Stumble off idle when cold, flat spot

[leideno]

Hi,

Just looking for a few suggestions as to what my problem could be, my vehicle is a Altezza RS200 3SGE+T

Basically on cold start the car will stumble with blips of the throttle, once it warm's up things are fine. I've also noticed that when accelerating from about 1000rpm's to about 2500rpm's in higher gears the car will bog.

For instance if I only accelerate with say 50% throttle it seems to have more power than if I accelerate with 100% throttle.

All I can think of is that it must be running lean in open loop. When its cold and the O2 sensor hasn't come online it runs open loop and when accelerating at WOT it runs open loop. This would explain why it doesn't stumble when warm and accelerates better with part throttle than WOT (since the ECU has adjusted the mixture via the O2 sensor)

Just wondering what could possible cause this scenario? On boost it runs perfect, so I'm confused.

[greywolf]

Restriction in the air intake somewhere ?

Perhaps a leaking injector pumping too much fuel ?

It sounds like for some reason the mixture is off at WOT.

[leideno]

I have a couple of idea's as to what it could be, It may be two different issues confusing the situation... Injectors shouldn't be at fault they are near new. Were originally off a 20,000km's motor.

1. cold start, possibly an intake leak or the motor pulls more vacuume at cold idle thus opening the bov letting in a little unmetered air. I adjusted the bov so it was closed at warm idle, will check this to make sure.

2. water temp sensor, possibly low resolution at cold so its not adding the extra fuel needed to maintain stoich at cold idle

3. knock sensor, could be causing sluggish response off boost, 'phantom knock' with the ecu pulling timing (which doesn't matter on boost so much)

4. DBW/TPS Sensor, possibly a dead spot in the meter so throttle movement at cold idle isn't being registered properly, thus no acceleration enrichment.

This morning it did much the same, it started and idle'd fine but any movement of the throttle caused the motor to stumble. If I opened the throttle very slowly it increased smoothly. I also noticed that cruising with the motor still warming up produced slight bucking which went away when it was warm.

I did a test not long ago and installed the MAF & Pod directly off the throttle body eliminating the turbo. The vehicle was barely drivable, normally it would pull pretty well N/A but this time it strugled up through the RPM's will a sort of muffled induction noise eminating.

[leideno]

Ok guys a bit of an update...

I've started to investigate the FPR being dodgy. It seems like the fuel trims were keeping things happy to a certain point which would explain it running better with part throttle.

On full throttle the bog seems to be as a result of a lean mixture, once boost hits (and the extra fuel is added) the tempo picks up and it seems to be running fine.

I did a little test by adding a global 16.5% more fuel to the entire injector map so whatever duty cycle the stock ECU spits out + 16.5% more. After an ECU reset its running seemingly better, before I had to drive it easy after a reset otherwise it would bog now I can drive it almost normally.

I'm thinking that if the FPR isn't reaching the target 62psi fuel pressure the extra fuel added is compensating for this. Another Altezzaclub member reported having 48psi fuel pressure (meant to be 62) and having to add more fuel to make it run smooth.

Does this sound like a likely scenario? Don't FPR's usually fail to the overfuelling state where they let 100% fuel pressure through?

Any ideas?