new fuel injectors. changed it all out this weekend and I got the same FT numbers as the original rail. High positive numbers around +18. So i went back to the 2012 rail where i get a rich condition around -13. at first the numbers were perfect around 0 however the ST ended up pulling the LT down and down to -10 to -13

oh i forgot to mention i tried the water test. i didnt see any steam coming out exhaust and no change in idle etc.,

update to rich negative fuel trims. highway driving the number read as it should. at idle they really go negative. -18 and worse in some cases.

Hi jscordo,

Just to discard. I have had problems with trim numbers when evap valve is leaking. I know is not related with injectors/rail, but maybe evap valve is "cheating" you.

I'll take a guess at it.
The exhaust manifold has a small crack in it allowing oxygen to enter and fool the O2 sensor into calling for more fuel.
Cost nothing to take a real good look at it.

i replaced the purge valve and hose going to it. unless its a leak back at the evap canister I should be in good shape. when i have the original fuel rail on the car and the positive fuel trim numbers...it doesnt act like a typical vacuum leak, the numbers dont get better driving. there are pretty wild swing s