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Old 09-22-2012, 01:49 AM   #1
Merko
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Default 95 WRX Power Cut at 5000rpm (Vid inside!) SOLVED!!

Hey guys does anyone have any idea what this problem I've been having on my car for a couple of months might be?

http://www.youtube.com/watch?v=CwhgC...ature=youtu.be

Basically the car hits about 5000rpm and cuts power... in the vid I've got my foot flat on the floor and it just stops revving at just over 5000rpm and the boost gauge drops into vacuum. It feels sort of like overboost fuel cut (but doesn't cut power quite as suddenly), but it's only at about 14psi so it shouldn't (Z4 ECU - 15.65psi fuel cut)... not to mention that I've got a one-way valve to the MAP sensor hose so the overboost safety feature should be removed completely. It happens in any gear, even in 1st, where I don't think it'd even be able to get to the overboost by 5000rpm! Any idea what else it might be? Relevant mods are VF34 turbo, Walbro fuel pump and 3" turbo-back exhaust. Also if I lower the boost to about 12psi it'll cut out at about 5500rpm... so the less boost, the later in the rev range it cuts out. This started happening a couple of weeks after installing the 3" exhaust, and about a day or 2 after I put in an oil catch can for the PCV valve (probably coincidence :P). Before that I was running about 16-17psi with no problems.

Spark plugs are new, coil packs are in excellent condition, no boost leaks, no dropped off vacuum hoses or anything. I am stumped! as is everyone else I've asked... One thing I thought it might possibly be is the fact that the new exhaust hasn't got anywhere to plug the EGT sensor, so I just left it unplugged. No check engine lights so I thought it was ok. But I don't think this is the problem. I tried the resistor mod that people do on newer WRXes on it but that didn't change anything.

Any ideas? This is really getting on my nerves now!

Thanks

SOLVED: Problem was high vacuum from higher boost sucking the soft turbo inlet rubber hose shut and starving the engine of air.
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Last edited by Merko; 02-27-2013 at 04:58 AM.
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Old 09-22-2012, 06:59 AM   #2
eg33GC
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off the top of my head the vf34 has more cfm output and a much different compressor map than the small 16g..

youre probably leaning out so bad you are getting ignition cut
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Old 09-22-2012, 07:06 AM   #3
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haha if anything I'm running too rich :P I have a wideband air/fuel ratio gauge and at full boost it's about 10.5:1, which is the same as it was with the TD05.
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Old 09-22-2012, 08:20 AM   #4
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I think it's ignition related, those coil packs do not handle higher boost very well.
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Old 09-22-2012, 09:35 AM   #5
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But why then would it still be doing it at my current 12-13psi? That's stock boost.
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Old 09-22-2012, 11:57 AM   #6
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Does an ECU reset fix it? If so, it might be speedo related since I have seen multiple cases of misfires about 5k caused by the speedo signal not getting to the ECU.
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Old 09-22-2012, 10:31 PM   #7
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ECU reset doesn't fix it Is that something to do with the 180km/h speed limiter? If so my car has a HKS speed limit remover.
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Old 09-24-2012, 01:00 AM   #8
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Any more ideas? I tried putting in my old W6 ECU and that didn't fix it so not an ECU fault. Could it possibly be a walbro pump issue? I don't like the little clip that holds on the sock, maybe it fell off and it starves of fuel at high revs? Unlikely I know lol but I have no idea!
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Old 09-24-2012, 02:20 AM   #9
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It's your tune.


edit: who ever did your tune... limited your ECU to 5K,,,,,,,, it's your tune.
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Old 09-24-2012, 02:26 AM   #10
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Quote:
Originally Posted by DHRDAN View Post
It's your tune.


edit: who ever did your tune... limited your ECU to 5K,,,,,,,, it's your tune.
yeah, those pesky Subaru engineers limited his z4 ecu to 5000.


your fuel pump may be dropping off at 5000ish due to a clogged sock or maybe stock filter
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Old 09-24-2012, 03:22 AM   #11
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Yeah it's factory Z4 tune :P

I think next step will be to check the fuel pump. I'm using the stock fuel sock with it, and maybe it's dirty (though it seemed clean when I did the Walbro swap). Fuel filter was apparently changed by the previous owner about a year ago.
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Old 09-24-2012, 08:40 AM   #12
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You got some buddies to swap over a coilpack? Sounds like it could be a proper contender for causing these issues.

Jasper.
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Old 09-24-2012, 09:52 AM   #13
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You could also try lowering the boost to wastegate by looping a vac hose from the compressor nipple to the iwg. If it improves, it's a good contender for proving some fuel issues
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Old 09-24-2012, 07:42 PM   #14
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What happens if you try to go past 5k at part throttle (still will some positive manifold pressure)?

Are you still running the stock bypass-valve?

-Rob
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Old 09-24-2012, 09:42 PM   #15
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At part throttle it will rev past 5k while boosting I think (have to double check tonight but I'm quite sure I've done it before).

I'm running a GFB bypass valve which should be a high quality unit. I've had no problems with it.

