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By C2K, November 13, 2014

Hi. My corolla (1.6 VVT-i liftback E11 ZZE112 240000km) hesitates at full (or near) full throttle..

I have the trouble code p0171 (System too lean on bank 1) and problem exist when the engine is cold or hot no noticable diffrence. My oil consumption is about a quart/1000km and the long term fuel trim is 26.56. The car has been hesitating for about 6 months and was hesitating very badly after a engine wash, about 3 months ago, but that went away and now is hesitating at full (or near) throttle only.

Here's what i've tried so far (not in order)

Changed new spark plugs (denso)

Changed intake manifold gasket

Checked for vacuum leaks with carb cleaner

Cleaned MAF (with CRC maf cleaner)

Backprobed MAF voltage while engine running (nicely between 1.2-1.7V)

Measured TPS (absolute) with touchscan+elm327 values between 12.5%-78.4% and no noticable gaps in between

Listened to fuel injectors with screwdriver and they all tick ok

checked fuel filter inside the tank and it looked clean

cleaned a dirty PCV-valve

Changed oil and oil filter

Used a bottle of stp injector cleaner in a full tank of gas

Cleaned carburator body

Checked air filter

What should i do next?

Could be the upstream O2 sensor at fault here. At lower throttle positions, it is setting up all the feedback, but at WOT, it will drop into open loop mode (preprogrammed air/fuel map). Toyota tends to be on the conservative side on that tune, so it will run a bit rich at WOT. Since you are running rich, possible the prebuilt map is no longer in range.

Could also point to bad injectors. A number of the ZZ family of engines have seen injector failures, especially with a lot of mileage on the engine. Sometimes it is as simple as a grommet on the injector leaking, Could also be the injector running outside of spec, ie, the ECM is forcing the injectors to run higher than normal duty cycles to feed more gas in than usual, can kill an injector faster.

Hi and thanks for the response!

I checked and at WOT it indeed goes into open loop mode (open loop due to engine load). Is this a corolla "feature" or malfunction of some sort?

I also removed the 02 sensor plug, but that made the car worse so i just put it back so i guess the sensor is still doing something useful ? default_huh

02 voltage in bank 1 sensor 1, according to touchscan, was min 0 and max 0.98.

I also found the old original toyota (denso) sensor from my trunk which was probably changed to a walker 02 sensor (it was in a walker box) is there a chance this old sensor is still useful?

I've have the impression that denso parts are preferred on toyotas and maybe the "new" sensor is just not performing correctly even though it seems to do so.

Nope - at WOT, it should go into open loop - pretty much every car I've seen does that.

Very possible that the O2 sensor is "lazy" - meaning that is still working, but not as optimally as it should. Many times, these sensors will not outright fail, but die slowly - many times, the CEL will not even illuminate.

Possible that the old sensor is still working. I know on this engine family - it is very sensitive to the brand of parts that you put on it. Especially with the O2 sensors, they recommend staying with Denso parts. I've used Walker and Bosch replacements, both caused engine drivability issues - ended up replacing them with Denso O2 sensors, problem solved.

I put back the old denso sensor and took a 20 minute drive, but no difference in performance.

Should i clear the trouble codes or reset the ECU the get some results?

It is always a good idea to reset the ECM and clear those codes - see if the codes come back or new ones pop in their place.

I cleared the trouble code and after that recorded some data while driving.

Here are the stats of about 25 min drive.

I look at the datalog for a while and the LTFT seems to go quite easily to 19.5% even at lower rpm (2000)

Is there something else going on in this chart that is out of the ordinary?

p0171 is back!

PID Description Value Units

--- ----------- ---- -----

0x02 Freeze frame DTC P0171

0x03 Fuel system status Closed-loop

0x04 Calculated load value 43.92 %

0x05 Engine coolant temperature 80 C

0x06 Short term fuel % trim - Bank 1 18.75 %

0x07 Long term fuel % trim - Bank 1 29.69 %

0x0C Engine RPM 2425 RPM

0x0D Vehicle speed 77 km/h

0x0F Intake air temperature 4 C


I cleaned OCV filter / VVT-i oil filter (no change in performance)

I just realized that the car hesitates also for ~1/4 sec if i'm not pushing the gas pedal and then push it fast to any position. After the hesitation it starts to accelarate.

If i push the pedal slowly then no hesitation not even in WOT.

Any thougts?

