Full boost surge/ backfire? Help please! :(

[Mrstevieboy]

Hi All,
I've got an issue that has begun arrising and i'm fearful.
The car has had a bleed valve on it for 4 months connected to the manifold pressure tap (Tube with red line on it)

I've tried searching and couldn't find anything last night or this morning...

Before the bleed valve went in the car was sitting on 11-12psi.
i was under the impression the car was stock but after some reasearch i found out it probably has the restricter removed from the boost solienoid.
Anyway, Currently the car has developed a like backfire
( i can hear a knocking sound from the engine bay something that sounds like lauch control backfires)
when at full boost. upon reading the boost gauge. is holding steady up at 15-17psi!!
But i can feel power being lost during the taps/ knocks similar to a rev limiter, but the revs are at around 4-5k

My question, is why aren't i getting boost cut at 13psi,
and what is the tapping/ knocking sound coming from the engine bay?
Hopefully no a rod/ piston giving a swan song...


**EDIT** Could this be the coil packs or plugs malfunctioning under high boost/ compression?

 

[VOLK]

Initial thought is blowing spark out. Incredibly common issue. Plugs, leads, coilpacks. Turn down the boost, does it go away? Boost cut threshold had probably been raised at one point.

What plugs you running?

 

[Mrstevieboy]

Initial thought is blowing spark out. Incredibly common issue. Plugs, leads, coilpacks. Turn down the boost, does it go away? Boost cut threshold had probably been raised at one point.

What plugs you running?

Thanks Volk, To be honest i'm unsure as the car was only imported 6 - 8 months ago.
I've turned the bleed valve all the way down at still see higher then normal boost... i will be changing this rubbish bleed valve with a EBC at first opportunity.

I first noticed the issue in winter before the bleed valve but it only happened on extreme load in the cold . this only happened once or twice.
If it is spark being blown out, will it cause any damage to the engine?
Also if boost threshhold has been raised, does this mean the car has previously been mapped?

I am most likely in the situation of servicing/ changing all engine consumables to get even playing feild.
I'm hoping its not coils, but i'll check the front banks of plugs and double check the gapping... see if theres any issues there.
Not too keen on the back bank unless im completely changing the plugs.
What iridium plugs should we be running? and from my understanding you can't re-gap irridium plugs?

 

[Hotwire]

The label of boost cut is incorrect, it is load related - more like fuel cut due to load.
Load is calculation of airflow so relative to boost, but if if you have stock exhaust can get higher boost pressure.
i.e blow out your mouth a certain volume of air
now try blow the same volume through a straw, you will have a far higher pressure.
so 15psi is possible not to get load levels high enough for cut if there is a restriction (like a cheap shitty or blocked catalytic converter)

 

[Hotwire]

Also your boost gauge could be fecked, have you checked with another gauge for its accuracy?

 

[Mrstevieboy]

So potentially Catalytic converter is blocked?
I've got a 2.5" High flow cat in my garage that is waiting to be put on was also thinking of putting an exhaust cutout
Stock exhaust pipe is 2.5" correct?

the original electrical Boost gauge was removed due to malfuctioning its been replaced with a pneumatic boost gauge that was on my golf.

 

[Hotwire]

No I'm not saying that the cat converter is blocked, that's just one possible scenario
The point I'm making is that the level of boost may well be 15psi, just because others say around 13-14 they get cut is not always relevant to your car. Compliance cats are sometimes very cheap restrictive units. However, before spending money changing that, ensure to see what boost you are running, get someone to log the vehicle to check for knock, airflow values, ecu load etc to see what is happening.

the forum won't help you solve it based on speculation sorry, you need more evidence of what is going on.

 

[Mrstevieboy]

Ah all good. I have an OBDII 409.1 KKL USB cable
Would i be able to use that with EVO Scan to see whats going on in more detail?

Looks like i'll be doing a bit more investigation when i get off work...

 

[VOLK]

If it's an FTDI based cable, yes, it will work. The winchip stuff doesn't sadly. Gotta bridge pins one and four though. I'll dig up the cheap data logging thread in a sec...

Way to confuse the guy Lee Regardless of what the underlying conditions are that cause it, it's called boost cut in ecuflash, so that's what I call it. But that's not what's wrong here!

Sounds like a clear cut case of ignition breakdown under load. Pull a spark plug out, check the brand, model and gap. Unless they're vk22s, I'd probably cut to the chase and buy some.

 

[Mrstevieboy]

Ok So update, Bought a set of VK22's.
Got the old plugs out, They're PK20PR-P8 Platinum's that were gapped to .89 (Didn't think you could gap platinum's).
So that might be the issue there... I haven't had a chance to change them yet, not too keen on the back bank...
Need a weekend where the girlfriend hasn't got a million and one things planned for me so I can work on the car.

 

[whats_doing]

Don't think .09mm will make a huge difference. They could just be old.

Just take it slow when doing it if it is your first time removing the plenum so you can remember how everything goes.
I don't remove the throttlebody off the plenum, I just flip it out of the way.
Be careful of your plenum gasket though that you don't break it. Also if you can, install studs into the left and right side of the plenum into the intake manifold. Makes lining up everything so much easier.

 

[VOLK]

PK20PR-P8 is 0.8 from factory. Probably just dead/old/wrong heat range for the boost or a combination of all three. Don't be scared of the back bank, you'll be pulling that plenum off A LOT in future Unbolt throttle body from plenum, remove y-pipe, unbolt plenum. Done.

 

[Mrstevieboy]

Well, Got back from our holiday today around 8:30 pm.
Decided at 9pm to attack it and was finished by 11pm taking my time and making sure I didn't do anything stupid.
the below are the plugs from the Galant, As you can see, the center 3 (which were all from the back bank) were pretty rooted.
The other 3 had a similar amount of meat left on them but all the same, they were getting tired.

Just from the Seat-o-pants dyno i've got, it feels like I've picked up at-least 30% power, possibly even close to 50% up top.
I never realized what the car was supposed to feel like. its insane the difference a tiny amount of metal makes...
So anyway, I also got proper boost cut in my drive, like proper throw you into the window boost cut.. it was NOTHING like the surge i was getting.
The surging was more like a hesitation, nothing violent. Which i'm so glad its fixed.
Just got to get a proper tune now! Can't wait for the next Melbourne day!

Thanks Volk for the quick answer!

 

[trotty]

Make u wonder how u were able to boost at all with that gap.

 

[Mrstevieboy]

Well i've only had the car for about 3,000km...
i knew the fuel economy of our cars was horrid. but it now makes alot more sense.
I just thought that was the power level. but I believe i was only runnings between 3-5 cylinders on high boost.

 

[VOLK]

You're welcome mate, easy fault to diagnose due to the frequency at which I've seen it occur on here. VK22s are great plugs. You can make them even better and extend their life with coil on plug from @naughtika.