Also running wastegate pressure will fix it as if I drop boost to 10psi it stops cutting.

P.S. Rob you have an email regarding the chip
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Old 09-24-2012, 09:51 PM   #16
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Yep, you need some fuel!
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Old 09-24-2012, 10:59 PM   #17
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This cut is so sudden, it seems like the ECU switches something off. Wouldn't a clogged fuel filter cause progressively worse sputtering along the rev range?

What happens when you disconnect the hose to the MAP sensor?
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Old 09-24-2012, 11:15 PM   #18
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Yes but if he can normally boost at 10psi, you can guess that there is either not enough spark or fuel for the demands beyond that.

This seems a lot like what my rotary does at 8000rpm when it runs out of gas.
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Old 09-25-2012, 03:58 AM   #19
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Ok just checked to make sure, no the car does not cut if I rev it up at part throttle. Only on full throttle.

I'm running a one-way valve on the MAP sensor hose to remove over-boost fuel cut so disconnecting it won't do anything.

Gonna check the the fuel pump/fuel sock/fuel filter next to see if there's any restrictions.
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Old 09-25-2012, 09:42 AM   #20
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It will be a real big surprise to me if a fuel filter sock or pump change will fix this. A cut that sudden is electrical. Testing the MAP sensor as suggested takes 2 minutes max. The fact that you have a one way valve shows that you've had similar issues before.

Just to confirm, are you sure what the MAP sensor looks like? Are you sure that your one way valve works at 15psi?
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Old 09-25-2012, 04:22 PM   #21
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Quote:
Originally Posted by Merko View Post
At part throttle it will rev past 5k while boosting I think (have to double check tonight but I'm quite sure I've done it before).

I'm running a GFB bypass valve which should be a high quality unit. I've had no problems with it.

Also running wastegate pressure will fix it as if I drop boost to 10psi it stops cutting.

P.S. Rob you have an email regarding the chip
Which GFB valve exactly? I did not see any bypass valves for the GC8 on their website.

Explain specifically how your one-way valve on the MAP sensor works.

-Rob
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Old 09-25-2012, 04:33 PM   #22
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iirc the gfb is nearly identical to the forge for our v1/2 , so it is a full recirc
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Old 09-25-2012, 04:51 PM   #23
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Quote:
Originally Posted by eg33GC View Post
iirc the gfb is nearly identical to the forge for our v1/2 , so it is a full recirc
I had assumed so, but after a quick check on their website, I only saw a BOV listed. That is why I wanted to get confirmation.
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Old 09-25-2012, 07:43 PM   #24
Merko
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Quote:
Originally Posted by pmugabi View Post
It will be a real big surprise to me if a fuel filter sock or pump change will fix this. A cut that sudden is electrical. Testing the MAP sensor as suggested takes 2 minutes max. The fact that you have a one way valve shows that you've had similar issues before.
The reason for the valve is to remove the factory overboost fuel cut. There were no issues before installing it, I just wanted to run more boost than the Z4 ECU would allow

Quote:
Originally Posted by pmugabi View Post
Just to confirm, are you sure what the MAP sensor looks like? Are you sure that your one way valve works at 15psi?
Yes I know what it is :P And I am positive it works at 15psi because it worked for a couple of months with the car at 16-17psi. The current issue is not an overboost fuel cut. Though originally I suspected it was.

Quote:
Originally Posted by rob View Post
Which GFB valve exactly? I did not see any bypass valves for the GC8 on their website.
I am using a GFB Mach 1. It's not listed for the WRX because it's a universal valve that I simply made fit. It's a Blowoff valve that can also be run recirc (as I am doing).

Quote:
Originally Posted by rob View Post
Explain specifically how your one-way valve on the MAP sensor works.
It's a very simple little trick (thought it was pretty common?). You put a one way valve in the vacuum hose that goes to the MAP sensor. By doing this, the MAP sensor can sense vacuum (if it even does that), but it is unable to sense boost, because the one-way valve prevents any boost from reaching the MAP sensor. This therefore eliminates overboost fuel cut because the MAP sensor isn't detecting any boost. This method works brilliantly. And before you ask, yes I have the valve the right way, and yes it is working :P (have tried another new valve). But my issue isn't overboost, as I am now running 3psi less than the Z4 fuel cut.


Here is my setup. You can see both my bypass valve and the grey one-way valve on the MAP sensor hose in this pic.


Last edited by Merko; 09-25-2012 at 07:49 PM.
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Old 10-12-2012, 09:10 AM   #25
Merko
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So... any more ideas?

Have now replaced the following:

-Spark plugs
-Walbro with another new Walbro
-Cam sensor
-Crank angle sensor
-Made sure there's no rusty grounds for fuel pump wiring (apparently common issue).
-Removed the one way valve on MAP sensor (car's only running 12psi now anyway so fuel cut not an issue)
-Plugged in my old (known working) W6 ECU

and it's still cutting

Please someone give me more ideas before I push it off a cliff!

Could bad air flow meter do this? mine looks pretty good, and was sprayed with AFM cleaner not long ago.
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