Why would you be boosting to 17psi? MP usually means megapascal, but if you're referring to the factory boost gauge isn't it mPa? Either way, from what I remember the factory boost gauage is roughly equivalent to bar.

 

On the MPa im not sure, i just went to the link i provided you, and typed in 17psi, then went through the M's that equaled what my boost gauge reads. None of the others were close besides Megapascal and Meganewton/meter. So i figured it would be either one of those two.

The only thing i can think of why it would hit 17psi after a TBE install would be because the turbo is allowed to breathe easier without the cats and larger diameter piping. Boost cut is set at 17+ psi. If the ECU sees 17psi for more than 3 seconds it cuts fuel just enough to drop boost off. Searching on Nasioc it seems like there are a lot of threads about this already, and everyone is confirming my thoughts of the boost cut.

One way to remedy this is to get an MBC or EBC, Im not sure if Access Port has the ability to do this or not.

 

And also, MPA isnt an equivelant to bar. 17 PSI is 1.1 bar, and .11MPA

 

Eep, those damn decimals always moving around. Regardless, stock boost should be 14.6, so that could be part of your problem.

 

Did you reset the ecu after the DP install? If not try that first.

I’ve never installed a DP without a reflash so I can’t say I know for sure if the DP itself is the problem but my guess would be that it isn’t. I would think it’s more likely that a loose vac hose, faulty WG or bad FBC is the problem. If resetting the ecu doesn’t work read my first post (#11), follow the steps and you might find your problem.

I doubt you are hitting fuel cut @ 17psi and I doubt the ecu would decrease fuel to reduce boost since a reduction of fuel would just lean the a/f ratio. My understanding is that the ECU has a set value of boost that it doesn’t want to exceed and since the WG is set to open at 7ish psi it needs the FBC to bleed off pressure to the WG solenoid until that target boost is reached. When the ecu sees the target boost it sends a signal to the FBC and it stops bleeding off pressure which makes the WG open. If the FBC is stuck open, a vac line is loose or the WG isn’t working the way it should you will see a spike.

I’m not sure how the ecu cuts boost other than fuel cut but I’m guessing it would be more to do with timing rather than fuel since (I think) fuel cut shouldn’t happen until around 20 psi.

A lot of this post is just what I think and not what I know so if you see something wrong, speak up.

Looks like I have some new question to post on the forums.

 

ECU has been reset 3 times, and i have driven the car ~400 miles since the TBE install. Still does it. And im positive it isnt some kind of boost leak, because nothing on that side was disconnected, and the car didnt have these problems prior to the TBE install.

The fuel cut is from the MAP, which stock is ~17 psi. This is just what it is, do a search on NASIOC for Boost Cut, and you will see tons of threads about this. This is, without a doubt IMO what the problem is.

The turbo is able to breathe more freely, which in turn creates more boost quicker. The boost hits .11 mpa which is ~17psi, the ecu cuts fuel (not entirely) but enough to reduce boost, and rpms. Boost falls to about 10psi, the ECU sees this is too low for WOT, and allows boost to increase again, where it will hit the boost cut, and continue the cycle.

This happened to me the other night going up a large hill in 4th gear. I went WOT, and my boost needle would go from .11mpa the fall quickly to .07mpa, then back up, literally swinging back and forth.

The only way to really fix this is to get an FCD (Fuel Cut Defender), or some kind of EM. Ive heard Access Port can stop this.

 

The problem that has been discussed in this thread is not fuel cut, it’s boost spike. Unless you have failed to mention some other symptoms your explanation is describing boost spike. If you were hitting fuel cut (your right it’s at 17-17.5psi) you wouldn’t need a boost gauge to know it because you would most likely see a CEL and your car would hesitate (a lot) then hit again when fuel was turned back on. Fuel cut is a 100% cut in fuel (temporarily killing the engine) not a partial cut in fuel (a partial cut would lean you’re a/f ratio). The turbine in a turbo is propelled by exhaust gasses so killing the engine stops exhaust flow which causes the turbine to slow thus decreasing boost.

I did a fuel cut search and your thread was the first one I read.

Cut and pasted

Question
Ok, so i have searched all over this forum, and have found tons of info on WHY fuel cut happens. But what does the ECU actually do during fuel cut? Does it cut just enough fuel to drop the boost down? Does it lean out the engine when it cuts fuel like that? Does it retard timing to reduce chances of detonation if it does run lean like that? Im sure Fuel cut isnt GOOD for the car, but does anyone have info on how bad it really is?

