I recently had my Forester engine serviced and afterwards it didn't feel like I was getting boost and was getting a whirring/spinning noise while throttling. I brought it back and they realized they left some loose turbo vacuum hoses. They put them back on and it's not making the noises anymore but I think it still doesn't feel like there's a lot of boost - at least not like before. They test drove it with me and say it does feel like there's some boost at the high end, but I certainly am not feeling the boost when I'm merging on the highway or hitting the throttle.

At this point, it's just seat of butt testing to know what's going on - is there a more definitive way I can diagnose whether I'm getting the proper turbo boost - or at least that the turbo boost is working at all - as a non-mechanic? Something I can point at and say yes/no it is working before I take it back to the mechanic?

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    Do you have a mechanical pressure gauge? I'm not sure how much boost you should be building nor at what RPM but it would be the easiest way to check peak and sustained boost. If you have a scan tool that can read live data you may be able to use the MAP sensor to get an idea of what's going on.
    – Ben
    Oct 10, 2016 at 21:54
  • No gauge that I can see - there haven't been any customizations to the car. Oct 10, 2016 at 21:55
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    Exactly what @Ben said. It would be very hard to check unless you know what the boost curve should look like and what the boost level should be for any given situation. If they left off vacuum/boost hoses, you may want to take a gander yourself and see if anything is out of place with the other hoses. It could be one of the pressure side (between the turbo and the intake manifold) is not attached correctly and you're losing boost out of it. It doesn't take much boost loss to feel a difference. Oct 10, 2016 at 21:57
  • Is installing a boost gauge something that a novice can install at little expense? I'm kind of a curious sort so would be interested in seeing that kind of info anyways. Oct 10, 2016 at 22:00
  • Generally the gauges aren't cheap, plus you need the vacuum line and a tee. Check your local autoparts store or amazon to get a feel for pricing.
    – Ben
    Oct 10, 2016 at 22:14

1 Answer 1


Without data, your diagnosis is about the best you can do. A poorly operating (or installed) turbo won't make as much boost (which you can feel), and they usually sound terrible when the turbo actually fails. In short, your senses are your best bet if you don't have tools to get data. Generally things are pretty bad by the time you see a check engine light.

If you're looking to do something to get data, I highly recommend an Accessport, especially if we're talking about a turbocharged Subaru. It can tune the car, read trouble codes, and even if you don't want to make a whole lot of power, datalogging can tell you if the car is making substantially less load (what the computer cares about rather than boost), if a cylinder is misfiring, or if the engine is running on the lean side.. loads of information.

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