Car is running lean on light throttle - any ideas?

[TAR]

I have an issue with my car running lean which may or may not be a problem. Any thoughts happily accepted.


The car has standard ecu, panel filter, 3" exhaust from turbo's back, standard intercooler, Walboro 533lph fuel pump, Sytec fuel pressure regulator (set correctly) and connected to vacuum line. I always run Tesco Momentum 99 octane fuel and have just installed reconditioned standard fuel injectors and new NGK BK7EIX spark plugs. \its currently got the cat fitted but I have experienced the same issue with a decat.

The car generally idles when cold at about 16 afr which gradually drops as the engine warms up, once warm at idle its running 14.7 or thereabouts.


If cruising just off boost, the afr's are perfect at 14.7 ish but if I put my foot down a little bit, such as to keep my speed up a slight incline, or hill, or just to overtake a slower car it shoots up to 16.5 and on occasion I have seen it as high as 17. Soon as I lift off its OK and also if I floor it to WOT, the afr's drop as expected. Its only on light throttle that I see the car leaning out.


On the rollers at Ben's last year he told me it was a bit lean and the spark plugs when changed were quite a light brown colour, just a bit too light for my liking.


I'm not sure what is going on but I don't initially think its an air leak as it would potentially affect the afr's at any boost and this only happens on light throttle?

Last thing worth mentioning is that the return to idle can be a bit lazy on occasion where it will idle at around 1000 rpm when warm.

[adaxo]

Its may been done, but may worth to mention, what about fuel filter?

[Humpty's Revenge]

Why is the car running lean from cold?

[Davezj]

my car is exaxtly the same, and so is adams if i remember rightly, on the idle front anyway.
My car starts from cold on idle and runs at about 15.5-16:1 AFR until the idle speed starts to drop from 1500rpm to the normal 650rpm. takes about 4-5 mins of just idle to warm up and drop back to normal.

when cruising at 70mph 14.7:1 AFR put my foot down slightly and i maean slightly, only enough to accelerate to 80mph very slowly AFR goes up to about 15.7-16:1 AFR, put it down any harder and AFR drops back to 14.7:1 and below.
i have a feeling it is entering a portion of the the fuel map in the ECU which is supposed to super economical.
you should be able to see the area of the fuel map that you are entering, very light load value, with rpm about 3000, if you use ECUflash.

i don't think there is anything wrong or out of the ordinary, apart from,

WHAT ARE YOU DOING PRESSING THE ACCELERATOR THAT LIGHTLY, SHAME ON YOU.

yes i was shouting LOL.

[Nick Mann]

I'm hoping to have a tinker with mine tomorrow if I get time - to try to increase the boost a bit before saturday. I'll try and notice what occurs.

[Humpty's Revenge]

my car is exaxtly the same, and so is adams if i remember rightly, on the idle front anyway.
My car starts from cold on idle and runs at about 15.5-16:1 AFR until the idle speed starts to drop from 1500rpm to the normal 650rpm. takes about 4-5 mins of just idle to warm up and drop back to normal.

when cruising at 70mph 14.7:1 AFR put my foot down slightly and i maean slightly, only enough to accelerate to 80mph very slowly AFR goes up to about 15.7-16:1 AFR, put it down any harder and AFR drops back to 14.7:1 and below.
i have a feeling it is entering a portion of the the fuel map in the ECU which is supposed to super economical.
you should be able to see the area of the fuel map that you are entering, very light load value, with rpm about 3000, if you use ECUflash.

i don't think there is anything wrong or out of the ordinary, apart from,

WHAT ARE YOU DOING PRESSING THE ACCELERATOR THAT LIGHTLY, SHAME ON YOU.

yes i was shouting LOL.

Those figures don't look that good TBH??

OK my Skyline is different but still has 6 cylinders which are told what to do via an ecu & albeit a different set up but mine reads the following:-

From cold start is 11.7

Tick over @ 650rpm is 15.4 - 16.2

Driving normally @40 - 60 mph is around 14.1

WOT is 11.6 - 12.2

That looks to me that the vr4 engine runs lean in general maybe ??

