MBC Issues...

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MBC Issues...

Postby Skurneha » Tue Aug 06, 2019 10:55 pm

Hey all! I just recently bought an alltrac in May and this is my first time owning one! I’m the fourth owner so it clearly isn’t stock anymore. It’s currently running 11-12 psi and I was told it was set with an MBC. I just wanted to turn it up a few psi, not much, but the MBC isn’t doing anything. I turned it down slightly and I think it did go down 1 psi so I don’t think it’s broke lol. It appears all hooked up correctly, at least to my noob eye, and I know the ct26 maxes around 20, way higher than what it’s at.

I’ve cranked it many rotations and there’s no difference. Every once in a blue moon it may blip to 13 but that’s for half a second. Anybody know how this can be fixed, or if there’s some kind of mechanism stopping it from boosting above 12?

I know the car also has a cracked exhaust manifold that I need to fix when I save up enough money, could that exhaust leak be affecting turbo spool? I work all week from early afternoon to late evening and as said previously I’m pretty new to the car scene and open to learn, but I don’t have any time to do a complicated checking of stuff or taking out things and reinstalling them if that’s an option suggested.

Thanks in advance and sorry for the block of text.


Side note: unrelated to MBC. Heard the tire clicking when turning and it seems like a CV joint going bad. How concerning is this and should I drop everything to get it fixed?
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Re: MBC Issues...

Postby Junctrac » Wed Aug 07, 2019 1:19 am

If you have free flowing intake and exhaust you could be making 12 PSI without a boost controller (or one that doesn't work). How big is the manifold crack? It could be reducing pressure if its bad enough.

Make sure your ignition and fuel system is sound before you crank it up. The power primer is somewhat outdated but it is a good read if you want to learn more about the 3s basics.

https://web.archive.org/web/20171224102 ... /power.htm
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Re: MBC Issues...

Postby Skurneha » Wed Aug 07, 2019 2:13 am

I don't know exactly how large it is since i can't see it from where it is in the engine bay, but you can definitely hear it when it starts up and at specific RPM points. Has a cold air and a SSAC 3" downpipe.
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Re: MBC Issues...

Postby underscore » Wed Aug 07, 2019 4:38 pm

For the CV I'd see how warm it gets after driving a bit but unless you can afford downtime I'd fix that first. Then fix the exhaust leak before trying to increase the boost since you'll get better power just from that. Then double check the plumbing on the MBC, if you want to see if it's working hook it up to an air compressor with the engine off and slowly increase the pressure until you see the wastegate arm move.
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Re: MBC Issues...

Postby Skurneha » Wed Aug 07, 2019 8:39 pm

I would’ve expected the MBC/plumbing to still be functional if it holds 12 psi but specifically 12 and doesn’t revert back to stock? Could be the exhaust crack...

I think I’ll install an EBC instead because there’s just more control and customization, but I think I’ll take the advice and work on maintenance before power, I don’t want to ruin the car.
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Re: MBC Issues...

Postby 93celicaconv » Thu Aug 08, 2019 9:55 am

I don't know what the acronym "MBC" means. Maybe it's the answer to my question back to you. But, what have you done to move the fuel cut boost pressure above the factory set-point of 12 psi? If you haven't done anything between the MAP and ECU (if you are running both OEM), then you can't exceed 12 psi (I don't think).
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Re: MBC Issues...

Postby Skurneha » Thu Aug 08, 2019 12:15 pm

93celicaconv wrote:I don't know what the acronym "MBC" means. Maybe it's the answer to my question back to you. But, what have you done to move the fuel cut boost pressure above the factory set-point of 12 psi? If you haven't done anything between the MAP and ECU (if you are running both OEM), then you can't exceed 12 psi (I don't think).


MBC is manual boost controller. The factory boost is like 7 psi but before I got the car the boost controller was used to push it to 12
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Re: MBC Issues...

Postby 93celicaconv » Thu Aug 08, 2019 1:12 pm

I don't quite think this is right - it's a bit more complicated.

Wastegate spring is set to start lifting at about 7psi. TVSV (bleed-type boost controller) is controlled by the ECU using the turbo VSV which caps boost at 11.8psi. If the ECU runs into knock sensor input, or cold intake (ambient) air conditions, it will deactivate TVSV which reduces boost to wastegate pressure of 7psi. So this engine has two systems that work together to control boost pressure. Fuel cut boost pressure is about 13psi on a USDM 3S-GTE engines on an ST185 (which is what all these values pertain to), which is the final fail-safe protection for the engine from over boost pressure.

Actual boost pressures vary as wastegate spring tension changes with age (will gradually reduce boost) and MAP sensor calibration shifts.
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Re: MBC Issues...

Postby Skurneha » Thu Aug 08, 2019 1:45 pm

93celicaconv wrote:I don't quite think this is right - it's a bit more complicated.

