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Diagnosing lean condition


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Hey Scotty and the community!!

Hope y'all are having a nice day

2003 Corolla, 1.8L, 135k miles, manual, no codes


At idle, my MAF reads 1.4 to 1.5 g/s

Short Term Fuel Trim is between 3% and 7%

Long Term Fuel Trim is 8.6% (after driving)

O2 sensor shows values below 0.45V more often


Can this lean condition be simply due to the MAF sensor?


I've cleaned it twice, replaced all the intake gaskets, got new OEM air filter from the dealer.


Car sat for prolonged periods of time before I bought it, fuel filter may be clogged


Cheers, sorry for the long write up. Scotty has said these can be made to run perfect, and I do want to keep the car as long as possible!

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5 Answers

Drop in MPG, most often than not, results when your variable valve timing start acting up a bit, … no longer act perfectly as designed! True, …

MAF sensor? I don’t think so, … on Toyotas at least, main computer is programmed to compensates for normal wear and tear, … and those sensors, pretty well never fail.

I would look into VVT proper operation: … dirty oil, electrical connection, inconsistent idle.



Oil is clean, new, engine inside has some lacque so I change it every 2k miles to clean it up. Using synthetic Mobil1 5W-30. I do get an inconsistent idle at times. The RPMs go up and down between 1100 and 1600 on their own, only when the engine is still cold. Most often after unplugging the battery

Another thing I must add,
At cold start, if I step on the gas, the RPMs would slowly creep up above 2100 and continue climbing up and up. The continue climbing, slowly. It appears that the target RPM value is that which it would have been had I kept the foot on the gas when I stepped on it. I will make a video and add it here to show what I mean

Bit more of generality on this: … you may have now a clean oil, but VVT system, beside solenoid valve and the mechanism that alters cam phasing, also includes oil passages, strainers and tiny filters in them. Gunk forms and gets trap there over time, causing blockage, restriction, et al. Consequently, VVT system responds lazily, erratically, … or not at all finally. And that, manifests itself in erratic idle, inconsistent response to the throttle, cams not advancing/retarding as called for, and eventually throwing code.

Anyway, VVT system is designed to improve efficiency to begin with, … and if these system not functioning as designed - deteriorates - result is dropping efficiency. Dropping MPG as you are observing, …


Typically a lean condition means you need MORE fuel, not less. It is when a car is running rich that MPG goes down because the injectors are dumping too much fuel. 


I would start by running a bottle of fuel injector cleaner through it to see if it helps. Also check the fuel filter to see if it is clogged.



My thinking is, MAF is indicating that there's less air than actually is, then the O2 sensor senses that there's not enough fuel and too much air, so it compensates by adding more fuel.

Scotty pointed out that MAF should be same in g/s as the displacement of the engine in L. So my value is 1.5 while supposed to be 1.8

What am I getting wrong here?

I've run all sorts of injector cleaners. Used two different LiquiMoly types, Techron, SeaFoam and others. Fuel filter could very well be clogged

1.8 is very close and not a big enough gap to worry about considering the mileage and age of the vehicle. It isn't going to be dead on.


Those short and long term trims wouldn’t indicate a lean condition.

What kind of symptoms are you experiencing?

Sudden and very noticeable drop in MPG. It was a steady 33, now its 25


Have you checked for vacuum leaks?



I will


If your not getting any codes it sounds like it’s running as it’s designed to. All electronics like the ecm have built in parameters or limits of sensor readings and specs and will trigger a code when something is seen above and beyond what it should be. I would leave it alone