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Rb26 Afr

Don Dada

By Don Dada,
in Engines & Forced Induction

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Don Dada Mentor

Don Dada

Posted
Posted

Quick question guys......

Is it normal for rb26 to be so rich on idle?

It sits anywhere from 11.6 to 12.5 on idle once the engine is up to temp.

Thought it was a little strange so i checked it out on another 26 what do ya know.... same thing!

These two 26 however are not in its "original" gtr chassis, so i just wanted to confirm if a original gtr behaves the same,or if there might be a problem with the wiring when it was transplanted.

Both are R33 engines running stock ECUs, 3" exhaust, 12-14psi, one with walbro pump,one with bosch, intake pods and using 95 fuel.

The car pulls very good ,no misfiring or hesitating, although i find the idle is a little lumpy and it uses a lot of fuel and my rear gets a little black soot after a while.

Is this normal?

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Don Dada Mentor

Don Dada

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OK some more info.....

Hooked up a pfc out of the box (UNTUNED) and got the following

With the ign on O2 (1) 0.04v

With the ign on O2 (2) 4.56v

On idle O2 (1) fluctuating between 0.04v - 0.88v

On idle O2 (2) constant at 0.12v

AFR at 12.5

At 1600 rpm O2 (1) constant at 0.92v

At 1600 rpm O2 (2) constant at 0.14v

AFR at 10.8 and couldn't rev any higher because it was starting to break up.

Also tested and got a 12v for the heater supply at the two sensors.

So i guess O2 (2) is screwed?!?

And why O2 (1) is only cycling at idle, and not when revved to 1600rpm

What do u guys think?

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Guilt-Toy Grand Master

Guilt-Toy

Posted
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Not sure what u mean???

you can switch off the 02 feedback to the ECU.

go into etc and function select and change the 02 option. if you are unsure look at the power fc faq.

or you can call me on skype or something and i can explain

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GTRNUR Grand Master

GTRNUR

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I installed my new wideband setup in my R34 and ran it around for a few weeks with the car 100% standard so I could get a picture in my head of how they japs tune them from factory.

Idle is 14.6-15.1:1 when warming up and when up to temperature. Idle stability when cold is achieved by idling faster, not richer.

It ran 13:1ish on mild boost too up to about 0.4 bar or so, and only got to 12:1 at full boost (0.8 bar). It dipped into the mid 11's toward the top end of the rev range at full load as well.

I have an FC in the car now. The load points around idle are at about 0.985 or something... That produces a 14.6:1 idle AFR. Check your temp corrections if this is way out, and as suggested turn off O2 feedback. It probably wont make any diference, but who knows the fuzzy programing logic that goes on inside these ecu's...

With my previous gtr and the tune I ran in it, I only ever turn that on after the car is tuned. And even then it was probably correcting very little. It only really comes into play when driving up hills to improve my fuel economy as at light load level ground cruising i'd run 16:1 @ 60km/h and 15.5:1 up to 100km/h. Above that its 14.7 at any sustained level ground speed.

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R31Nismoid Grand Master

R31Nismoid

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Ok i understand now.

But what difference should it make to tell me if the sensor is bad, being that the pfc is running on its base map?

Turning them both off will stop them referring to the O2 - what good is it having them looking to a dead O2 for a reading? Its no good.

Hence turn them off and watch what happens :laugh:

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Don Dada Mentor

Don Dada

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Turning them both off will stop them referring to the O2 - what good is it having them looking to a dead O2 for a reading? Its no good.

Hence turn them off and watch what happens ;)

well i hooked up the pfc again today switched off the feedback control and nothing happened!

Afr remained the same and the O2 voltage (1&2) remained the same.

Also with it on or off O2(1) kept constant at 0.84v where as yesterday it was cycling between 0.04-0.88v

I pulled out the vac line for the brake booster thus creating an air leak so the wide band went lean and while peddling the throttle it was cycling the afr from rich to lean and the voltage on the O2(1) sensor bounced between 0.0-0.98v which seems up to scratch.

O2(2) however remained at 0.0-0.2v

I know there must be a few stock gtrs well with pfc installed.

Could some one just run outside, and check their hand controller and tell me what the voltage reads for the O2 sensors when the car is now started and after warming up. :D

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GTRNUR Grand Master

GTRNUR

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With closed loop off the idle afr regardless of what it is should be stable. Lets assume you have a 14.7:1 afr at idle, that should be 0.5v on the narrow band. The factory sensors have a very narrow operating band where 0.45-0.65 is the only usable voltage range where the afr can be read accurately. The limits of the sensors range is 0v = lean and 1v = rich.

Yesterday when you saw it cycling it was the computer operating in closed loop mode trying to hunt around the 14.7:1 afr of 0.5v. It will stay on the rich side of things with the o2 feedback turned off.

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