[Davezj]

as soon as i plant my foot i am down in the 10's low 11's AFR straight away, my car runs rich most of the time, only ever goes marginly lean is in the above condition.

[Humpty's Revenge]

as soon as i plant my foot i am down in the 10's low 11's AFR straight away, my car runs rich most of the time, only ever goes marginly lean is in the above condition.

Silly question but why are these engines running so lean when cold?

Surely it should be richer when cold then leaning of once the temperature starts to rise slowly ?

[swinks]

Tim, your assumptions are right, i should run 12.5 - 13 ish afr on cold startup.
I guess there may be some air leak past maf, before turbos, not significant but affecting low idle revs.

[Davezj]

there is a temperature enrichment table in the ecu so yes it does sound like should be richer at initial warn up but my car has always been lean at initial start up.
and i know adams is the same.
The dash temp works OK and this is driven from the small single wire temp sensor on the thermostat water manifold the ECU does not know about this temp sensor.
the other temp sensor on the thermostat manifold is the one the ECU knows about and if i can log coolant temp with evoscan then it must be OK.
so what does this tell me?
the premier enrichment table must not be being looked at for some reason, i don't know why.
i have a innovate MTX-L wideband.
maybe the calibation is off on the AFR gauge is out, i re did it about a month or so ago, by disconnecting the wideband from the gauge power up the gauge which deletes the calibration, switch off the gauge reconnect the wideband power up the gauge so it does the calibration. then switch it off to save the calibration then start the car, and the gauge should be colibratied.
the only thing i did not do was remove the wideband from the exhaust pipe. i figured the car had been left all night so there would be only fresh air in the exhaust pipe, so it would be OK.
do you think this could be wrong.
but this would mean adams and tims gauge setups must be the same as mine.

if i blip the throttle when cold then the AFR comes down to 14.7:1 much quicker in about 30seconds, but if you just start it and let it idle then it always goes lean for the first few minutes until it idle drops to 650 and 14.7:1 AFR.

i will try and recallibrating it tomorrow morning

[TAR]

Thanks for the replies guys....

Adam - fuel filter not that old but I have another one to fit soon.

Dave - you appear to have a similar scenario.

Steve - no idea why its running lean on cold start.

I wondered if I had a lazy lambda sensor, I suppose I could prove that by hooking up the narrowband emulatipon from the wideband.

Its very odd but great for fuel conservation!

Tomaz - I will have another look for a leak, but I'm pretty sure I dont have one, unless the butterfly is worn? I think someone on OZVR4 found an issue with that?

[swinks]

Tim, I meant air leak between maf and turbo, low pressure one. Only that one may give similar symptoms. Typical when rubber 3-way splitter is worn or not tighten up enough. That will give you lean idle and rough low revs.

[mike74]

a dodgy engine temperature sensor could give a similar symptom, couldn't it? ie when engine is cold it actually thinks its hot and doesn't put as much fuel in

[Nick Mann]

To confirm, my car doesn't do this!

[Davezj]

what does it do nick?
Is it the same as tomasz, and go rich down to about 14:1 then as the coolant temp rises it goes back to 14.7:1
this is assuming you just start the car and do not rev it at all, just let it idle until it comes up to temperature.

a leak in the MAF to turbo inlet pipework.
or a faulty temp sensor.
seem reasonable causes, is there anything else?

