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This is a discussion on Engine cutting out at full throttle within the Engine Modifications forums, part of the Tech & Modifying & General Repairs category; Hi all, new here I bought a 2002 WRX from someone about 6 months ago, and there is one issue ...

  1. #1
    Registered User Kush's Avatar
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    Engine cutting out at full throttle

    Hi all, new here

    I bought a 2002 WRX from someone about 6 months ago, and there is one issue that is driving me nuts.

    At 3/4 - full throttle, with boost between 0.1 and 0.15 Mpa (I think thats around 15-17psi) the motor just cuts out for a second and comes right back - I mean it totally shuts off and re-fires, not just stumbles. There's a manual boost controller on the thing, could it be that the previous owner had the boost set too high and there's not enough fuel at that amount of boost to keep it running?? It's also got a full exhaust, BOV and air filter if that helps.

    Thanks alot for any insight, I'm afraid that I'm damaging the motor, so any help would be appreciated.

    Aaron

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    Registered User cheeseybacon's Avatar
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    Yes, I'd be willing to bet that the manual boost controller is the source of your problem. WRXs general don't take well to MBCs, and while some people will argue that the older 02s could handle a MBC better than the new models, they still cause can issues like PTFB (part-throttle, full boost) which is still dangerous and not working risking if you can help it.

    Stock boost is around 13 psi, and 16.5 psi is about the highest you'd want to boost the stock turbo. If you're smart, you'll immediately ditch the boost controller and get yourself some proper engine management instead. Engine management will instruct your injectors to provide proper fueling to match your increased boost. As it stands now, your boost is increased, but your fueling has not been increased to match it, causing you to run lean, so yes you are correct in assuming there isn't enough fuel. Running lean makes the engine run hotter, and can certainly cause damage, especially for extended periods. When you run lean enough, the ECU must intervene and completely cut the flow fuel to your car to stop the lean condition, causing the cutting out effects that you are presently experiencing.
    Last edited by cheeseybacon; 03-07-2006 at 10:50 AM.
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    Registered User Kush's Avatar
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    Thank you! I was somewhat right then

    I see what you're saying about why it's cutting out. Can you recommend an engine management unit? Also, would engine management also control boost, or would I need to get a more sophisticated boost controller to work with it?

    Thanks alot,
    Aaron

  5. #4
    Registered User Kush's Avatar
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    Thank you! I was somewhat right then

    I see what you're saying about why it's cutting out. Can you recommend an engine management unit? Also, would engine management also control boost, or would I need to get a more sophisticated boost controller to work with it?

    Thanks alot,
    Aaron

  6. #5
    Registered User cheeseybacon's Avatar
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    With proper engine management you do not need a boost controller. The engine management will control your boost for you. The Cobb AccessPort is very popular, I suggest you take a look at that. The TurboXS UTEC is also another option, or a Ecutek custom tune/reflash. Any one of them will do the job nicely.

    Also, I see you have listed "full exhaust". What do you mean by full exhaust? Just a catback? A full turboback? Is there an uppipe included in there anywhere? A catless uppipe is super important to have, and I'd be willing to bet that the idiot (I say idiot because MBCs, BOVs and intakes are the #1 installed mods by people who don't know what they're doing) who previously owned your car didn't put one in because they're tricky to install.
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  7. #6
    Registered User Kush's Avatar
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    You gotta forgive me, I'm a 70's camaro guy so all of this technology is somewhat new to me.

    I'll take a look at the controllers you mentioned. You said something about part throttle/full boost - I thought that was normal for these cars, I guess I was wrong haha. I definately get boost up to 0.1 Mpa at maybe half throttle or less, depending on my gear and speed. The acceleration is great, but I feel like it's not as controllable as it should be (sudden accel, delayed accel, etc.)

    As for the exhause, I'm not too sure on the terms, but I guess it would be an up-pipe. I'ts aftermarket all the way to where it connets to the turbo. It does have a cat, though that might be aftermarket? Also, there's no front or rear o2 sensor so I'm constantly throwing codes for it

    I'll take a look at the exhaust in a little bit and give you a more detailed explanation.

    Previous owner an idiot? Yeah, I'll agree with that

    Thanks again,
    Aaron

  8. #7
    Registered User cheeseybacon's Avatar
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    If I were you, I'd do the following (in this order)

    1) Immediately remove the boost controller and sell it.

    2) Sell your BOV and put the stock bypass valve on if you can bring yourself to part with the "psssh" sound. BOVs cause you to run rich in between shifts and can cause stumbling and other driveability problems if they aren't adjusted properly. Even when adjusted properly, some problems may still exist.

