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Old 09-09-2020, 05:56 AM   #1
AfroSlyStone
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Question Low boost on 92 740 turbo

Hello all I have been trying to find a solution to this problem. I have posted on Facebook and discord and got suggestions but no good outcome. I was told that I have a vacuum leak basked on the video people. My speedo doesn't work so I am going by rpm. I did a pressure test and a fuel injector o ring was bad so I replaced it but still the same outcome. Does anyone have any other suggestion?

Here is a link to the video
https://photos.app.goo.gl/CjCS7LnpLoSDffQs7
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Old 09-09-2020, 08:24 AM   #2
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i watched your vid and your turbo does seem to create boost. When you let go of the gas pedal you can hear the CBV dump the boost. If the car has no aftermarket boost controler then it all seems pretty normal to me.

Maybe spool-up could be a bit better? You could try to dial in some more pre-load on the wastegate actuator rod. That way boost will build a bit faster.
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Old 09-09-2020, 09:14 AM   #3
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I do have a boost controller
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Old 09-09-2020, 10:12 AM   #4
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I do have a boost controller
I wasn't able to load your video, but assuming you don't have boost leaks and your wastegate puck isn't leaking, then you likely have a bad turbo. We went through 2 'known good' 13c turbos on my buddy's 245+T. Both took forever to spool and I was almost positive his car had underlying issues.

Then we found a NOS 13c turbo for $300 and it fixed all our problems.
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Old 09-09-2020, 12:29 PM   #5
ZVOLV
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I bet cracked turbine housing. Take off the downpipe and look in there with a mirror.
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Old 09-09-2020, 12:42 PM   #6
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I bet cracked turbine housing. Take off the downpipe and look in there with a mirror.
Ah yes, this too. Though I would think the crack would need to be a significant size to make a difference, as I've run a handful of cracked turbos without issue.
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Old 09-09-2020, 02:11 PM   #7
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The vacuum on the boost gauge in idle seems quite low from what I can tell?
The needle is much more to the left on mine - I have a numerical boost gauge on mine as well and it shows about 0.65 during idle and 0.85bar vacuum during cruise (if I remember correctly).
Are you sure you don't have any vacuum or boost leaks?

Under boost it doesn't seem to be far off than what it should be I guess?
The stock boost pressure is 0,55bar (I think - about 8psi) and by the looks of the gauge it seems to be there +/- I'd say.

Last time I saw a post from you on the Facebook group the wastegate actuator was disconnected or something, is everything alright on that end?
As said above, does the actuator arm have enough preload? Without any preload on it, the wastegate flapper thing inside the turbo isn't fully closed and is creating a boost leak.
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Old 09-10-2020, 01:30 PM   #8
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The Facebook group the wastegate actuator was disconnected or something, is everything alright on that end?
As said above, does the actuator arm have enough preload? Without any preload on it, the wastegate flapper thing inside the turbo isn't fully closed and is creating a boost leak.
The wastegate actuator is on. How can I check the preload? I will check the wastegate flapper to see if it isn't fully closed. I have to ask but can you tell me what the wastgate flapper is?
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Old 09-10-2020, 01:49 PM   #9
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Take the downpipe off. Inspect for a MELTED turbine housing near the flapper/puck inside there.
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Old 09-10-2020, 02:15 PM   #10
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The wastegate actuator is on. How can I check the preload? I will check the wastegate flapper to see if it isn't fully closed. I have to ask but can you tell me what the wastgate flapper is?
Here is a picture of the wastegate flapper.
https://images.app.goo.gl/ofgMTJpCJhWnjYJs7
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Old 09-10-2020, 04:17 PM   #11
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Originally Posted by AfroSlyStone View Post
How can I check the preload?
You just make sure the flapper is closed and then connect the actuator arm by pulling it out a little bit from the actuator housing.
The preload needs to be just enough so that the flap is staying closed. The arm has a thread on it on the front part where it connects so you can adjust the preload.
If it's too loose, turn the front part clockwise so the whole arm gets shorter.

Basically just like shown here:
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Old 09-10-2020, 07:01 PM   #12
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A clogged converter can reduce boost response and performance.
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Old 10-03-2020, 10:35 AM   #13
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i'll piggyback off this thread.... i'm running straight stock on a 94 940T and driving last night, I was not really getting the boost gauge above 1:30 or so (if we think of the black/white split as noon). I need to adjust the throttle cable so the pedal is smoother, but I wonder if my problem is, as described, the wastegate arm needs shortened. I replaced the vac lines from the compressor housing, and between the wastegate actuator and the outlet. is that typically a job that the turbo comes out for? just eye-balling it, looks like a PITA to do with turbo in...
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Old 10-08-2020, 03:22 PM   #14
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Quote:
Originally Posted by Fa182 View Post
You just make sure the flapper is closed and then connect the actuator arm by pulling it out a little bit from the actuator housing.
The preload needs to be just enough so that the flap is staying closed. The arm has a thread on it on the front part where it connects so you can adjust the preload.
If it's too loose, turn the front part clockwise so the whole arm gets shorter.