[fassi1]

there is a temperature enrichment table in the ecu so yes it does sound like should be richer at initial warn up but my car has always been lean at initial start up.
and i know adams is the same.
The dash temp works OK and this is driven from the small single wire temp sensor on the thermostat water manifold the ECU does not know about this temp sensor.
the other temp sensor on the thermostat manifold is the one the ECU knows about and if i can log coolant temp with evoscan then it must be OK.
so what does this tell me?
the premier enrichment table must not be being looked at for some reason, i don't know why.
i have a innovate MTX-L wideband.
maybe the calibation is off on the AFR gauge is out, i re did it about a month or so ago, by disconnecting the wideband from the gauge power up the gauge which deletes the calibration, switch off the gauge reconnect the wideband power up the gauge so it does the calibration. then switch it off to save the calibration then start the car, and the gauge should be colibratied.
the only thing i did not do was remove the wideband from the exhaust pipe. i figured the car had been left all night so there would be only fresh air in the exhaust pipe, so it would be OK.
do you think this could be wrong.
but this would mean adams and tims gauge setups must be the same as mine.

if i blip the throttle when cold then the AFR comes down to 14.7:1 much quicker in about 30seconds, but if you just start it and let it idle then it always goes lean for the first few minutes until it idle drops to 650 and 14.7:1 AFR.

i will try and recallibrating it tomorrow morning

The problem is that ECU stays in open loop and doesn't want to switch to closed loop and you get AFR more than 14.7,
Look at oxygen fuel trim when lean it's 0%.
As you tap accelerator it suddenly goes in to closed loop and targets AFR 14.7 as well as oxygen fuel trim
starts working.
I don't know why doesn't it want to go in to closed loop when oxygen sensor is warm and ready
like most ECUs I had experience with do.

[TAR]

Tomaz - I will have another look at that pipe.

Nick - thanks for checking.

Does anyone know at what point the ecu switches to open loop? is it rpm based, tps or something else?

[Nick Mann]

I have been playing a bit today with ECU flash and I am under the impression that it is based on TPS vs RPM. i.e. if tps is over x value when rpm is over y value, ecu will switch to open loop.

[Davezj]

Right let me tell you what I have found out and what I think might have happend to cause it.

Well I did not recalibrate my wideband sensor before going to work today, I did it when I left work this afternoon. I did not remove the sensor worm the exhaust.
So I did the calibration of the MTX-L ,
Disconnect the sensor from gauge
Turn ignition on for 30 sec.
Switch ignition off.
Reconnect sensor to gauge
Switch on ignition wait for htr to flick to cal and wait until gauge reads 22.5:1 AFR.
Switch off ignition
Start engine

This what to the AFR over the period of 20 seconds,
10.6:1
11.6:1
13:1
14:1
Next 30 seconds
14:1
14.1:1
14.2:1
14.4:1
14.5:1
14.7:1

So after a the calibration was carried out my car seems to be acting the same as nicks and tomasz cars.

Now for the reason why my calibration was out.

Last time I calibrated the MTX-L it was first thing in the morning and I did not remove the sensor from the exhaust as it was left over night and the gases around the sensor would have dispersed to fresh air over night, I have no doubt of that.
But it was quite damp and cold, so I can surmise the sensor in the exhaust could have been covered in water vapour/dew or the air in the exhaust was just very dense and it could be this that has made my last calibration incorrect.

What do you think to my theory, does it sound plausible?

[fassi1]

I had my MTX-L calibrated in dry air environment and got it fited in my car.
After having AFR gauge fited noticed the same problem which is lean idle mixture.
Some time ago recalibrated MTX-L the same way you did it Dave leaving the car over night and nothing changed.
ECU notices too much air while in closed loop and tries to adjust fueling to hit about 14.7
ECU stayes in closed loop as long as fuel trim in use (long term fuel trim) doesn't exceed about 4.69.. % as far as i remember
and oxygen fuel trim (short term fuel trim) doesn't exceed about 16.7.. %.
If these limits are achieved, ECU switches to open loop and uses fixed fuel trim in use 5% and at that moment AFR starts increasing and we are getting lean idle.
As soon as we tap the accelerator ECU will start everything again.
The question is what causes it, is it wrong TPS signal, air leak, wrong MAF signal ?????
Pezza's new leggy seems to be fine cause idle fuel trim in use (long term fuel trim) < 4.69.. % which means there is still some room for adjustments in case leaner mixture. Didn't have enough time to compare other live values with my leggy.