Answer
Fuel cut is what it sounds like. The injectors are shut off so that the engine can no longer accelerate on its own. Once it gets back into a happy range (from overboost or rev limit conditions ect) it will continue as normal. Timing isn't a factor during fuel cut because there is no fuel to ignite. I am not positive, but I imagine the wastegate is opened up to bleed off turbine speed. It is the speed limiters in the newer ECUs lower the boost.


This answer does not contradict my explanation. I wasn’t saying that timing is pulled during fuel cut I was trying to make a guess as to how the ecu could decrease boost without fuel cut or a working FBC.

If your positive there isn’t anything mechanical wrong with the car I’d go out and get an accessport or a dyno tune and hopefully that will fix your problem.

Before you spend your hard earned $ you should try a boost spike search and I think you will see where I’m coming from. Posting your own thread that explains your problem and includes the make of DP you installed along with the year of your WRX would also be helpful.

Good luck and keep us updated.

BTW .11mpa=15.95415psi and 0.11 bar = 1.595 415 118 pound/square inch not 17psi and fuel cut is at 17ish psi not 16. http://www.onlineconversion.com/pressure.htm

 

Ok, i dont know about anyone else, but im not getting an CEL's from this. So if this isnt Fuel Cut that is making my boost swing like ron jeremy at an orgy, then what is it?

Going up hill, in 4th gear at WOT my boost hits the ~.11 then falls to ~.07 repeatedly. Even spirited driving in 2nd hear around town will yeild the same results.

Im not fully convinced that Fuel Cut completely shuts the injectors down. I have heard at a higher boost level (i believe it was 22psi) on the stock ECU is when a full shut down will occur, but at 17psi it is cut out enough to lower boost, but not completely shut the motor down. There really are no good threads anywhere describing how this actually works, so im having to dig through some garbage, and try to find what is going on.

I am pretty sure that the boost spiking and fuel cut after this new TBE have to be related somehow since this wasnt as big of an issue before.

BTW: This is on an 02 wrx with TBE.

 

I did my TBE about 2 months ago and have had no problems with boost. If the TBE is your only mod, you shouldn't be boosting above .09-.095 mpa. If you can I would suggest switching ECU's with someone and drive it around for a couple miles and see if you get the same results.

here's a good conversion table.
http://www.chapelsteel.com/psi-mpa.html

 

Boost spike under 17psi will not give you a CEL but I would think fuel cut which doesn’t occur @ 16psi should and that is one of the reasons I’m almost positive you are not hitting fuel cut.

I think I’ve given you all of the advise I can on what is wrong with your car.


I'm not sure if you know this or not but fuel acts as a coolant in the combustion chamber and air heats things up. If you are overboosting (more air=more heat) the last thing you want to do is take some of the fuel out of the equation.

Like I said before you need to do a boost spike search and stop with the fuel cut searches.


Are you saying that you had a small spike before the TBE install?

 

CRAP CRAP CRAP! I jinxed myself. On my way in to work I noticed the boost went to .1 mpa then went to .06 mpa and hovered between them until I let off the gas. I have a CEL that I will try to check out tomorrow morning though.

 

this is the same problem I've had from the beginning of time. Then I set up my stage 2 and got tuned by Ed. Everything was fine for a long time. Then out of the blue, it started happening again. I notice it in the cooler weather more than at any other time. For what it's worth, the car was road tuned in like 95 degree heat. It really does run well in the hot weather. I don't really notice the boost fluctuation. but when it's cool out it really hinders performance and I have to stay off the throttle.

So to reiterate:

When car was new it ran well for a long time.
Then I noticed the fluctuation every once and a while while still stock.
Went to stage 2 it ran fine. Got custom tuned and car ran really well.
Just recently started seing fluctuation again. Really bad in cool weather.

02 wrx sedan

 

Have you tried any of the troubleshooting I mentioned at the beginning of this thread? If so what did you come up with?

Spike will be more drastic in cold weather since cold air is more dense.

 

Another thing that might be worth a try is lengthening the WG actuator arm but doing that might increase flutter.

Lengthen rather than shorten. http://www.scoobymods.com/forums/showthread.php?t=3654

 

I had, and still have this issue after I was running a UTEC and then went back to stock. I think the factory BCS is screwed up. My fix was to use an MBC and I've been fine at 14.5psi with it.

I'm going to reinstall a UTEC and use a perrin BCS with it. I don't think this has anything to do with wastegates, boost spikes, or boost creep. It's the factory BCS going south.