Bucking at low rpm boost..S5 TII
 

S5 TII pretty much all stock

Regrounded
New spark plugs

Runs fine 3000 rpm+ with boost or without, no hesitation. Part throttle and full trottle, no problem.
5-9 psi...Idling fine, starting ok.

Problem is when i hit boost (about 5psi) in lower rpm (2000-2500). Then it starts bucking and does not want to go.

Any ideas?

Swapped boost sensor from perfectly running S5 TII, no change.
Adjusted TPS, no change.

it sounds like Fuel cut,but I am new to Turbo engines Higgi,so take that advice with a grain of salt(just a guess).
My engine setup is pretty Finicky too.It will buck,etc if I have the TPS ON the TB.I take it off but still have it connected to the wiring it Drives Excellent.
so,that means try a different Tps if you can.

You check both TPS? Sounds like a narrow-range TPS problem.

Fuel cut happening at 5psi in 2500 rpm and not happening at 9psi at 3000+ rpm does not make sense to me. And it does not feel like it. As far as i remember, fuel cut was firm feeling, like on/off.
This is more gradient.

I will try another TPS, another ECU, AFM and see if anything will helps...

swapped TPS - no change
swapped ECU - no change

What sort of condition are your primary injectors in?

There have been cases of this problem in which other posters have had and their problem was caused by the fuel pump resistor relay. To test this possible cause just disconnect the plug (should have 6 wires) to this relay and jumper the Blue/Red wires to the Blue/Green wires and take the car for a spin.

Replaced MAF, no change.
Replaced fuel pump resistor relay (under airbox), no change.

As Bamato mentioned, my last resort is injectors itself. Car runs great when secondaries are on, there is no hesitation but u can feel that it really comes alive once it will turn the secondaries on.

I guess i ran out of basic swaps and i will have to get dirty (injectors swapping etc..)

I'll be replacing my primaries this weekend... not a job I'm looking forward to....

I've been having the same problem with my car. I have checked everything but the fuel resistor relay. Let me know if you find the problem please.

Replacing fuel pump resistor relay did not helped, relay works, but signal to relay was somewhere lagging. Above 3000 rpm , relay was switching from 9 to 12V as it should.
But in low rpm's and in boost, it was not.

Bypassing resistor and keeping it at 12V permanently solved problem with bucking.

So bypassing the resistor completely solved the problem but it still bucks or it no longer bucks?

my car has been doin the same thing. and ive done just about everything you have minus the ecu swap... but would like to know more about this "resistor" method??

I believe he's saying that replacing the unit brought about no change but bypassing the relay did make a positive difference.

I see. Is it bad to bypass the relay like that?

It negatively affects gas mileage but not sure if there is anything else wrong with the bypass procedure.

How would someone go about bypassing it?

Disconnect the plug to the relay and jumper the Blue/Red wires to the Blue wires. This is just to see if there is an improvement and not necessarily to be an end all repair. You can situate the plug from the relay properly such that the upper right terminal is terminal A and below that is terminal B. Top center is terminal C and below that is D. Upper left is terminal E and below that F. With no key necessary check the resistance between A and B (0 ohms), C and D (68-92 ohms) and E and F (.64 ohms).

no longer bucks

Thank you.

Ok. Thank you for the help. I just wanted to be clear on it. I will give this a try to see if it helps.