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  #1  
Old 3rd May 2008
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JMarkert JMarkert is offline
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Default What limits boost?

I have an ' 04 Linear (2.0t). A few days ago I put in new spark plugs (official saab spec NGK's). Yesterday I took a little road trip. Early in the drive I was playing with the throttle a little (testing out hot the new plugs felt), and the boost gauge would go into the red now and then like normal.

After a while, I stopped playing and put it in M5 and cruised along for a while.

When speeding up after I hit a little traffic, I noticed that even when I floored the car, the boost gauge would only go up to the half way point. I wondered if this might have something to do with being in M5, so I switched back to D and had the same result (except of course that the trans downshifted when I floored it).

On the drive back, the boost gauge was back to normal, and I had the usual amount of pep.

All this has been on one tank of premium gas (93, I think), so I don't think it was the knock sensor.

What else can limit boost? Could this all just be the computer adjusting to the new plugs (the old ones were probably original, the gap was pretty big).
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Old 3rd May 2008
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ECU limits it. But physically, the wastegate.
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Old 3rd May 2008
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OK, but what ECU parameters?

Can wastegates get temporarily jammed open?
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Old 3rd May 2008
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waste gates are actuated by vacuum pressure, they mostly malfunction when something is broken. ex: flapper, spring, ...
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Old 3rd May 2008
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The logic for this (boost control) is rather sophisticated. There is pretty much detail in the WIS. The ECM determines the need for boost based on a number of parameters including engine load, vehicle speed, throttle plate position, accelerator pedal position, manifold pressure, ambient temperature, etc.

The wastegate is actually controlled by positive pressure, not vacuum (which is negative pressure). The source of positive pressure is the turbo compressor outlet pressure routed through the boost control solenoid. The solenoid turns on and off at high speed based on a control signal from the ECM. This allows the wastegate to be opened or closed incrementally with precision.
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Old 4th May 2008
Dach Dach is offline
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My curiousity would be, how did you get the boost gauge with the linear unless you installed an aftermarket one?
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Old 4th May 2008
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Quote:
Originally Posted by Dach
My curiousity would be, how did you get the boost gauge with the linear unless you installed an aftermarket one?

I think it's standard for North American models.
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Old 4th May 2008
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Quote:
Originally Posted by ctrlz
The logic for this (boost control) is rather sophisticated. There is pretty much detail in the WIS. The ECM determines the need for boost based on a number of parameters including engine load, vehicle speed, throttle plate position, accelerator pedal position, manifold pressure, ambient temperature, etc.

The wastegate is actually controlled by positive pressure, not vacuum (which is negative pressure). The source of positive pressure is the turbo compressor outlet pressure routed through the boost control solenoid. The solenoid turns on and off at high speed based on a control signal from the ECM. This allows the wastegate to be opened or closed incrementally with precision.
With a OEM BCS yes, but not with a boost controller.
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Old 4th May 2008
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Quote:
Originally Posted by JMarkert
I think it's standard for North American models.
The stock boost gauge is only a rough estimate calculated by the ECU, not actual pressure.
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Old 4th May 2008
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if you were too include atmospheric pressure on a PSIG gauge. you would be adding 14.7psi
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Old 5th May 2008
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Default Update

OK, so today, three days later I'm merging onto the highway with a fairly heavy foot. Next thing I know, the CEL comes on, and once again the boost never goes above the middle.

So, I zoom up to AdvanceAuto and pick up a code reader (always wanted one, anyhow) and find....

Code P0244 = Turbo/Sup Wastegate Solenoid A Range/Performance - so, we're on the track anyhow.

I cleared the code and all is normal again - Advice most welcome
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Old 5th May 2008
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WIS does not have much to say specifically about p0244. Basically a charge air circuit problem... one that results in excess boost.

Has this car been modified in any way? Manual boost controller or adjustment of wastegate actuator locknut?

If not, I would first check wiring and plumbing to boost control solenoid.
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Old 5th May 2008
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Thanks for checking Ctrlz! Nope, no mods at all.

I checked alldata, and it's a little vague too. There is a possible software and/or control module update, but I imagine this would have been done by now. Otherwise it says to check the wiring and connections.

Last edited by JMarkert; 5th May 2008 at 08:52 PM.
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