So when I bought my car one of the first things I did was a get boost leak test and found out that my old BoV (Greddy) wasn't holding boost 100% of the time due to a sticky spring.

(No overboosting when this BoV was on it)

So I ventured on evom and bought a used evo 9 BoV that was in pretty good shape, and claimed by the guy who sold it that it wasn't leaking (but I havn't had this one leak tested)

So what happens is I get on the throttle and boost builds to 20-21 PSI holds there for a bout 500 rpm and then spikes to 25 PSI ish which brings me to a fuel cut and I get a slight "pop" from the engine.

What could be causing this?

I figured to be safe I would turn my MBC down (Forge) and I don't know how these things work that well so I turned it down slowly (about 25 to 30 degress first) then more and more. Now I've turned it about 180 degrees from where it was to begin with and the car still overboosts.


Trying to get some feedback here, could my MBC not be holding boost? I had a thought that the car was tuned with this leaky BoV and those over boosting when I put a non-leaking BoV on it... Possible? Could this evo 9 BoV be leaking?

I'm looking for a hardware fix, I will be getting the car tuned as soon as I can.

 

Let's get something straight. When you state that you turned the adjustment knob "down", just exactly what did you do? Did you turn the knob "in" clockwise? Or did you turn the knob "out" counterclockwise?

Turning the adjustment knob inward(clockwise) will increase boost. turning the knob out(counterclockwise) will decrease boost.

I just want to make sure that I understand what you are explaining to us.

 

When you state that boost spikes do you mean that boost jumps up suddenly by 4 PSI, or does it instead creep up gradually from 21 to 25 PSI? What gear are you road testing this in.

 

A hardware fix? What's that? Do you mean that you prefer to haphazardly throw some more parts at it? Don't you want to tinker with it instead just to see if we can get your present boost control circuit to function in geomantic harmony? You're no fun! LOL

 

To clear up i turned the knob counterclockwise (towards the negative on my forge MBC)

The PSI JUMPS very quickly to 25PSI, to the point if I saw it happen i wouldn't be able to avoid it, almost like something is "letting go." Testing is usually done in 3-4 where the car has more RPMs to stay at full boost. Let me be clear too it stays at 21 pounds for a little while, there is a hesitation, before it kicks up.

And if there is a fix without having to buy anything, i'm all for it!!

 

Could be you just fixed the boost leak lol. Keep adjusting the mbc. I have a forge too and its 3 clicks per full rotation. It won't be the same for every car, but its usually 2-3 clicks per psi. I'd turn it down 2 full rotations and see where your at. You should hit a certain boost point and it will either hold there or taper down a bit...it shouldn't creep up.

Creep can be caused by wastegate actuator problems (which is connected to the other hose coming off the mbc). Check and make sure all vacuum hoses are attached correctly to the mbc. It should be connected to a boost signal (either from the intake manifold or the black pipe coming off the turbo outlet) and then to the wastegate actuator (metal thing bloted to the turbo, facing the radiator, bronze or silver in color). I think the bottom nipple should be to the WGA and the side to the boost source...check connections are secure first, then adjust the boost down several clicks.

 

Tomorrow when it's nice out I'll take a look and snap a couple of pictures.

When you say "clicks" does your MBC actually make a "clicking" sounding? Mine just slides

 

Just keep turning it down until you get to your desired boost level..if you go to far you will just hit low boost. After you see that you are only getting to say 15 PSI..start going back up, then you will know for sure whats going on.

N

 

Back the adjustment knob on your MBC out all the way counterclockwise to full soft which would be the lowest boost level attainable with your MBC at your WGA's current spring pressure.

Take it out for a test drive in 4th gear and do a WOT pull and get back to us as to what peak boost you see on the dial with the MBC set on full soft and, whether or not the turbo is still creeping/spiking at this minimal MBC setting. This test will tell us what the WGA's spring pressure is set at. That is, whether it is preloaded or not. I want to see how your boost behaves with the MBC temporarily out of the loop.

 

No it doesn't actually click...more like a notch. The knob will move freely, then you will feel it slide into place...sort of locking, but its obviously not locked lol. I would call it a notch for every 1/3 turn you make. Just turn it down 2-3 full rotations and see where your boost is at...

 

Sparky's advice is good too. By removing the mbc from the loop we'll be able to tell you if you have a simple mbc adjustment to make or if your wga is on the fritz. If your mbc adjustments yield a lower boost level, but its still creeping up as you escalate rpm, you've most likely got a wga problem. Again, confirm all vacuum hoses are attached to proper destinations. Pics will help us help you

 

I seemed to have fixed it. When I was turning it down before I was turning it down so slow, I don't think I actually turned it down because I went and do a full 1.5 turns and I felt it like 'clicking' per say. I got in a did a 3rd and 4th gear pull. boost peaked @ 16ish. got out and adjusted it up till i got to about 20-21, holds there nice and steady.

It feels so nice to actually be able to rev the car past 5-5.5k.


One other question. I noticed that boost builds really smoothly from 0-15pounds or so, then after 15 pounds it... is more or less skippy in a way to 20-21pounds. nothing super dramatic just enough to notice when my eyes are peeled to my boost gauge.

It was also really warm out today and I started having the issue during the winter, so the colder denser air might have had something to do with it aswell.

edit* BLAST! forget to take pictures, ill get on that ASAP

 

So, is that the peak boost level that you want to run at for th time being(21 PSI)? Is your diverter valve stock, or do you have an aftermarket blow off valve installed?

 

I have a metal 9 BoV on an evo 8.

So it started happening again.

I fixed the issue when it was 65 degrees out and went for a cruise yesterday when it was like 39 ish and it over boosted.

The weird part is that it doesn't spike very high before I get fuel cut. It'll spike to like 22-23 and then fuel cut.

I turned the boost down some more and it seemed to have stopped. Any ideas why isn't so sensitive to temperature and the boost leak. Like I figured you could turn the boost up on a car, regardless of a tune, to like 23-24 if you wanted, for whatever reason. But with mine, if I go past a fine line it'll overboost. The car doesn't want to go past 21 ish PSI

 

have you changed anything else on the car recently? I assume you have good gas and spark plugs? just break down and learn how to tune it yourself and ECU controlled boost