dannytwo

iTrader: (5)

since iv cleaned my boost sollinod the scoob does'nt seem to be boosting properly.

its driving me mad and i hav'nt a clue to what it could be.

any ideas

the car is a 97 model forester import same engine as the same year impreza turbo.

cheers

ReggieMY99

could be a pipe leaking somewhere, they tend to get tender and crack up when you mess about with them since its very hot down there and i asume you have since you cleaned the solonoid


i had the same problem after cleaning the solonoid, leaking pipe

hope it helps

dannytwo

iTrader: (5)

how can i find a leak?
if there is one

plus what is the boost solleniods job?

cheers

dannytwo

iTrader: (5)

anyone

ZIPPY

How many times have you cleaned the solenoid, i have on many occasions had to clean them at least twice before the car will boost correctly, agreed you should check the associated piepework for leaks but dont presume cleaning it once is good enough.

Zippy

pabs

I had the same problem until yesterday when I swapped two pipes round which I think I must have put on wrong and the car was only boosting to 0.6bar - the pipes were the ones coming off the small white t-piece vacuum line, one of the t-piece outlets has a smaller size bore than the other two, where this was previously feeding the boost guage, I swapped it with the pipe feeding the map sensor, not sure if that was the right thing to do but seem to be getting more boost now.

pwhittle

Finally traced the fault on my car to the intake pipe to the intercooler, which wasn't secured properly. Made a huge difference, and cost me 35p to fix.

Graz

Have you got a boost gauge? Invaluable in diagnosing these sorts of problems.

Hope I get this right

The final output boost pressure of the turbo is under closed loop control.

The boost pressure is controlled by the wastegate, opening the wastegate allows some of the exhaust gasses to bypass the turbo and flow directly into the exhaust. This mechanism is used to prevent overboost, if the wastegate didn't open the turbo would spin faster and faster with engine output and could eventually tear itself to bits, hence the fuel cut mechanism in the ECU. If the ECU sees too much boost (overboost) it will cut the supply fuel to the injectors to prevent the turbo (and engine) from damage.

The wastegate is controlled simply by a pressure activated actuator, the air pressure from the turbo outlet is connected directly to the wastegate actuator. According to how heavily sprung the wastegate actuator is dictates how much pressure is required to open it. About 0.5 bar on an Impreza (hence the 0.5 bar limp home mode).

Now here's the clever bit Obviously we want a higher boost pressure then 0.5 bar and for fast spool up the wastegate needs to be held shut until the target pressure is nearly reached. So while the outlet of the turbo is connected directly to the wastegate actuator a tee-peice is also stuck in the middle leading off to the boost solenoid. The solenoid can open and close under control of the ECU to bleed away some of the pressure from the wastegate actuator. The excess air is bleed back into the air intake. So while spooling up the solenoid will be open to allow the pressure to bleed off and keep the wastegate shut. When the target pressure is close to being achieved the ECU will start to close and then open the solenoid (duty cycle) to maintain the target boost and prevent overboost by allowing the wastegate to open. The ECU boost map and programming dictates how quickly the boost solenoid should open and close to maintain the target pressure. The ECU also sees a signal from the MAP (manifold pressure) sensor enabling it to close the loop, i.e. it knows the actual boost pressure vs. target pressure and can dictate boost solenoid control accordingly.

Think that's more or less how it works