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Low boost on 92 740 turbo

A

AfroSlyStone

New member
Joined
Sep 12, 2019
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
 
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.
 
AfroSlyStone said:
I do have a boost controller
Click to expand...

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.
 
ZVOLV said:
I bet cracked turbine housing. Take off the downpipe and look in there with a mirror.
Click to expand...

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.
 
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.
 
Fa182 said:
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.
Click to expand...

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?
 
Take the downpipe off. Inspect for a MELTED turbine housing near the flapper/puck inside there.
 
AfroSlyStone said:
How can I check the preload?
Click to expand...

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:
wg_arm_350-jpg.2364
 
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...
 
Fa182 said:
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:
wg_arm_350-jpg.2364
Click to expand...

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?
 
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.
 
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.

<a href="https://ibb.co/MPsCY9y"><img src="https://i.ibb.co/XFbChVf/DSC-0017.jpg" alt="DSC-0017" border="0"></a>
 
Last edited:
mschultz373 said:
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.

<a href="https://ibb.co/MPsCY9y"><img src="https://i.ibb.co/XFbChVf/DSC-0017.jpg" alt="DSC-0017" border="0"></a>
Click to expand...

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.
 
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.
 
mschultz373 said:
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.
Click to expand...

Is the waste gate plumbed to the nipple on the turbo outlet?
 
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