Boost Cuts Out At 5000Rpm...

[Deza3000]

Got this weird boost cut thing thats been happening since last night, ran out of ideas, not sure what to do

At WOT, i get the full 1.3-1.5 bars of boost, but once i hit 5-6k revs, the boost drops off completely, and the revs climbs slowly (basically like a n/a), If i shift before i hit 5000 revs, the boost is still good (roughly 1-.1.5 bars), however once i push it to 5000revs, all the boost just drops again, and the car barley accelerates. Once i shift, the car gets back onto boost till i hit 5000revs again. No clue why its happening ==;

It happened around 15 minutes after i filled up, so thought maybe it might be a bad batch of fuel??? But then i thought, if thats the case it would be pinging as soon as i hit boost...

Boost range:

- 1st gear: 1 bar, then drops off at 5000revs to 0 (or below)

- 2nd gear: 1.2 bars, then drops off at 5000 revs to 0 or below

- 3rd gear WOT (from a 40kph pull) 1.5 bars till i hit 5000 revs then drops off again.....

If its hitting high pressure of boost, i have no clue what the problem is anyone got any ideas?

Things ive done:

- Check MAF sensor cabling

- Cleaned MAF sensor with maf sensor cleaner

- Checked for boost leaks, tightened all clamps

- Chucked in some octane booster
- Unplugged battery to reset the ECU

Things to note:
- Spark plugs are 5000ks new
- Evo is tuned for 98 octante fuel
- Car runs 100% from 0-4900 revs
- No knocking (cant hear anything)
- Idle is still smooth
- Car starts with ease

Any help will be much appreciated

[luke gtr]

I'd be leaning towards fuel lean out or cas/maf sensors malfunctioning.

So, load maxing out or signal going haywire from that point on.

It's where I'd start.

[luke gtr]

Oh, of its only since you fueled up, it could be a bad batch.

[scotty nm35]

Bad fuel isn't going to drop boost.

I would suspect the BOV is blowing open, or you have a major boost leak. Pressure testing the intake up to 1.6 bar would be my first check.

[Deza3000]

#@luke gtr
Yea i was thinking the MAF sensor was playing up, took it out and cleaned it thoroughly, yet to test it out... Also noticed the bolts came off the air box on the maf sensor, so there was a little gap between the airbox and maf.

Could it be a fuel pump on its way out by any chance?

#@scotty nm35
Ill have a look at that today, will do a smoke test to check for leaks...
The BOV is pretty new, i doubt its opening early, its a mr edition bov.

[Deza3000]

Update:

Just took it for a drive, ran a bit better, but boost is still dropping off at 6000 revs ==;

After the boost dropped on 1st @5900ish rpm, i switched to 2nd @ WOT, then as soon as the boost began to drop, i quickly switched to third before the boost completely dropped off, at which point the boost quickly shot up to 1.5bars @4000rpm, It feels like the car really doesnt want to rev past 5000-6000. Feels like the its cutting out boost for a reason. I also have a check engine light now Will need to check the error code tomorrow.

[Deza3000]

This is exactly whats happening! No clue what it is

[GH05T]

Fuel pump making any new noise?
i had the same issue on my 32 when my bosch died.

if it is dropping rail preassure and leaning out it can do damage quickly.

[Deza3000]

#@GH05T
Ill check today, it still has the factory fuel pump, maybe it is on its way out.

Yea thats true, im a bit worried now, pushing my luck forcing it to boost.

[Deza3000]

Ended up fixing the boost cut finally Turns out there was no cut, it was just the intake pipe collapsing in on boost at high revs, hence why i had no boost. We stretched out the airbox a little to create tension, which resulted in no more collapsing

Very hard thing to look for when having boost cuts, cause the intake pipe always rises when you pull over and check, so you'd think its all fine ...

Cheers for all the help

[Tonba]

^ Time to change it out for some hard piping!

[luke gtr]

Good to hear you got it sorted.