V7 STi boost fluctuation issue - Ran out of ideas!

[BajamanCam]

Hey ClubSub! New member here, with an issue that me and Google can't solve by ourselves. I'll try keep it short.

My STi fluctuates it's boost, under WOT between 15-20psi - up, down, up, down, up, down... until I let off the gas. Occasionally it boost spikes to over 20psi before I can react and take my foot out.

Car details:

- Stock. Yep.

What I've checked/replaced:

- Checked wastegate, opens smoothly at 8(ish)psi

- New vacuum lines and boost pill (all in correct place, I've looked at many different diagrams over and over again)

- New boost solenoid

- Checked for leaks in intercooler and hoses

The thing that's really digging at me, is that it has always had a manual boost controller and held steady boost fine at 18psi. I don't like mbc's and decided to safely go back to the stock boost control system. So why is it holding boost fine with a mbc, and not the stock set-up... even though I seemed to have gone over everything?

Any help appreciated.

Cam

[Guest]

Faulty stock solenoid by sounds of it. Mbc is as simple as you get doesnt break and generally 100% dependable. Look into adding a 3 port mac valve its the best ebc for money that works with stock ecu. Need to do some reading on how to set it up or hit up kwi_fozze on here hes mucked around with open source a lot.

[Guest]

And welcome to cs.

Ps top marks for posting in the right place.

[BajamanCam]

Thanks for the welcome! I thought the standard boost solenoid as well, but I replaced it with an identical one and the same issue, so can't be? I've read many other forums with a similar issue and it's always the same answer - boost solenoid. But that doesn't seem to be my issue in this case, unless I bought an equally dud solenoid.

[Guest]

Possibly dud. Where are you located? Might be someone who can log your ecu or swap a known good bs for a test drive. Or could wire up an led to see if its actually working.

[BajamanCam]

Taupo mate. Both solenoids open and close perfectly with no air leaks on a 9V battery, unsure what they'll be like under normal conditions though, as they do pulse. Still, the chances of both being dud and having the exact same issue, yet work fine off a 9V seems highly unlikely.

I'm wondering if the ECU has been played with before I took ownership, and not knowing about it? Has to be an electrical issue, seen as that a manual tap works fine?

Unfortunately with me being in Taupo, there's no one close (that I know of) that can log my ECU.

Edited July 16, 2015 by BajamanCam

[Guest]

Bugger. Doubt testing with a battery will tell you if its an intermittent issue. If you could get your hands on a cable you could at least try it at a low cycle to see if it holds boost vs what its doing now. Boost control is a pretty dumb system not much to go wrong. Ecu requests a certain map then pulses the solenoid to make it happen. So pretty much either ecu is sending it funny requests or the bs is fekt. Or map sensor is fekt causing skewed readings.

[BajamanCam]

If the ECU is given an incorrect map (causing boost controller to go nuts), this can cause knocking with a manual boost controller can't it, as the air/fuel ratio will be fluctuating as well?

I think I need to take a roady somewhere for it to get logged, may get expensive!

[Guest]

If it knocks with mbc or ebc it will put it into limp/pull timing. Could try dtech or roadie to an auck meet and get one of those guys to plug in a cable and possibly swap a different solenoid in. Could always buy a cable and do a bunch of homework...

[boon]

Personally I wouldn't just whack an MBC in there, that's a Band-Aid on a different issue.

How does it feel when it's doing it? Pull smoothly or jerking? And chance it's just the gauge being ****?