Twin Turbo Problem

[mike]

Hi
I have a '96 Legacy GTB & having turbo problems.
It runs normally on the primary turbo at around 12psi boost, but when it
changes to the secondary turbo the boost drops to 7psi and stays there. -
usually it would rise to about 15 psi.
Any thought what the problem. Is there any common twin turbo problems that
would have such an effect?
Cheers Mike

 

[noone]

Hi
I have a '96 Legacy GTB & having turbo problems.
It runs normally on the primary turbo at around 12psi boost, but when it
changes to the secondary turbo the boost drops to 7psi and stays there. -
usually it would rise to about 15 psi.
Any thought what the problem. Is there any common twin turbo problems that
would have such an effect?
Cheers Mike

is it possible that the wastegate is partially open on the secondary turbo
or perhaps the boost controller solenoid isn't bleeding off boost like it
should so it is running strictly off of wastegate spring pressure?

 

[mike]

Hi
Thanks for the suggestion.
This model dosn't have a wastegate on the secondary turbo (unlike pre'96) it
uses the primary turbo wastegate to contol boost during the primary &
secondray phases.
The primary wastegate could be a little stuck open preventing boot build up
at higher loads, the primary turbo seems a little sluggish as well. I'll
take a look
Thanks
Mike

 

[Guest]

Hi
I have a '96 Legacy GTB & having turbo problems.
It runs normally on the primary turbo at around 12psi boost, but when it
changes to the secondary turbo the boost drops to 7psi and stays there. -
usually it would rise to about 15 psi.

It doesn't *change* to the secondary, it uses the secondary as well.

Any thought what the problem. Is there any common twin turbo problems that
would have such an effect?

There is a valve between the output of the secondary turbo and the
intercooler with a differential pressure sensor across it.

It is supposed to open when the pressure from the secondary turbo matches
(exceeds) the pressure in the intercooler.

If that valve (or pressure sensor) isn't opening you share exhaust gasses
between turbos but only get boost from the primary. No idea about common
problems but the effect matches your symptoms.

--

 

[mike]

It doesn't *change* to the secondary, it uses the secondary as well. Thanks for the clarification.


There is a valve between the output of the secondary turbo and the
intercooler with a differential pressure sensor across it.

I measured using my boost guage the secondary turbo pressure by tapping
into the airline that goes to this sensor and measured low pressure - it
never exceeded 10 psi.
In a seperate experiment disabled the pressure relief valve (AKA BOV) on the
seconday turbo to see if this was leaking resulting in pressure loss but it
had no effect.

It is supposed to open when the pressure from the secondary turbo matches
(exceeds) the pressure in the intercooler.

If that valve (or pressure sensor) isn't opening you share exhaust gasses
between turbos but only get boost from the primary. No idea about common
problems but the effect matches your symptoms.

--

The only things I can come up with is faulty exhaust valve - either primary
wastegate or changeover control valve or the secondary turbo bearing have
died.
But there are no audio indication of the latter.
BTW Here is a good website for twinturbos if you are interested
http://www.veloce.pwp.blueyonder.co.uk/