The freeze frame data looks OK to me - mine are pretty similar. Nothing in there screams as a potential problem yet - except for the fuel trims. Short term can fluctuate in sub-seconds scale, but usually tries to stay within a couple tens of percentage. Long term fuel trim shouldn't exceed double digit percentages. Rule of thumb, add STFT and LTFT together, should not exceed 10% total trim, if it is more (positive or negative) - then the ECM has to pull or push an excessive amount of fuel to get the engine to run within spec. The absolute max range between them is also important - like to keep them less than 10% trim variance in total.

As for the weird hesitation, it does sound like a feedback issue to me - the engine cannot react fast enough to changes in input. That points to MAF, O2 sensor, TPS, or even the ECM. Hard to say for certain without being right infront of the car and doing some more diagnostics. If it is electrical - could be a nightmare to diagnose, as they can be very intermittent and/or hard to track down.

Might try capturing data when you keep the RPMs steady at 2500 RPMs, see what the trims do. If the STFT bounces around a lot, that is OK, but the LTFT should stabilize at a fairly low percentage. If it doesn't - really points to a possible MAF issue. If neither one of them changes much (low or high) - that could point to a potential O2 sensor issue. Possible it could be multiple ones issue compounding each other.

LTFT stabilized at 2500rpm to 7.8%, at 2600rpm to 8.8%, at 3000rpm to 14%

Perhaps i should mention that when i push the throttle fast i hear a swoosh type sound which comes from the intake manifold. Since i have not found a vacuum leak after multiple attempts, i think it's just the air get sucked into the manifold and not able to go further. Does this make any sense?

Noise could be from the VVT-i - changes the valve overlap on the intake side, so it could cause that induction noise. Vacuum leaks are usually a higher pitched, hissing kind of noise.

LTFT valves at 2500-2600 RPMs look good, at 3K, they look a bit high, but still within reason.

Whining, high pitched noise could be from a noisy bearing from the accessories - like the alternator. little too high pitched to be a vacuum leak, but shouldn't rule out that possibility.

Those other recordings are pretty helpful. I can hear that very pronounced "dead" spot right before the engine revs up. That is really unusual. Almost like the ECM is lagging in its feedback to the rest of the engine. Almost like it is overshooting the amount of fuel needed for that amount of throttle application - only once it figures out what the right mix is, then it stabilizes and the engine revs up like it should.

I'm assuming by the model year that this car still uses the cabled throttle body and not the DBW setup?

Unfortunately, lots of things can cause what you are experiencing. Sensors to electrical - to induction and exhaust issues. Since you are consuming a fair amount of oil - possible plugged catalytic converter is a real possibility. Also would do a number on your O2 sensors, more so if you run a higher mileage oil, as their additive packages, though great for older engines - protection and lubrication wise, can slowly kill these O2 sensors and catalytic converters more quickly.

Cabled throttle yes.

I unplugged MAF and the engine revs nicely to 3000rpm and then starts to stumble.

Perhaps the MAF is bad after all and the engine limits to 3k?

Possible that the MAF is bad. That stumbling at higher RPMs is just a feature of not having the MAF connected, car runs in limp mode to save the engine. Another possibility, is that with the MAF off, the car is running off of preset air/fuel maps and other feedback sensors (O2 and TPS) - could point to those sensors being offset, causing the MAF to "appear" faulty.

But you could be on to something. That sort of behavior when you disconnect the MAF, usually points to either bad MAF or vacuum leak.

I think i'm gonna check for those leaks once more before buying parts.

What others methods there are for finding vac leaks besides carb cleaner?

And is unplugging o2 sensor similar to unpluging MAF?

Vacuum leaks can be nightmare to track down. Some only leak dependent on temperature - some only under movement. Spraying something on the hoses and unions is the quickest way to check them. You'd almost have to pull each hose off and inspect it. Check around the induction system and look for cracks in the airbox, especially close to the MAF. Gaskets, might have to pull off the TB and intake manifold if you can't see all the way around.

Unplugging the O2 sensor would have a different effect than the MAF trick. Some cars, makes it completely undriveable - some will fall under a limp home mode. Don't remember what the Toyotas does off the top of my head, their tuning is pretty conservative - might drop into a limp , degraded performance mode.

I tried to vacuum cleaner method and created nice suction in the intake but didn't hear any hissing. Maybe it's not leaking then?

Is there a reliable way the check the maf, or should i just get a new one?

Just replace - electrical parts like that can be intermittent - this is one of those swap and see sort of part. Even if it diagnoses OK, it could still be faulty. Double check the return policy on the replacement parts, some retailers won't take back electrical parts once they touch the car - just in case the MAF is not the culprit.

I got a used original MAF from ebay and WROOM! The car accelerates like never before! No hesitation or bucking whatsoever.

Haven't reset any codes or looked at the data, but i have a feeling that the problem is solved. default_smile

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