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'89 740 Turbo will not make boost

Alex Buchka

Alex Buchka

8 cylinders
Joined
Apr 8, 2004
Location
Los Angeles
I've been diagnosing buu142's '89 740 turbo for the last few days. It has a strange problem where it won't make any boost at all, regardless of load or rpm. The engine is bone stock save for a gutted cat and a TurboXS boost controller set at 12psi.

I pressure tested the intake plumbing and it all checks out. The turbo spins freely and is not seized up. I tried disconnecting the wastegate signal line completely and the car still wont make boost. This leads me to believe the CBV is somehow stuck open, causing a massive boost leak. Any advice is appreciated.
 
wastegate

If the wastegate is open it wouldn't matter what you did with the signal hose. Have you reached under there with the car on ramps and checked to make sure the wastegate is closed and being pulled shut by the rod on the actuator? Another thing that happens that is sneaky is the bracket that holds the actuator to the turbo can come loose which releases all the tension on the rod so the wastegate can stay open all the time.
 
dl242gt said:
If the wastegate is open it wouldn't matter what you did with the signal hose. Have you reached under there with the car on ramps and checked to make sure the wastegate is closed and being pulled shut by the rod on the actuator? Another thing that happens that is sneaky is the bracket that holds the actuator to the turbo can come loose which releases all the tension on the rod so the wastegate can stay open all the time.
Click to expand...

X2 .... my previous 84 760 was this way...those 2 bolts were very loose on the bracket and it would not make any boost.
 
I should have mentioned, the bracket for the actuator is tight and the wastegate rod is securely attached to the flapper arm with the correct pre-load. I pressurized the actuator and it cycles back and forth just as it should.
 
no boost

Then of all things that might have happened. It's possible that the puck that opens and closes in the wastegate came loose off the arm inside the wastegate. Because of all things that control boost the wastegate is the main thing. A massive boost leak would have to open up whenever the turbo starts to pressurize yet close up so the engine idles and runs fine when not in boost. That's a pretty tall order. I'd think that it would also be a bad vacuum leak then, too. If you've already pressurized the intake I would have thought you'd find a leaking bypass valve then.

It could also have an internal crack that is causing no boost. All the turbos on these cars can get a cracks in the turbine housing. With the results worse in some cases than others.
 
dl242gt said:
Then of all things that might have happened. It's possible that the puck that opens and closes in the wastegate came loose off the arm inside the wastegate. Because of all things that control boost the wastegate is the main thing. A massive boost leak would have to open up whenever the turbo starts to pressurize yet close up so the engine idles and runs fine when not in boost. That's a pretty tall order. I'd think that it would also be a bad vacuum leak then, too. If you've already pressurized the intake I would have thought you'd find a leaking bypass valve then.

It could also have an internal crack that is causing no boost. All the turbos on these cars can get a cracks in the turbine housing. With the results worse in some cases than others.
Click to expand...

The puck coming loose sounds like it could be fairly likely. Next time I take a look at it, I will remove the downpipe and take a peek. I pressurized the intake after the turbo, so I'm not certain if the bypass valve really is leaking. The problem came about suddenly with no fanfair or strange noises while cruising on the highway, so I don't think it is a crack in the turbine housing. I'm pretty damn stumped on this one...
 
Well after a couple painstaking hours we found out the issue. The wastegate flapper wasnt in all the way, it slid out so it wasnt fully closing. It would close but still be open half way.

Sounds like a simple fix though no? Well it would have been but the studs connecting the downpipe to the turbine housing all sheared off, Alex was able to drill one out. The designers of these engines have the skill to place everything in front of everything. It felt like if you had to take out part A you had to take out part B, but in order to get part B you had to move part A lol.

He's gonna finish the car tomorrow, he was nice enough to let me borrow their car for now, just need to pick up the car tomorrow. So with that fixed and the gutted cat it should be running fine now, I just hope it stays that way.

I really appreciate the help he's been giving me. If you need a reliable/dependable person Alex is your man.
 
It felt like if you had to take out part A you had to take out part B, but in order to get part B you had to move part A lol.
Click to expand...

Ain't that the dam truth.

And to get part C, you need a 20 ton press, a 1.3 meter precision torque wrench, three 1 foot men on steroids, and some factory tool number 23477532-A, only available in Sweden for the measly sum of $658 not including shipping. :w00t:

Me... I just use a big fat hammer or go buy another dam car. :lol:
 
I buttoned it all up this morning. The car spools super fast and pulls pretty good at 14psi. Next step is a 15g, ported 90+ and a thelostartof 3" turbo to tail.
 
clipster said:
Ain't that the dam truth.
And to get part C, you need a 20 ton press, a 1.3 meter precision torque wrench, three 1 foot men on steroids, and some factory tool number 23477532-A, only available in Sweden for the measly sum of $658 not including shipping. :w00t:
Click to expand...

...Or you can fabricate your own using JB Weld, PVC pipe + nylon rope and there are exact instructions for how to do this in the FAQ ;-)
 
easiest car ever to work on, if this gave you a hard time never touch an acura, or anything other than a bug
 
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