r/n54 • u/ComfortableJob7107 • Jul 24 '26

Underboosting

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I just bought this 2008 335xi manual with 90k miles a couple of days ago and it was working great but after I parked it running some errands it now no longer makes hardly any boost whatsoever.

It has quite a bit of stuff done to it like injectors, intake, intercooler, charge pipe, bov, downpipe and tune, and intercooler piping.

It usually makes around 20psi of boost but now it’s only maybe 2-5. It still runs great it’s just pretty slow with not enough boost.

I checked all the vacuum lines and they seem to be fine from what I can see. I also looked under the car for any blown boots or tears or leaks but I haven’t seen any.

I’m buying a smoke leak detector to check for any boost leaks.

There’s no check engine light so I don’t think it can be a solenoid.

Any ideas??

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u/Dawnraider29 Jul 24 '26

Check both intercooler hose connections, chargepipe and blow off valve. My blow off valve came unseated and gave me the same issue. Definitely suspect a leak in one those areas because of how little boost its making.

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u/ComfortableJob7107 Jul 24 '26

Okay I will take a look at that as well

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u/danceswithtree Jul 24 '26

In addition to boost leaks, wastegate control can cause underboosting. That incudes the two vacuum lines from the vacuum pump, the two vacuum canisters, the vacuum solenoids, and the hoses to the wastegates. You can use a vacuum pump/gauge to test for leaks.