Dangerous power cut/jerks during WOT + high boost

[ARGH34]

I have an R34 GT-t and experience a very strange (and dangerous) power cut whenever I floor the throttle or run up high RPMs.

Usually happens around 6000rpm in 1st, 5000 in 2nd, 4500 in 3rd, 4000 in 4th and 3500 in 5th. What happens is when the car is spooling up and when I accelerate, the car suddenly loses all power and jerks forward harshly and then hits boost again, then jerks, then accelerates and so on... so you end up jerking your head forward and back pretty violently.. its pretty dangerous because you sort of lose control of the speed of the car and the whole balance is upset.

Once when it happened, I kept on trying to accelerate and the orange oil lamp on the far left of my dash flicked on for about 3 seconds...

I got an airfilter, exhaust, external venting BOV, bleed valve, RSM..

I have a feeling the car's boost is too high and perhaps there is an auto cut-out protection of some sort if you boost spools up too high? I notice on my boost gauge it happens when the boost is around 1.2-1.5 x100 kPa.. I don't have an intercooler and I'm not sure what boost the car is running at the moment (perhaps 1 bar)?

also the car previously had an FMIC and Power FC installed, so I'm thinking perhaps the engine management has been changed and they didn't revert it back when they pulled it out.

Any comments, advice or opinions would be greatly appreciated, thanks.

[whistla]

Boostcut/Fuel cut

i never reached it with my stock computer, but i know what your symptoms are

some funny little shit set my boost cut at 12pound on my aftermarket engine management, and everytime i reached 12pounds my injectors would shut down, re start, shut downa nd jerk the car violently

good chance is your reaching factory boost cut, its a safety measure implemented in the computer so as when your wastegate vaccum line decides to come off, the turbo/engine is still protected from over spooling

find out what the stock boost cut/fuel cut is on the factory r34 computer (i dont know it personally)

anyone else?

[Zahos]

Yeah, I had the same problem when I set my boost too high...

How did you set your bleed valve?

1. dyno?

2. on street with after market boost guage?

3. on street with stock boost guage?

4. guess?

When I set mine using #3, I hit the same problem. Then I got a boost guage and set it using #2, no problems....

So wind your boost down a little...

What is is currently set at?

[ARGH34]

Thats the thing I'm not too sure what the previous owner set the boost at. I think it would be approx. 1 bar, and from what I've been told this isn't healthy at all for the stock turbo/internals. When the cuts happen, my aftermarket boost gauge reads about 1.2-1.4 bar.. which is far to high (also from what i've been told).

In which case I think I'm gonna have to wind down the boost.

Any tips on how to lower the boost on a turbosmart bleed valve, and how to check it myself?

[Zahos]

That is WAAAAAAY too high if ur turbo internals are still stock.

Wind it all the way down so your boost is something line 0.5bar.

Then take it out, and turn it just a little, and you will see your boost go past 0.5bar.... then keep going until you get it to about 0.7 ~ 0.75 bar. Any more than this on a standard turbo can cause major damage in a very short period of time....

[ARGH34]

I'll go home and do that tonight. But I'm very green to all of this.

Specifically how do I go abouts changing the boost? Do I just turn the little knob down all the way? Do I have to pull the valve out from its place, or can I just leave it in and turn it?

After that, do I take the car for a run and keep adjusting so that when I'm on full throttle the boost maxes out at 0.7?

[Zahos]

there is no need to remove the valve... just leave it where it is.

The valve will have arrows on the knob, indicating how to lower and raise boost. Lower it all the way...

Once you have done that, take the car for a spin, and note what full boost is.

If it is below 0.7, turn the knob on your blled valve to raise the boost a little, then go for another drive. Keep repeating this until you hit 0.7bar or *just* over. 0.7bar is approx 10psi, and 0.8 is approx 11.5 psi. at 0.75 it will be arounf 10.7 which is about as far as you should go. To be safer, stick to about 0.7bar.