Basically just like shown here:
Not seeing an adjustment nut on the arm for my mitsu 13c turbo? is that normal?? if so, how does one adjust the arm on the 13c?
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Old 10-08-2020, 04:51 PM   #15
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Could be, I don't remember top off my head.
I believe it's adjustable on my 16T but I didn't work on a 13C for so long so I don't remember.

If you can't adjust it and you're sure this is your issue, you could put a washer on each bolt between the actuator bracket and the turbo to move the actuator a bit away from the flapper.
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Old 10-08-2020, 05:10 PM   #16
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not sure what the deal with the 13c is. got the turbo out and there's no obvious way to adjust the arm. additionally, the wastegate in the turbine housing seems to be securely shut when at the length of the arm without adjustment. but I was only getting 4lbs of boost on the car with the wastegate actuator hose attached, and the car was overboosting to 15+lbs with the hose removed. so is my wastegate actuator just toasted? am I not getting proper vacuum to the wastegate actuator?

EDIT: and regarding cracks in the turbine, how concerning is this crack? it doesn't appear on the exterior of the housing and is the only crack present.

DSC-0017

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Old 10-08-2020, 05:51 PM   #17
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if it makes any difference my gauge is t'd off the main line to the stock turbo gauge in the cluster.
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Old 10-09-2020, 11:26 AM   #18
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Quote:
Originally Posted by mschultz373 View Post
not sure what the deal with the 13c is. got the turbo out and there's no obvious way to adjust the arm. additionally, the wastegate in the turbine housing seems to be securely shut when at the length of the arm without adjustment. but I was only getting 4lbs of boost on the car with the wastegate actuator hose attached, and the car was overboosting to 15+lbs with the hose removed. so is my wastegate actuator just toasted? am I not getting proper vacuum to the wastegate actuator?

EDIT: and regarding cracks in the turbine, how concerning is this crack? it doesn't appear on the exterior of the housing and is the only crack present.

DSC-0017
Sounds like your waste gate is working properly, it only sees vacuum when you're not boosting the car, pressure when the gauge is over 0. I'd be looking for boost leaks somewhere else, maybe on the outside corners of the rubber elbows.
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Old 10-09-2020, 01:39 PM   #19
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but why does the car overboost with the wastegate disconnected then? I guess there could still be a boost leak but with the wastegate disabled the turbo just won't dump any boost.

i am getting 16-18 inHg vac at idle, 20 in D. not really hitting boost unless I mash the pedal and then I top out at 5lbs. i'll keep poking around for the leak.
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Old 10-09-2020, 01:51 PM   #20
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Originally Posted by mschultz373 View Post
but why does the car overboost with the wastegate disconnected then? I guess there could still be a boost leak but with the wastegate disabled the turbo just won't dump any boost.

i am getting 16-18 inHg vac at idle, 20 in D. not really hitting boost unless I mash the pedal and then I top out at 5lbs. i'll keep poking around for the leak.
Is the waste gate plumbed to the nipple on the turbo outlet?
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Old 10-09-2020, 01:55 PM   #21
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Is the waste gate plumbed to the nipple on the turbo outlet?
yes. and the CBV is running to the T under the intake manifold.

the wastegate in the turbine housing is sealing when the actuator is NOT engaged - at rest, basically.. I verified when I took the turbo out yesterday. so i wondered if the actuator could be faulty; but since it's just a mechanical part, I don't know how it would fail in such a way as to always leave the wastegate open once the car is running.
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Old 10-09-2020, 02:03 PM   #22
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When your engine is cold. Remove the intake hose to the compressor and check the turbo shaft for play. It may be you have a worn out turbo as suggested. You are looking for very little play. If it moves around enough to impact the housing. It's done. You should have a small amount of side play and a small amount of play lengthwise.
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Old 10-09-2020, 02:04 PM   #23
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When your engine is cold. Remove the intake hose to the compressor and check the turbo shaft for play. It may be you have a worn out turbo as suggested. You are looking for very little play. If it moves around enough to impact the housing. It's done. You should have a small amount of side play and a small amount of play lengthwise.
I checked the play on the bench yesterday and both the turbine and comp wheels appeared fine. very little play and both spun freely.
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Old 10-10-2020, 11:12 AM   #24
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confirmed that the compressor shaft doesn't have play.

would there be any benefit to hooking up a boost controller and trying to boost the PSI? I.e, if the boost controller is adding more boost but turbo is still loosing boost, might it confirm a fault wastegate actuator or CBV or boost leak? or is that just asking for parts to break?
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Old 10-10-2020, 02:24 PM   #25
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Pull the CBV and make sure it isn't torn.

I guess you could wire the wastegate arm shut and go for a drive and see what happens.

We have a new high pressure smoke machine at the shop for testing for intake/exhaust leaks. They can be tricky to find.
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