    3) Check your intake. What do you mean by "air filter"? If if you just have a high-flow air filter in the stock intake box you're fine, but if you have a full blown aftermarket intake you're gonna want to remove and sell that as well because 99% of intakes out there mess with your MAF sensor readings and also make you run lean. Cobb makes the only intake that is gauranteed not to mess with your readings.... not that an intake gives you any power improvement to begin with, they're all basically expensive noisemakers.

    4) Check to see if you have a catless uppipe installed. This is very important. If you don't have one, then now is the time to buy one. The uppipe is located between the exhaust manifold and the turbo and feeds the exhaust from the engine into the turbo. Everything after the turbo leading to the tailpipe is called the turboback, which consists of a downpipe (which contains two cats in stock form) and a catback (the remaining piping and muffler). The stock catted uppipe has the potential to fragment apart and fly into the turbo if subjected to too much heat. Under normal circumstances this usually isn't a problem, but given the conditions that your car has been running in, I'd be willing to bet your uppipe cat is a lot worse for the wear than most. Besides, replacing the stock uppipe with a catless uppipe will snag you a safe 10 hp, and eliminate a boatload of turbolag. Uppipes are cheap, you can get one for around $100 if you look around, but you should have no problems with that after you sell your MBC and BOV.

    5) Buy some proper engine management. In addition to providing proper fueling and timing to match the increased boost, EM can also get rid of exhaust related CELs. There are a lot of options, but the AccessPort is probably what you'll want. It's easy to use, and has a variety of maps to match different levels of modification. Then later on if you want to get a bigger turbo, you can get your car professionally tuned and have a custom AccessPort map made just for your particular car/setup.
    Last edited by cheeseybacon; 03-07-2006 at 10:36 AM.
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    Registered User Kush's Avatar
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    The air filter is just a cold air system that ends at the actual intake, so the intake itself is stock.

    I'll definately get rid of the MBC and (probably) the BOV (though I've sorta gotten used to driving a space jet )

    I double checked and there IS a cat on the uppipe, so I guess I should find a used one to stick in there, new ones that I've seen are upwards of 400 bucks Should I look for one with a flex pipe or is that not necessary?

    EDIT: Just found the Helix and Invidia uppipes for under 200 bucks.

    One question - if I take the MBC out, how much boost will the EM allow without having to do other major mods like injectors, major tuning, etc.? I've read that the fuel system is good to 300hp - will the EM take me to that point (along with exhaust, intake, etc.)?

    Thanks alot for the help, this is extremely educational, I definately have respect for you guys that know how to mod these high-tech cars (well, more high-tech than I'm used to lol)
    Last edited by Kush; 03-07-2006 at 02:25 PM.

  10. #9
    Registered User Kush's Avatar
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    One more question - lets say I had an opportunity to buy another WRX, modified marginally, but properly, and sell mine as it is. Would you say that my motor has the potential to be damaged from running so lean for so long to the point where it may be better to start with another car, or would you say that it is safe to stick with my motor and fix the current problems (MBC, uppipe, etc.)?
    Last edited by Kush; 03-07-2006 at 02:51 PM.

  11. #10
    Registered User cheeseybacon's Avatar
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    Quote Originally Posted by Kush
    The air filter is just a cold air system that ends at the actual intake, so the intake itself is stock.
    When you mean, cold air system, are you talking about just a cone filter on the stock intake tubing, like this...


    or something different?

    If your MAF is attached to any kind of piping other than the black ribbed piping like in the picture, then you need to change that out. The MAF sensor is mind-bogglingly sensitive, and it's the piping itself that the MAF sensor mounts to that causes all of this trouble.

    Quote Originally Posted by Kush
    I double checked and there IS a cat on the uppipe, so I guess I should find a used one to stick in there, new ones that I've seen are upwards of 400 bucks Should I look for one with a flex pipe or is that not necessary?

