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Discussion Starter #1
I posted a thread about a week ago regarding boost creep issues in my stock CT-26. A few suggested that it could be my GReddy T.V.V.C. MBC so I disconnected it and tried shimming the wastegate instead – same thing. Others suggested that it might be a leak in the IC piping so I checked that over, too – no problems. So, I’m back here with more details on this annoying problem in hopes that someone might be able to shine a light into this benign mystery. I am running a wide-mouth downpipe with no cats and a straight-thru 3” exhaust system. Intake is a K&N FIPK but otherwise the car is pretty much stock. I’m doing most of my test drives in 3rd gear and here is what’s happening with my car:

Running with an MBC:
  • I mash the gas around 2000 RPM and wait for the turbo to spool.
  • Around 2800 RPM, the turbo is fully spooled and I see the needle at 0.45 BAR.
  • It holds it at 0.45 BAR with NO signs of creeping until the revs hit 4000 RPM.
  • Reaching 4000 RPM and beyond, the boost starts to creep to 0.7 to 0.8 BAR depending on where I’ve set the MBC to.
  • It holds 0.7 to 0.8 BAR to 6000 RPM and I shift.
This is already very strange because theoretically, it should achieve 0.7 to 0.8 BAR or my desired boost setting at 2800 RPM. However, it doesn’t seem to matter where I set the MBC to; it’s always 0.45 BAR until 4000 RPM.


Running with a shimmed wastegate:
Thinking that the MBC was the culprit I tried shimming the wastegate, but the results were also very strange and similar.
  • When using 1 washer, I now see the turbo spool to 0.55 BAR at 2800 RPM.
  • But again, once it nears 4000 RPM the boost takes a leap to 0.7 to 0.8 BAR. =(
  • Now, by using 2 washers, I’m now able to hit close to 0.65 BAR at 2800 RPM and the car picks up very fast. But things start looking a bit scary around 4000 RPM.
  • Entering 4000 RPM, the boost creeps all the way to 0.8 to 0.9 BAR and I really don’t like that.
I haven’t hit fuel cut yet but I’m sure I would in 4th or 5th gear at this rate.

It seems that by using the washers I am able to attain a higher boost pressure initially. But both MBC and washers exhibit an increase in boost once the engine hits 4000 RPM. This leads me to believe that neither the MBC nor washers are at fault. So, if the problem isn’t the MBC, washers or even the IC pipes – which I double checked – what could possibly be causing this? I’m really lost. Please help. Sorry about the lengthy post.
Thanks,

Jason L.
 

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Not ur jizz, not ur bizz
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6,571 Posts
you have a faulty wastegate actuator...probably the spring....time to replace it.
 

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Discussion Starter #4 (Edited)
BTZ said:
leaky vacc line???
I thought about that too so I checked it over and, to my knowledge, nothing seems to be leaking. But even if a vacuum line was leaking, it would have to be a pretty significant leak to exhibit a 0.2 BAR discrepancy - that's 3 PSI. I have a question to clarify things: the wastegate starts opening up as soon as the actuator sees a set amount of boost, correct? So then, as boost increases does the actuator push the gate to open more and more or is it a fixed position? Because if the opening is "dynamic" then I'm beginning to think that the culprit IS the actuator - like Johnny said. However, if the opening is static then my car is either making a surge of power at 4 grand or making not enough at 2800 RPM. But I somehow doubt that it's the latter because my engine was rebuilt 2 months ago and it's both running and sounding great. By now you must be thinking "then it MUST be the actuator!" right? But I somehow doubt that, too. I bought one - although it was used - not too long ago and I did check to see if it had a good, strong spring. It did and it still does, unless it's not supposed to be THAT strong. But it is pretty stiff, I must say. So, what do you guys think? Should I try another actuator? I feel like I'm on a wild goose chase...

Thanks,

Jason L.
 
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