Rb25det - Flat Spot Just Off Idle

[Color_Of_Green]

Hi all.

I have a really annoying flat spot just off idle. My car idles fine and has nice power etc but if i just tap the accelerator lightly and keep it there (say, try to go from 850rpm -> 1000 rpm) the engine stalls.

The funny thing is that when it was stock it was doing this and even after many mods its still doing this.

Mods include: ( PFC, Plazmaman FMIC, Plazmaman Plenum, HKS 2835ProS, e-boost2, plumb-back bov, z32 AFM, pod filter + CAI, Splitfire coils, HKS 264 cams, lightened flywheel, even brand new AAC, nismo fuel pump, nismo 740cc injectors, and more.. ).. making about 268rwkw with nice power from 3200rpm -> 7500rpm.

A PowerFC map trace (air,rpm) sees it going from idle (2,2) to around (3,3) or (4,4) then increases along the airflow axis to around (7,2) then stalls. TPS is measuring .45v on idle and increases up to 4.7v at WOT. O2 sensor is original I think, and I'm replacing this anyway.

Normally I would just think retuning the PFC first 5x5 cells would fix, but because it happened when it was stock I am kind thinking its something else.

Whats left to check?

I would appreciate any help / suggestions.

Cheers

[danielson]

are you using the factory plumb back bov still??

[Color_Of_Green]

Nope, I its a turbosmart bov with plumbback piping supplied by plazmaman.

[paulr33]

The mods you have listed arent likely to cause that sorta problem. The only one I could think would be the plenum change. You could always try DATA INIT and perform self idle adjust to maybe help iron it out. Youll need to save your tune before you do so first.

The 02 sensor wont affect it.

A datalogit log may help identify it

[Color_Of_Green]

Well, it was happening before and after the mods ;-) so I'm thinking around the stock (electronic) parts that I have remaining:

o2 sensor

TPS

PTU

but seems like needs some work so I'll take it into the shop next week.

[paulr33]

if TPS was shagged car would really stuggle to run and work correctly.

if 02 was shagged you would have shit fuel economy (and nothing else).

whats PTU ?

[Color_Of_Green]

PTU = Power Transistor Unit

Drives the coil packs.

[Cubes]

I had this problem it was extremely evident when the car was cold and got to the point where it would stall the car. Once warm it was still there but wouldn't stall the car.

Then one day.... the car started stalling at the lights.

Turned out to be my injectors.

Not saying thats your problem but it was definitely mine. Whats fuel consumption like?

When my was doing this it was averaging around 13-14L/100km's up from ~10.5-11L/100km's.

Dropped the new injectors in and fuel consumption was back down again.

The end.

[Color_Of_Green]

Hey thanks for the reply.

I have brand new nismo 740cc injectors & nismo fuel pump.

I'm getting around 16L per 100kms ... city driving, with some flogging here and there.

The problem was there before the injectors & fuel pump.

It would be unlikely that both my stock injectors and my new nismo's would be faulty...

Q: If it was injector related but not my injectors and not my ECU what else could it be? fuel pressure?

I had a fiddle with the PFC correction table this morning to see if it was just a simple matter of fuel or timing. Timing made no real difference and leaning it out made it slightly better... and altering the PFC inj accel correction table @ 1000rpm also made no difference .. its obviously something else.

Edited May 8, 2006 by Color_Of_Green

[luke_1987]

Check for exhust leaks

[Cubes]

Another suggestion..

Did you replace your injector plugs? The old boots may have a small crack letting in a little water that causes corrosion.

[MKDR33]

try checking ur intake piping for leaks or maybe retune ur pfc dude..

[Baz]

Check for exhust leaks

I'll second that..

[Color_Of_Green]

Hey thanks for all your replies. I had thought my post went stale until I just checked.

I have managed to remove 80% of the flat spot by fiddling, which I dont like doing but its another month or so before I get the car back to the shop for more mods & full PFC tuning.

Ok, so I reduced the amount of air coming in on idle by adjusting the idle screw at the back of AAC. This definately helped.

I also modified the acceleration enrichment in the PFC at 1000 & 2000 rpm. This also helped, but I have the feeling that I might be treating the symptom and not the cause.