    EDIT: Just found the Helix and Invidia uppipes for under 200 bucks.
    You will hear people argue the virutes of both solid and flex uppipes. Some people claim that solid pipes flow better with less turbulance than a flex pipe, while others says solid pipes are more leak prone. Whether or not solid pipes have a definate performance advantage over a flex pipe cannot be confirmed or denied. Both flow 1000 times better than the stock catted uppipe. The primary reason why flex uppipes are more popular is because installing an uppipe without a leak can be somewhat difficult and the flex uppipe allows for some slop and a slightly larger margin for error during the instalation. If you do everything correctly however, you can install a solid uppipe with absolutely no leaks at all. 9 times out of 10 an installation-related uppipe leak is due to using an inexperienced person using improper torque sequence and slapping on sh*tty gaskets. The majority of gaskets that come with most aftermarket uppipes aren't that great. If you're smart, you'll get yourself a set of genuine Subaru gaskets. Aside from the exhaust manifold, the uppipe is under more heat and pressure than the rest of the exhaust system, and even slight leaks are absolutely not an option. Furthermore, the uppipe is somewhat of a pain to get to, so making a 2nd trip under the hood to replace the gaskets a year or so from now is the last thing you're gonna want to do. The trick to a leak-free install is when you're torquing everything down, first torque down the exhaust manifold to the head, then the uppipe to the exhaust manifold, then the uppipe to the turbo, in that order. Doing it any other way is just asking for leaks. Oh yeah, a oxygen sensor socket is a HUGE help for an install like this.

    Quote Originally Posted by Kush
    One question - if I take the MBC out, how much boost will the EM allow without having to do other major mods like injectors, major tuning, etc.?
    The ECU with stock programming, will allow a maximum boost level of 13.5 PSI in third gear. This means by removing the MBC you might actually feel a bit of a performance decrease as your boost drops from 15-17 psi back to 13.5 psi. You'll be safer though, you won't have PTFB anymore, and you won't be running lean as hell. When you're ready for engine management, the stage 2 map (which is what you'd be running for your present level of modifcations) via the Cobb AccessPort will safely increase your boost up back to 16.5 PSI with all the proper fueling and ignition timing to match it. 16.5 - 17 psi is about all the stock TD04 turbo is good for, any higher and the turbo begins to move out of it's efficiency range and at that point you're just blowing hot air. The stock injectors and fuel pump will do everything you need them to do on the stock turbo up to 16.5 PSI. You don't need to be think about upgrading them until you're ready for a bigger turbo.
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  12. #11
    Registered User Kush's Avatar
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    Quote Originally Posted by cheeseybacon
    When you mean, cold air system, are you talking about just a cone filter on the stock intake tubing, like this...
    or something different?
    Instead of the rubber ribbed piping after the air filter element in the picture, I have something this (not my car, but the same type of system):



    If your MAF is attached to any kind of piping other than the black ribbed piping like in the picture, then you need to change that out. The MAF sensor is mind-bogglingly sensitive, and it's the piping itself that the MAF sensor mounts to that causes all of this trouble.
    So I should actually go back to the stock intake system with an upgraded filter?

    The primary reason why flex uppipes are more popular is because installing an uppipe without a leak can be somewhat difficult and the flex uppipe allows for some slop and a slightly larger margin for error during the instalation. If you do everything correctly however, you can install a solid uppipe with absolutely no leaks at all. 9 times out of 10 an installation-related uppipe leak is due to using an inexperienced person using improper torque sequence and slapping on sh*tty gaskets. The majority of gaskets that come with most aftermarket uppipes aren't that great. If you're smart, you'll get yourself a set of genuine Subaru gaskets. Aside from the exhaust manifold, the uppipe is under more heat and pressure than the rest of the exhaust system, and even slight leaks are absolutely not an option. Furthermore, the uppipe is somewhat of a pain to get to, so making a 2nd trip under the hood to replace the gaskets a year or so from now is the last thing you're gonna want to do. The trick to a leak-free install is when you're torquing everything down, first torque down the exhaust manifold to the head, then the uppipe to the exhaust manifold, then the uppipe to the turbo, in that order. Doing it any other way is just asking for leaks. Oh yeah, a oxygen sensor socket is a HUGE help for an install like this
    haha yeah, got one of those. If they aren't much of a difference performance-wise, I might go with a flex pipe just for the added ease of installation, and I'll definately pick up the Subaru gaskets.


    When you're ready for engine management, the stage 2 map (which is what you'd be running for your present level of modifcations) via the Cobb AccessPort will safely increase your boost up back to 16.5 PSI with all the proper fueling and ignition timing to match it.
    Thats great, so I'd be getting the same performance levels, just the right way.
    Do I need a bigger MAF to compensate, or will the stock maf be alright running the high boost levels?

    So here's my list:
    replace intake with OEM ribbed type, high-flow filter element
    replace Uppipe with catless, using OEM gaskets
    remove MBC and BOV to fix lean condition/drivability issues
    use AccessPort to safely achieve higher levels of performance with the stage-2 map!
    Last edited by Kush; 03-07-2006 at 03:37 PM.

