Negative fuel trims – Mazda 3 2010, 1.6 petrol

[Cosmin Dinca]

Hi to all!

Due to some drivability issues I’ve decided to verify/diagnose my Mazda 3 engine’s management. Car has 54.000 km, spark plugs, air filter, oil were changed recently, no cell.

With the help of a diagnose tool, I’ve managed to read the Long/Short fuel trims and air/fuel ratio values. So, I’ve discovered that:

– Long fuel trims were -35%
– Short fuel trims were -28%
– air fuel/ratio was 0.96 (at idle).

All the sensors seem to work fine, no errors.

Therefore, I’m guessing that my Mazda’s lengine runs rich, due to the negative values of the fuel trims (ECU is subtracting fuel).

What can be the causes of such negative values ?

Regarding the drivability issues:

– sometimes I feel that the engine power cuts down (especially when in 3 and 4th gear) and after a while (more driving) the power comes back.
– i feel slight vibration at idle (which comes and goes). There are times when it vibrates, and times when I don’t even feel that the engine is on.
– non linear acceleration/deceleration at low speed.
– there are moments when I can’t drive at constant speed (it’s either accelerating, either decelerating).

This behavior started just after I cleaned the throttle body (which is DBW). I did not change its gasket.

The other day I’ve decided to remove the bolts of the throttle body and re tight them at a lower torque. After this, the drivability issues mentioned above have 90% disappeared. Engine response has become really nice and got rid of that non linear acceleration/deceleration. So, I’m guessing that there were some problems with the gasket (air loss ?) or something to do with the tightening torque of the throttle body to the intake ?

Could this be the cause of the negative fuel trims ?

Thank you.

Topic

[Its me]

Running rich can be caused by:

Faulty evaporative emissions components
High fuel pressure
Leaking fuel pressure regulator diaphragm
Leaking injectors
Defective O2 sensors
Exhaust leaks/pinholes before the O2 sensor
Coolant temp sensor problems
Low compression
Incorrect camshaft timing.

The unusual thing about if there were a leak between the throttle body and the intake is that it should manifest itself with a vacuum leak which would cause the opposite, a lean condition.

My guess based upon what you have said is if you have a electronically controlled throttle body; is when you were cleaning it you may have knocked it out of its proper setting and/or it may just need to be re-learned (throttle calibration). May try to disconnect the battery for 30 seconds and re-connect to see if resetting the ECU Helps (this will also re-set the fuel trims)… There may be a re-learn procedure as well, but I do not know it… Sometimes a factory scan tool is required on some makes sometimes not.

An example of a re-learn without a scan tool is to turn the ignition switch to On (don’t crank the engine),
then step on the gas pedal until it bottoms to the floor, release gas pedal, then turn off the the ignition key. Then do the process the again…

-Karl

[Cosmin Dinca]

Thanks for the reply.

I’ve also replaced the DBW throttle body (with new OEM one) to make sure the original one isn’t toasted. It was recalibrated through Mazda scan tool after replacement.

But the fact that the engine runs rich, isn’t because it doesn’t get sufficient air ? So in that case the ECU starts to subtract fuel, because there is to much unburned gas ?

O2 sensors were fine, ECT as well. No errors. Don’t know about fuel injector leak…though. How can I check that? If it was one leaking I thing the engine would run really bad.

It’s really strange on how the engine started to gain power after I’ve re torqued the throttle body bolts.

[none]

[quote=”djkoss86″ post=130594]But the fact that the engine runs rich, isn’t because it doesn’t get sufficient air ?[/quote]

Your air filter would have to be plugged really well to say the engine’s not getting enough air. An O2 or a bad MAF sensor could be misreporting the air it’s measuring and that can cause a rich condition.

[Its me]

Are you using an oil impregnated air filter? Those have been known to cause issues…

-Karl

[Cosmin Dinca]

Hi Karl.

No. I’m using an OEM air filter.

Now, as I remember, in fact, the new throttle body was not recalibrated exactly at the moment when it was installed on the engine but later (i.e. 2000-3000 km after install).

So, I’m guessing the negative long fuel trim percentage which appears now is a consequence that the engine ran for a while with a non calibrated throttle body.

[Its me]

The fuel trims will adjust over time (easier to disconnect the battery and have them re-learn from 0) but you should be seeing positive short term fuel trim in order for it to get back to a more normal number and 2000 kilometers should have done it by now…

Let us know how it works out…

-Karl

[Cosmin Dinca]

I think I’ve solved the rpm issue (I hope).

I’ve replaced the throttle body gasket with new OEM one and the 4 bolts of the throttle body.

I think the other gasket didn’t seal correctly because it was over torqued.

Now, it’s something like this: https://www.youtube.com/watch?v=lMvg7pEplNk

Which I think it’s normal.

[Author]

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