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2006 Chev Malibu 2.2 Ecotec engine, not DI, no turbo. 198,000 miles. Uses 1 quart oil/3,500 miles. Engine is reasonably well maintained, air filter and plugs, oil changed on schedule.
Occasional code P0171, lean bank one. I'm sure this code gets set at idle.
STFT varies up and down a few percent but is usually positive at idle.
LTFT +%14 at idle engine warm, this number varies + - %2 or 3.
LTFT at 1500rpm no load, +%9.
LTFT at 2500rpm no load, +%4.
Driving under load, LTFT is at most + %2-3
O2 sensor voltage appears correct. Upstream cycles .9 to .1 volts about every 1.5 seconds.
After cat O2 sensor sit around .7 or .8 unless throttle is changed rapidly.
Suspect an air leak but try as I may, none to be found. Evap purge solenoid seems to pass a lot of air but there are times when it does not operate and the fuel trim remains high. It opens sometimes at idle. Is this right?
MAF reports 2.7 g/s of air at engine warm idle, 700 rpm. This figure varies from 2.25 - 3.1 g/s.
MAF reports 9 g/s at 2,500 rpm, no load.
Engine runs very well but sometimes will hesitate when applying throttle after backing off for a couple of seconds, such as gliding to a red
light and then applying throttle as it turns green. This happens with engine hot or cold.
I have cleaned the MAF sensor [hot wire type] a couple of times and the hesitation has gone away but only for a few days and then it returns.
I don't have a decent scanner I'd post some charts.
If the MAF sensor is under reporting air, why is the idle g/s so high? Don't want to just throw parts at it but I'm going to have to. MAF and O2 sensor?
14% Long term fuel trim at idle that goes better under load
most of the times sounds like a vacum leak ...
Vacum leak at any direction ... EGR, intake gasket, hoses, evap valve etc ...
Dirty MAF ... as you already clean it, the point here is to see for difference in grams per sec, before cleaning and after ...
And in some more rare cases low fuel pressure
Or even a leak at exhaust
Did you check vacum hoses with water as Paul does ?? or smoke machine ??
You can test your O2 also by driving it lean or rich to see how fast is switching
I have checked for vacuum leaks three different ways. Water, WD40 spray and a smoke machine [ me smoking a cigar ]. This includes the vacuum lines to the brake booster and EVAP purge solenoid and PCV line from valve cover to intake.
I have not thoroughly looked at the exhaust for leaks. I assume you mean before the O2 sensor. The exhaust is quiet and the O2 sensor is tight no leaks that I can see. The exhaust is in a hard to see position.
I don't understand "test your O2 by driving it lean or rich"
The upstream O2 sensor does oscillate as per what appears to be normal.
I hate to be a parts changers but the MAF and O2 sensor are original so I'll probably just change them soon. Rather figure it out though.
Last edit: 6 years 5 months ago by Drunken Elvis. Reason: Spellin n such
How did you check the purge solenoid? Unplug the electrical and disconnect the canister side line then start the engine. It should not have vacuum on the open port. If it does plug the port with a cap or? And watch your STFT
With the electrical connected to the purge solenoid, sometimes at idle, it opens and vacuum is applied to the evap line back to the canister. Sometimes the solenoid is not active and no vacuum is applied to the canister.
Doesn't seem to matter, as when I remove the line to canister an plug the purge valve opening, the fuel trims don't really change much.
Question? Should this purge valve ever be open [draw vapours from the canister] at idle?
The O2 sensor swings up n down about every 1.5 seconds at idle. I think this is normal.
I just returned from a test drive with the scanner hooked up. [scanner is just an iphone] The LTFT is highest [%14 OR MORE] while gliding, such as when you are slowing a bit to time the next traffic light. When descending a hill in a lower gear where the engine is revving a little high as the engine is doing some braking, the LTFT drops to maybe %5. I would think a vacuum leak would produce even higher fuel trim during this condition.
Both the MAF and O2 sensors are old. I'll probably just replace them both.
I felt all around the exhaust manifold when I cold started it this morning. Could not hear or feel any exhaust leaks. I have checked the fuel pressure at the Schrader valve on the fuel rail and all was normal. 55-60 psi if I recall correctly. This problem, hesitation/high LTFT and lean code seem to show themselves at idle or coasting. On occasion it has idled a little rough as well. I don't think it's a fuel pressure issue.
Perhaps an injector is not functioning well at idle but functions at higher dwell times?
Twice in the past the hesitation has temporarily cleared up after cleaning the MAF sensor and I have put far more time into this diagnosis than the parts are worth. The car runs fine so I'm not really in a hurry but I may as well change the MAF and o2 sensor. Mean while I just wait for the solution to magically present itself.
I really do like to first figure out the exact cause of a problem but don't have any fancy scanner tools or any other way to trouble shoot further. I'll post the cure should I get lucky.
My vehicle was a cobalt but it is the same engine. The reprogram did not fix our issue and in part 2 a new MAF was installed for the fix. But your's is a little different as your trim numbers improve at higher RPMs suggesting a vacuum leak.
It may be worth researching this reprogram
Well that looks very much like it could be the problem. I may have been chasing a ghost. It's going to be fun tomorrow when I call GM and ask them about this TSB to upgrade the ECU programming to recalibrate the MAF on an 06. They will just say it's already been done, but it can be re-done for $$$.
SB #06-06-04-033
Thank you for the info. Maybe the sun will shine on me tomorrow and they'll offer the reprogramming for free.
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