  13. #12
    Registered User cheeseybacon's Avatar
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    Quote Originally Posted by Kush
    Instead of the rubber ribbed piping after the air filter element in the picture, I have something this (not my car, but the same type of system):

    So I should actually go back to the stock intake system with an upgraded filter?
    Yepper, that looks exactly like you have a cold air intake! I just knew you had to have one.. the previous owner botched up too many other things on your car (BOV, MBC, no uppipe) to not screw up the intake as well. Get rid of that sucker and put the stock airbox back in place, but don't worry you don't need a conical air filter on the end of the intake like the picture that I posted, just the original stock airbox will do nicely. Aftermarket intakes can really help naturally aspirated vehicles, where the engine is breathing on it's own, but for a turbocharged car like the WRX where the turbo is doing all the breathing for the engine, intakes typically don't make any noticable difference at all, and in this case the skewed MAF sensor readings that aftermarket intakes create are DEFINATELY not worth whatever little gains they might have. The stock air box is efficient up to a surpringly high amount of horsepower.


    Quote Originally Posted by Kush
    Thats great, so I'd be getting the same performance levels, just the right way.
    Exactly.

    Quote Originally Posted by Kush
    IDo I need a bigger MAF to compensate, or will the stock maf be alright running the high boost levels?
    You might need a bigger/different MAF if you have a huge watermellon sized turbo that couldn't utilize the stock turbo inlet tube, but other than that the stock MAF sensor is fine, and more than adequate for most aftermarket turbos as well. Actually, after a certain a boost level/rpm is reached, the ECU stops making fueling decisions based on it's sensors (closed-loop) anyway. At that point, the sensors that the ECU has been using to adjust the A/F can't react fast enough to provide accurate data, so the ECU switches over to open-loop operation, where it provides fueling based on a table of preset fueling values called a map. That's why engine management and new mappings are so important. The boost controller you have might be forcing your turbo to provide 17 psi of boost without any problem, but your ECU is still only mapped out to provide enough fuel to keep things safe for 13.5 psi of boost.

    Quote Originally Posted by Kush
    So here's my list:
    replace intake with OEM ribbed type, high-flow filter element
    replace Uppipe with catless, using OEM gaskets
    remove MBC and BOV to fix lean condition/drivability issues
    use AccessPort to safely achieve higher levels of performance with the stage-2 map!
    Looks good. Just be wary of some of the high-flow filters, like the K&Ns. The filters are good quality and all, and a drop-in filter on the stock airbox won't screw with your A/F, but just be aware that they sometimes tend to be over-oiled right out of the box. This often means that after a while your MAF may get coated with oil and cause your car to act a little funky. If this happens, you can clean your MAF sensor by carefully spraying it down with non-chlorinated brake cleaner. If it were me, I wouldn't worry about a high-flow filter. Like I said it's more important for NA cars more so than turbocharged cars, but that's completely your call.
    Last edited by cheeseybacon; 03-07-2006 at 04:54 PM.
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    Moderator   Sasquatch's Avatar
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    Damn Cheesey ... you are on a roll! Are you an aspiring writer? Good info!

    Welcome Kush!
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    Registered User cheeseybacon's Avatar
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    Quote Originally Posted by 04.SPT.WRX
    Damn Cheesey ... you are on a roll! Are you an aspiring writer? Good info!

    Welcome Kush!
    Haha, no I couldn't think of being a writer, just I can't stand seeing people flouder about with incorrectly setup vehicles, especially if it wasn't of their own doing. Unbeknownst to him, Kush bought a car with a CAI, BOV, no catless uppipe, and a MBC set at 17 PSI, which are THE ABSOLUTE WORST mods you could do to it. It's no wonder he's having problems with fuel cut, it's like the noob-car from hell. I think we've got everything well on it's way to being straightened out though, right Kush?
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    Moderator   Sasquatch's Avatar
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    Quote Originally Posted by cheeseybacon
    Haha, no I couldn't think of being a writer, I can't stand seeing people flouder about with incorrectly setup vehicles, especially if it wasn't of their own doing. The guy bought a car with a CAI, BOV, no catless uppipe, and a MBC set at 17 PSI. It's like THE noob-mod car from hell, with the absolute worst mods you could do to it, it's no wonder he's having problems with fuel cut. I think we've got everything straight though.
    You took a lot of time to explain in detail and sort it all out for the new guy. Very nice of you.

    Hi hew guy ... 70's Camaro? How old are you? I am old enough to clearly remember 70's Camaros when new.
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