[Mk2] [Turbo] Not holding boost

[markstevieandmads]

Been running my rev 2 turbo today and been running well but since ive refurbed it the boost doesnt feel right.
Ive fitted a gt4 chargecooler system, jimgts old hybrid ct26, a decat and a walboro pump.
Car runs really well and hits the 0.9bar i want but then boost just falls till no boost when accelerating.
As soon as i come off the throttle and back on the boost hits the required boost then tails off again.

Im running a gtschris mbc which ran fine on my old standard setup.

I checked the wastegate flap didnt foul on the decat before installing it and it wasnt .

Id have thought if i had a boost leak it wouldnt hit 0.9 bar at all?

Any iseas?

[markstevieandmads]

No one?
If the arm from the actuator comes of the wg would it provide this sympton?

Theres also a rattle at a certain rpm just as the boost comes in then goes away?

I did have to wind my mbc in quite a bit to get it to hit 0.9bar then it tails right off to 0.4ish bar

[chrismit]

Hi

If the actuator arm had disconnected from your wastegate your wastegate would just be flapping around. You would be seeing the car spool really slowly and it would take a long time to get to your peak boost of 0.9 bar. as the wastegate would be opening immediately causing some exhaust gasses to bypass the turbine and not just opening to divert at the desired boost level.


Just from the way you are describing it, seems like the boost pressure is building up fine. Then at the point you want the wastegate to open to prevent it boosting any higher the boost is just being lost?

Almost sounds like when your wastegate is opening its bypassing too much of the exhaust gasses from the exhaust turbine and the boost is coming back down. Not something I have seen before.

If your wastegate was fouling youd probably see the opposite (not able to bypass enough exhaust gasses to stop boost creeping up)

Id probably try checking and replacing the vacuum lines on your actuator, and turn the manual boost controller off. See if you can hold the stock boost level without that tailing off then slowly increase your MBC.

[markstevieandmads]

Thanks chris. Ive never really known how it all works tbh.

Its building up to 0.9bar fine, and hits it fine just not holding that boost at that level.
Will experiment with it hopefully later today and see if i can narrow it down.

Need to get this sorted before sunday tho as want to go to japfest.

Not sure if it has anything to do with it but i do get a rattly sound from the engine bay just as it comes on boost. Maybe just something vibrating as need to make a support bracket for the core as its just currently held in place by the boost hoses.
It seems solid but wouldnt want to keep it like that.

[tiff_lee]

Your boost controller is basically a 'dawes' device (ball and spring non return) is there a bleed hole on the actuator side to vent pressure off?
Once that line is pressurised causing the wastegate to open the pressure is remaining trapped in the line holding it open hence the drop in boost.
Pressure should bleed off allowing the wastegate to close and regulate boost, I dare say the bleed off is insufficient hence why after fully backing off you can then boost again (proof that it is indeed bleeding off).

It may not be the above but that would be my first guess based on what you've said.

[markstevieandmads]

Thanks tiff lee.
Actually ive mounted the mbc to a rubber pipe and im wondering if that bleed hole is blocked?
I will give it a blow out with an air line.

[markstevieandmads]

Problem solved!!
I had the mbc connections round the wrong way!
Swapped them around and now happily holding 1bar. Thanks everyone.

Also solved the buzzing noise.....bloody heatshield!