Hi i have a 1990 subaru legacy rs BC5 single turbo it seems to blow turbos all the time, the blow off valve opens up on idle even through the boost gauge reads zero and when it drives and hits boost it takes off then loses power although it still reads boost almost like it drops a cylinder, i have replaced igniter, turbo, manifold, ecu, intercooler, sensors ect any suggestions or help would be awesome cheers

It sounds like a lack of fuel to the injectors. check the fuel manifold pressure and the regulator. run fault codes to check for faulty sensors.

a common part over looked when a turbo is bad the drain tube gets plugged and oil cant drain

hi i would invest in an electronic boost controller, they remove the dead spot in the revs before the big turbo winds in, they are worth a few hundred dollars but are well worth the money. You also have the option of turning up the boost safely a few psi with these but you will love the difference without the dead spot it will seem like a new car to drive. If you do that and want more still i would get a turbo upgrade, ecu upgrade, fuel pump upgrade etc do all that you can before going with the expensive motor rebuild

T3/T4 Turbo
Lubrication Kit "oil pump and lines"
Turbo Bracket
Intercooler
Intercooler Brackets
Aluminum Intercooler Piping
Silicone Couplings
T-bolt Clamps
Type RS Blow-off Valve
Stainless Steel Headers
Stainless Steel Downpipe
Rising Rate Fuel Pressure Regulator
Auto-meter Boost Gauge
Vacuum Harnesses
Cone Style Air Filter
PVC Filter

And to be able to Dyno Tune the engine for the extra boost. Nothing more then 8 PSI with out replacing the Pistons.

Might of forgot something but this are the major things needed.

There is a differential pressure sensor mounted beside the 2wd fuse link with a couple of 5mm pressure lines going in each end. These lines connect to each turbo output. The two pressures acting on each side of this sensor produces a voltage read by the ecm which tells it the differential pressure between both turbo boosts during acceleration. Normally the primary turbo leads the boost and the secondary turbo plays catch up until the pressures are even then air valves get opened and shut to bring the second turbo compression in parallel with the first turbo. The two turbos simply act in sequence but function in parallel. Its just like having one big turbo but operating down two smaller paths.
The sensor measures the changing boost pressures as the second turbo rises to equal the first.
If the generated voltage is not what it expects it shows as a code 66. As you can start to see there could be many different reasons why there might be an imbalance in the two pressures.
It could be either turbo causing the imbalance or a faulty bypass valve or pressure relief valve or a number of other issues. It doesnt necessarily have to be a turbo or a solenoid which is faulty. All it means is that the pressure difference between the two boosts is wrong.
The real question is why is it wrong.
There is a lot you can do yourself to diagnose the cause. If you are able to study the manual which is available free off the internet (do a search for the link) then you will see that you can test many of the component parts quite easily at home and also temporarily replumb the turbos to bypass aspects of the system control to figure it out for yourself. To do this you will have to gain an understanding of how the twin turbo system works. However its not hard if youi strip away the complex jargon hiding simple functions.. There is a mystique of fear concerning working on these motors which is not justified.Just use common sense and logic.
Or you take it to a shop and pay someone else to do it for you.

check wastegate(round valve on turbo)is working it may be loosing vacuum,or you have an electrical issue with the boost controler,but check wastegate first as its easier and could be the problem

The turbos are two different types.One is designed to start a bit lower in the revs and the other to come in when the revs are higher.Its called compounding boost.It can be changed but to put it on a sinlgle turbo would cost a fair bit.If you hot up the ones you have by recalibrating them and exhaust,you do get VERY big gains.The car is more driveable and you can get big boosts in power if the tuner knows what he is doing.Some really big horsepower units come from triple compounding turbos and the torque levels are extremely high.Hope this helps

I don't know much about Subaru but I expect the turbo wastegate is controlled by vacuum which is switched by the engine management system.

In the old days the turbo wastegate was normally closed and the manifold pressure was fed into a servo which opened the gate at the predetermined pressure.

The new way of doing things is to have the wastegate normally open and the turbo producing no pressure until the engine management decides the time is right and it switches a vacuum supply to a vacuum servo which then closes the wastegate. With this system the wastegate is constantly moving and so is unlikely to give trouble though the vacuum servo sometimes leaks and vacuum lines give trouble.

A fellow I know had a similar problem where the engine check light would light up and he would lose performance. In theory this was because of an overboost condition but the repair shop after fiddling around with it for a week fitted a new EGR valve to produce a cure.

have ur sensors check 02/ cam angel etc and may b a.f.meter

replace the wastegate vacum lines or wastegate acuator. if they leak it raises the boost. Let me know if its not that.