'04 Impala running rich

[Karpman1000]

Hello again,

It's been awhile but I'm a bit stumped.
I have a 2004 impala with a 3.4 that had starting issues after engine was warmed up and sitting for half an hour or so. Started perfecly when cold.
Discovered a leaking fuel pressure regulator so replaced that.
After repair it still had warm start problem.
Checked fuel pressure which was fine but dropped to zero after shutting off engine.
Replaced pump and injectors and fuel filter. Now holds pressure all the time but still has starting problem when warm.
Fuel trim is negative 30% at idle, and doesn't change much with increased rpm's
MAF has been cleaned but has a higher than normal reading (~4 gms/second at idle with no load from a/c etc.
Has new plugs and wires and both crank sensors and IAC valve. EGR has been cleaned.
Vacuum is steady 18 in HG at idle
New PCV valve and MAP sensor and air filter and coolant temperature sensor.
Can't think of what else to try.
Runs well, plenty of power, but the hard warm start must be from the learned -30% LTFT which is from running rich...or so I understand from the research I've done.
Fuel pressure doesn't seem excessive (about 54 at idle) so maybe one of the new injectors spraying too much fuel? Or over-reporting MAF?
It still starts instantly when cold, as it always did.
It's a mystery to me...my other Impala starts first time evry time!

[Blue Bowtie]

Are we certain that the injectors had the correct flow rate? I believe the 3.4 used 17 lb/hr injectors stock.

[Karpman1000]

Yes, according to the supplier.
They look identical to the original injectors.
I suppose that putting another set in would be the way to confirm?

[Tech II]

What happens when you slightly step on the gas pedal with the engine warm on startup?

There are no codes?

What happens if you disconnect the MAF before a warm start up?

The usual fix for this is the F/P regulator, which you already replaced.......

Air filter dirty?

[Karpman1000]

Starts faster with throttle partially open.
No codes...green light on scan tool
New air filter and mileage just over 140000 kms.
Pulled vacuum line off FPR to create a lean condition with not much change to fuel trim.
Haven't tried disconnecting MAF yet but will try today.
Thanks for the suggestion 🙂

[Karpman1000]

Correction:
Fuel pressure is 45 at idle, not 54 as previously stated.

[Karpman1000]

Update:

Unplugged MAF and let it sit for 40 minutes.
Fired up instantly without foot on gas pedal!
Seems to drive down the road more smoothly too, with smoother shifts.
The MAF was new last summer, installed with the arrow towards the motor...is this correct?

[Karpman1000]

Now I'm thinking I should remove the throttle body and try cleaning it.
Did spray it while in place, but maybe some blockage in the idle passage?

[Blue Bowtie]

It can't hurt to clean the TB and air passages, and don't be afraid to clean off the two thermal junctions on that MAF sensor.

[Karpman1000]

I removed the TB and cleaned it, also the MAF again.
Seems slightly better, but still a bit rich (almost 0.8 volts at downstream O2).
I pulled the hose off the PCV valve and plugged it and the O2 dropped to 0.6 volts and fuel trim close to 0%.
Maybe oil mist making a rich mixture?
Going to go outside in about half an hour to see if it starts better.
Idle air passage in TB wasn't that bad, just a bit black.
Thanks to those who took the time to read and make suggestions.
Big help to me ��.

[Karpman1000]

I think it's a bad pcv valve...sucking in blow-by when valve is closed. Disconnected from valve cover and fuel mixture returned to normal. LTFT and STFT are 0%. Could have a weak lifter or worn valve guide or rings causing excess blow by? Partially blocked cat? Doesn't seem to be much pressure out the tail pipe, but has lots of power.
Might have to run a breath hose under the car for now.
Vacuum is now steady at 20 "HG