As far as exhust goes, the stock manifold has been on and off a couple of times when I had my 2835ProS fitted. Studs were replaced, new gaskets etc. The HKS dump pipe was all deburred & checked etc. Had a new race cat installed. From there back is the original 3" pipe to twin -outlet muffler, which yeah, isnt muffling much any more as the car is quite loud these days I have a nice 6boost low mount manifold coming soon, and considering a new muffler then.

Cubes: I dont think the boots were replaced, but will look into it.

luke_1987: when you say exhaust leaks do you before the cat / after / anywhere?

MKDR33: yeah, thought maybe getting some rogue air so when through and checked every hose & clamp - they were all good.

Can anyone tell me the correct procedure for adjusting the ACC idle correctly and also the PFC acceleration enrichment table - how is this calculated? I understand theres an initial injector ms value followed by an adjustment (decay). Is this something that can only be done with a wide band o2 sensor + dyno or is it possible to do them yourself?

Cheers

[Cubes]

AAC.

Disconnect the plug connected to the aac, adjust idle so that it doesn't go over the pfc's idle value.

Reconnect, init the pfc and let it learn the idle, turn a/c on let it learn again. It will take a little while for it to be 100% perfect but it will eventually get there.

The accel. pump values need to be done on the dyno. You could fiddle and try to get it right by feel but on the dyno is much easier as you can see what the afr are doing.

I assume you done done the simple overall inj. trim? 370/720*100 = 50%

Have changed the inj. lag values to suit the larger injectors?

Double check all the vacuum lines, especially the ones at the carbon canistor.

[Color_Of_Green]

I also assume this has been done when the new injectors were installed.

The current Injector data currently shows:

1: 96.0 0.00

2: 97.5 0.00

3: 98.0 0.00

4: 98.0 0.00

5: 99.5 0.00

6: 100.0 0.00

I assume the first column is to accomodate pressure drop across the fuel rail? and the second is the injector lag value?

I could do a full init, then key all the values in via the H/C but I'd rather not ;-) was considering getting a datalogit. Also considering getting a AFR guage and wideband sensor that can also feed narrow band signal to ECU.

If I could see the AFRs then a somewhat silly idea I had was to put car on a robust jack and apply slight handbrake at certain RPM to achieve particular load point, etc etc..

Edited May 28, 2006 by Color_Of_Green

[Gambit]

Hey guys,

Newbie to this forum but regular user of the UK forums.

Had this same flatspot on my 33, would hesitate and verge on stalling everytime I went to pull away. Checked for leaks, cleaned the aac etc etc etc.

Turned out to be the O2 sensor was shagged, changed it for a generic 3-wire one and it cured it instantly.

May or may not be your solution but worth a try I reckon.

[Color_Of_Green]

Hey guys,

Newbie to this forum but regular user of the UK forums.

Had this same flatspot on my 33, would hesitate and verge on stalling everytime I went to pull away. Checked for leaks, cleaned the aac etc etc etc.

Turned out to be the O2 sensor was shagged, changed it for a generic 3-wire one and it cured it instantly.

May or may not be your solution but worth a try I reckon.

Yes definately going to do this next week. I've read on this forum that they should be replaced every 50,000 kms or so.

As for injector lag, I've googled and found that 740cc nismos have a 0.66 msec lag time.. can anyone confirm this?

[Color_Of_Green]

hmm

Shouldn't the injector settings in the PFC be:

1: 50.0 0.08

2: 50.0 0.08

.. etc for my setup?

or as I think cubes was saying: 370/740 * (original trim value) @ (0.66 - 0.58) lag

correct / incorrect ?

Edited May 28, 2006 by Color_Of_Green

[Color_Of_Green]

I thought about this a little more last night.

So, from moving from stock (370cc) to nismo 740cc injectors:

I think the shop has applied the 50% to the inj correction map, hence my values are around .54, .56. rather than 1.08, 1.12, etc..

Interested to know whether people usually do this, or leave the inj map alone in favour of applying the 50% to the injector trim table.

As I am new to all of this, I really do appreciate everyone's comments.

Cheers