Wastegate spring is set to start lifting at about 7psi. TVSV (bleed-type boost controller) is controlled by the ECU using the turbo VSV which caps boost at 11.8psi. If the ECU runs into knock sensor input, or cold intake (ambient) air conditions, it will deactivate TVSV which reduces boost to wastegate pressure of 7psi. So this engine has two systems that work together to control boost pressure. Fuel cut boost pressure is about 13psi on a USDM 3S-GTE engines on an ST185 (which is what all these values pertain to), which is the final fail-safe protection for the engine from over boost pressure.

Actual boost pressures vary as wastegate spring tension changes with age (will gradually reduce boost) and MAP sensor calibration shifts.


Any way to work around this and push it higher than 12? Sounds like it needs to be tuned for it?
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Re: MBC Issues...

Postby 93celicaconv » Thu Aug 08, 2019 3:11 pm

I can't help you with what to work around......I'm one who is meticulous on keeping things stock and keeping stock things working as they were designed to work.

You can try disconnecting the wiring harness on the MAP sensor (will trigger a check engine light, not sure how it will affect engine operation - most "experts" say the MAP on these engines only provide data for the stock boost gauge in the instrument cluster and the input to the ECU to trip fuel cut - I think it also affects fueling with the AFS but not certain). If this works and you don't mind a non-functioning stock boost gauge and a check engine light, you can try it to see what happens.

Or you can remove the vacuum hose connected to the MAP (and plug the hose) - similar to above except the check engine light will not come on because it is still electrically connected.

Just know your engine will be at risk by removing a safety device designed to protect the engine.
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Re: MBC Issues...

Postby underscore » Thu Aug 08, 2019 5:56 pm

93celicaconv wrote:I don't quite think this is right - it's a bit more complicated.

Wastegate spring is set to start lifting at about 7psi. TVSV (bleed-type boost controller) is controlled by the ECU using the turbo VSV which caps boost at 11.8psi. If the ECU runs into knock sensor input, or cold intake (ambient) air conditions, it will deactivate TVSV which reduces boost to wastegate pressure of 7psi. So this engine has two systems that work together to control boost pressure. Fuel cut boost pressure is about 13psi on a USDM 3S-GTE engines on an ST185 (which is what all these values pertain to), which is the final fail-safe protection for the engine from over boost pressure.

Actual boost pressures vary as wastegate spring tension changes with age (will gradually reduce boost) and MAP sensor calibration shifts.


This is my understanding as well, except I thought fuel cut was at 14.7psi (1 bar). I know the 205 and 185RC have the cut at 17psi. You can bump the cut up to 17psi with a FCD or the diode mod but beyond that it'll have to be disabled.
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It's for sale! http://www.alltrac.net/phpBB2/viewtopic.php?f=64&t=52181
Build thread http://www.alltrac.net/phpBB2/viewtopic.php?f=44&t=44216
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Re: MBC Issues...

Postby Skurneha » Thu Aug 08, 2019 9:01 pm

This is my understanding as well, except I thought fuel cut was at 14.7psi (1 bar). I know the 205 and 185RC have the cut at 17psi. You can bump the cut up to 17psi with a FCD or the diode mod but beyond that it'll have to be disabled.[/quote]

Is it possible the MBC hit a limit and not the turbo? I’m mostly just confused why I can’t even push past 11 and hit fuel cut?
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Re: MBC Issues...

Postby underscore » Thu Aug 08, 2019 9:54 pm

Well if your exhaust is leaking pre-turbo that's gonna hurt your ability to make boost. Beyond that you've got the usual boost leaks, worn out wastegate, worn out turbo, leaking MBC, etc.
★ 1991 GTFour RC ~ "Rebel Scum" ★
It's for sale! http://www.alltrac.net/phpBB2/viewtopic.php?f=64&t=52181
Build thread http://www.alltrac.net/phpBB2/viewtopic.php?f=44&t=44216
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Re: MBC Issues...

Postby Skurneha » Thu Aug 08, 2019 10:16 pm

underscore wrote:Well if your exhaust is leaking pre-turbo that's gonna hurt your ability to make boost. Beyond that you've got the usual boost leaks, worn out wastegate, worn out turbo, leaking MBC, etc.


The turbo is a ct26 with a 46 trim upgrade and was rebuilt in 2016(?) is what I was told, so it seems to be either the MBC, or maybe wastegate. Also I definitely think the exhaust leak is hurting it, because every once in a while it does blip to 13 psi
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Re: MBC Issues...

Postby Junctrac » Wed Aug 14, 2019 9:59 pm

There's a good chance you have the mbc from twosrus or atleast a spring and ball one that works the same, I would look through these install directions and check the routing and see if the tvsv line is capped.

https://www.twosrus.com/Articles/Instal ... ctions.PDF

Basically the the mbc goes inline between turbo compressor outlet and wastegate. When turbo pressure is greater than mbc spring pressure the ball moves and allows pressure to go to wastegate which then opens. I have this mbc and after the car sat for along time i think its spring became slightly weaker because I had to turn it in to get back to the same psi I was at before. Like underscore said you could hook up a compressor and regulator to the line going to mbc and see where it opens. If its above 12-13 then its probably the exhaust leak holding you back.
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