Zahos

[ARGH34]

Cheers for that. I'll try it tonight and let you know how it goes

[ARGH34]

Hm. My bleed valve was wound all the way down already.. went for a drive last night and under heavy boost the same thing happened. Running at over 1 bar, up to about 1.2.

Is there another way to affect boost besides the bleed valve? I don't have any electronic boost controller.

If this keeps up I'll have to see a mechanic I think..

[Zahos]

It couldn't have been. It might have been wound all the way up...

Did you try turning it the other way to see what happens?

If it the same, then you have a leak in the line that goes to your wastegate.

[ARGH34]

I looked up the turbosmart website and found instructions on my valve:

http://www.turbosmart.com.au/pdf/FG-GBCV-001.PDF

I checked it with a screwdriver and it was wound clockwise all the way already (i.e. it was screwed down as far as possible). Turning it anti-clockwise makes the screw come out looser and looser. I didn't try loosening it coz I'm worried that the boost will run even higher.

Is it possible that the computer settings have been changed to run higher boost?

I'll try loosening the valve tonight and taking it for a drive, but I'm pretty sure that I been screwing the valve down the right way... :

[Zahos]

Hmmmmm

Well then I am guessing a leak... Or maybe even a blocked hose???

I cant really tell without looking at it, to see how it has been setup...

Do you know anyone that could have a quick look at it for you?

[NismoR34]

How long have you had the car? Just wondering whether it was like this when you test drove it?

AFAIK - the boost cut on our 34's is approx 1 Bar. I have hit the boost cut before and what u described in 1st post is basically what happens [jerking etc].

Even if you could turn that valve more you are not going to develop anymore boost cause you are already hitting the limiter / cut out, you will only hit the cut earlier if possible.

Perhaps the valve was plumbed in reverse somehow - JUST turn the knob and take it for a test drive.

A fully OPEN bleed valve should run stock boost, a fully CLOSED valve will give maximum boost!

Where abouts in "Australia" are you?

What colour is your 34?

BTW: stock boost on a 34 is .55 bar which is 7.9 psi!

Cheers.

[ARGH34]

I bought the car recently, and didn't notice the jerking but was told any jerkiness would be due to the TCS kicking in (which I've found to be false last night in the wet, because the slip light flicks on).

While I'm by no means a pro on this, I had a look at the plumbing of the valve last night, and the arrow on the valve is pointing towards the round silver wastegate actuator, so it looks like its plumbed the right way. I'll post a photo on the weekend if that helps. I'm in Perth btw.

I'll try twist the knob the other way a few rotations to see what difference that makes. Been a bit hard to test lately coz its been raining over here and whenever I try to boost I'm losing traction bigtime. Damn I wish I had ATESSA.

Zahos:

Yeah I got a few tuning shops in mind I might pop into over the weekend or next week when I have the time. For the moment I'm keeping it under 4000rpm coz I don't wanna fry anything... and I'll post a photo if that'll help.

[ARGH34]

Oh by the way you may have noticed I've run the same thread on skylinesdownunder, and the people here have so far been a lot more helpful than the other board... wonder why eh?

[ARGH34]

Nah turning bleed valve either way hasn't worked at all. Think its time to take it to a shop...

[Duncan]

Originally posted by nismoR34

. A fully OPEN bleed valve should run stock boost, a fully CLOSED valve will give maximum boost!  

mmm, my understanding is the opposite, opening the valve gives you more boost as air is allowed out thru the valve.

clockwise = close = less boost

[NismoR34]

i think its hard to compare in text. my boost valve is mounted upside down for starters so clockwise doesnt mean much.

definately on my car a reduction in the airflow to the factory boost control valve results in more boost!

Read 21st century performance book or look up *boost control* on www.autospeed.com.au there are articles in both that demonstrate that a *restriction* is needed to increase boost!

[ix9]

If adjusting the bleed valve does nothing, you probably have it on backwards...

There should be an arrow indicating pressure flow -> wastegate, check this before you take it to a shop.

[Zahos]

If you go back up to post 14 , he already said he checked it is on the right way.....

Most likely it will be either:

- a leak in the line somewhere

- a dodgy valve

- a dodgy setup