Need Help : Ongoing Hesitation Issue !

[psytz]

Hey guys ,

I am in need of some dire help , I've had enough of swapping out parts and wondering what on earth is going on. I've swapped out as much as I can think of to eliminate this issue.

Upon acceleration , the sound is like It's dosing / forcing air back through the turbo / the BoV is going off and the car jumps / coughs if you may ? , much like a L plater learning a clutch but not an aggressive bunny hop. It's a hesistation like a back pressure of some kind maybe ? ( un educated guess )

It started when I had my mechanic install my HKS GT-RS Turbo kit on my 1998 R34 GTT. I'm going to rattle off a list of my modifications and parts changed and I'm hoping someone with extensive diagnostic experience will help point me in the right direction. I'd really appreciated this communities help.

Low mount stock manifold , HKS GTRS Turbo kit.
Stock Intake , stock throttle body

Blitz return flow FMIC , all tight hoses and clamps.

3" Bellmouth full exhaust , no leaks that I'm aware of.

Apexi Pod Filter

Z32 AFM with tomei plug

Manual Conversion ( issue was happening before the conversion )

Splitfire coil packs ( recently changed to stock ones , issue was once again slightly worse and a little more frequent ( splitfires don't appear to be faulty , yet to be tested ))

Bosch 040 fuel pump ( changed to walbro 255 pump recently and grounded the pump aswell ( helped the issue alot when at operating temp but did not fix it for cold ))

Might I add , with a fuel pressure gauge hooked up. When driving the card under load when cold the fuel pressure gauge dips dramatically , essentially to 0. I will presume its meant to sit around 3 - 3.5 bar/kpa ? I don't know the type of measurement. But then drops to say 200 kpa , essentially 0 its hard to tell as its very quick. ( sorry for the lack of knowledge , I'm told the 3 - 3.5 is good / the normal )

Profec B Boost controler , all hooked up correctly.
800cc SARD injectors swapped out for 550cc Bosch injectors , Tomei FPR.

Blitz VTA BoV ( replaced recently with turbosmart plumb back BoV as the ECU has a Nistune and it likes to program in fuel on the de-cel map which caused very loud and offensive backfiring( no longer backfires because of plumb back))

When the car is dead cold , I slowly accelerate and the issue is very much present. As the car warms up to say (block temperature wise) 40°C the issue starts to disappear or becomes less frequent , this is also still granny driving to work. I'm not one to ever flog a car cold.

When the car reaches 50°-60° the issue is pretty much gone , once again the issue disappears as engine temperate increases. When at operating temperate. Sitting on the dyno It made 361hp + 434nM @ 17psi. Nice figures out of the turbo I have. Also the Issue is not replicated when on the Dyno , I repeat , when put on the dyno cold and run even put underload on the dyno , the issue is not present. After that tune , the issue was barely there atall , I think it did a mild hesitation once in a week but has slowly come back over the last 2-3 weeks since the tune.

The fuel mapping is correct , what we discovered though on the real time screen of the software its adding in fuel randomly and leaning out randomly during idle and sometimes when cruising. On the programmed software it says its doing its 14.7 then in real time it will over fuel down to 11 - 12 then sit normal at 14.7 again with no change in RPMS , not keys touched or any programming done , or randomly choose to lean out on the real time software but it shows no sign of doing so on the program software when changes are made ? What could cause it to change things by its self ? Would this indicate an electrical issue ? Water temp sensor playing up ? ECU freaking the F' Out ?

When on the road , it happens the most when accelerating up an incline when the car is I suppose under actual load , and less on a flat and essentially not at all when granny driving down hill.

Side note : I recently was having a discussion with a young bloke having the same issue and he said that he knows he has an exhaust manifold leak at the gasket and his turbo is very much rooted. Mine how ever is not rooted as it would not make that power , but I have not checked my exhaust could this of be of any effect ?

Ask any questions , in request to any other mods I may have missed , just request please.

Thankyou so much to the people taking the time to read this and offer some possible advice.

Richie.

[GTScotT]

Two things:

Your fuel pressure issue is a major problem, but not the cause of your light throttle bucking. it will either be your reg, pump or wiring to the pump causing the low pressure. Attend to this before you blow le motor.

The bucking is caused by your BOV and the tuners ability to tune around it. Best recommendation is to go back to a stock bov at least as a test and see if I was right. I bet my ass I am.

[psytz]

My only issue with the suggestion is as the car warms up ... the issue goes away slowly , So why the paramater of temperature affect the reg or pump ? The pump is wired up correctly and grounded. Confirmed by an auto electrician , but I guess I can get it rechecked. See when I'm on full noise it tends to slightly overfuel , leaning out isn't even on the cards and the car only goes on full noise when at operating temperature ( completely warm 80°C + Oil temp ) as I do care for my car as much as possible. I have a plumb back recirc valve , It's just turbosmart. Why would the stock recirc make the difference ? Would it pay to set the spring pressure indentical to a stock one ?

Thanks for your suggestion ,

get back to me.

[psytz]

Also can you shed some light on the low fuel pressure ? What do you believe it should be sitting at ? Should it be more ? I'm told the pump is more than sufficient ? should I look at the physical hard lines in the fuel system ? the fuel filter is brand new ? would it make a difference ?

Bucking is a term that I like for what is happening. So the the BoV is the cause of the bucking in turn results in that very momentary plummet in fuel pressure ?

Richie.

[wolverine]

Soften the BOV as much as possible. The stock BOV is nicer to use with a turbo like the GT-RS as they crack open at idle and have a small recirculating leak designed into them from factory which dampens the tendency to surge.