BMW N54 30FF Underboost Code – Boost Leak Testing – DIY


We decided to share this great DIY on boost leak testing that was contributed to the N54 community a long while back and is still very useful for those running into issues with underboost codes, limp mode due to a boost leak in their charge piping.


Spent a lot of time the past week trying to diagnose a 30FF. (Boost Log is NOT matching Target)
Since 30FF can be from a leak in the pressurized section of the intake track and/or the turbo just isn’t putting out enough, I broke this up into 2 areas:

1. Turbo compressor (COOL side) to intake manifold/head interface.

a. Test entire intake system.

b. Test Diverter Valves

2. Turbo compressor operation(HOT side) –

a. Vacuum system & line integrity

b. Wastegate actuator & line integrity

c. Solenoid functionality

The sequence of the tests is not so important. You can start with the tests you already have the tools or time for and stop if you are lucky enough to detect & fix the problem. Just remember that there can be more than 1 source for the 30FF. You’re not done till the 30FF’s stop popping up.

OK, I had already made several fittings to tap into, plug and pressurize sections of the OEM intake system.
After removing sections of the intake system and checking them independently, it was obvious they could leak when they were put back together.
So after playing with all the new gadgets I wanted to test the system w/o removing any parts that are exposed to boost pressure.

Step 1a Tools & Equipment
• Compressor w/ regulator, GOOD pressure gauge, hose
• Very quiet garage to listen for leaks – beware of neighbors watering the lawn
• Squirt bottle w/ Dish soap & water
• (2) – 1-1/2” PVC pipe cap ($0.77 each @ Menards)
• (1) – ¼” NPT male hose fitting ($0.25 @ Harbor Freight)

Prep work:
Remove any bumps (injection molding gates) on the OD of each cap w/ a file (if necessary, depending on mfg.). Make it smoothed.
Drill & tap 1 cap to install air fitting. (7/16” drill, ¼” NPT tap)

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Get front wheels off the ground – ramps, jack stands or lift
Car off and cool
Remove the 14 screws & engine splash pan
Remove the DCI’s (or air filter box) to access the 2-1/4” accordion intake tubes.
Install plain PVC cap in the F tube and the other cap w/ air fitting into the R tube and tighten hose clamps.
**If running OEM crankcase venting system you must Plug the rear air intake tube at the PCV return tube heater element**-edit on 8/25/11
Set the air pressure regulator to ZERO and connect air hose to cap fitting.

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You are now “in Theory” ready to pressurize the whole intake & exhaust system from the air inlet to exhaust tips. Not sure why but didn’t need to use exhaust tip plugs I made cause air was not exiting the exhaust. Only thing that comes to mind is no overlap between intake & exhaust valves. Ideas welcome…

SLOWLY increase regulator air pressure to 10 psi (it’s high enough to find a pretty small leak).
Filling up the whole system takes some time and can sound like a leak when filling. Give it time & let pressure stabilize.
The air intake tubes are not designed to hold pressure but they do hold some. Mine held to 12 psi B4 rear turbo inlet started hissing. This is NOT a boost leak but the weakest joint in this test method.
Don’t go leak hunting if a turbo inlet is leaking @ 10 psi. You may need a buddy to hold intake tube onto turbo @ 10 psi if leaking there or get creative and wedge it in place.

Reference Picture from Hotrod’s post, I did NOT remove my engine!
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Listen & look, squirt soapy water anywhere/everywhere you suspect & look for bubbles to confirm leak. Get under the car & check both turbo connections, piping, connectors, FMIC, elbow, DV or BOV & recirculation tubes, vacuum/boost lines to DV’s, Charge pipe, Throttle body, intake manifold to engine head. Fix any leaks & retest. If air tight @ 10psi you are done with this step.

Here’s what I found!

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Step 1b.Test DV’s

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I also rigged up a way to pressurize the DV’s , CP & elbow as an assembly. Used syringe w/ Tee to both DV’s.

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Step 2a. Vacuum system & line integrity – Tools & Equipment
• Vacuum gauge
• Pick or small pry tool to remove vacuum hose

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Vacuum to Wastegate solenoids
Follow hose from the top a canister to a solenoid. Remove hose at the solenoid and connect vacuum gage. Start car, gauge should read about 26” Hg at idle. Don’t know acceptable limits but if it’s much lower than 26” trace ALL hoses all the way back to vacuum pump (behind oil filter). Replace leaking or pinched hose.
Reconnect the hose and repeat the vacuum test on remaining solenoid.

Step 2b. Wastegate actuator & line integrity – Tools & Equipment
• 100cc or larger syringe
• 5/35” Tee
• Towel Clamp or long nose vice grip

Test can be done w/o a Tee by connecting syringe directly to braided hose going to wastegate actuator. (Easier said than done)
Since a Tee is needed for Step 2C you can save time & aggravation if you:
Cut the 5” long hose that’s between the 2 OEM tees (see below)
Use clamp to Pinch the short hose that runs down from the Rear Tee to the Rear Solenoid
Connect syringe to the Hose you just cut
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Apply vacuum by pulling syringe plunger and check for smooth movement of actuator rod and a metal to metal sound when the wastegate closes.
Once wastegate is closed it must remain closed and the force needed to hold the plunger steady must remain constant. If it gets easier to hold the plunger in place it means the hose and or actuator is leaking.
Repeat for front actuator connecting syringe to remaining 1” of OEM tube & pinching front line between Tee to Solenoid. The front actuator that is NOT visible from above engine. If you don’t have access to a borescope to watch the rod movement, listen closely to the sound and compare to the front. If you can’t see the actuation rod you will have to make the call (by listening) if it’s operating like the rear actuator.
Connect Tee and go to next step or plug the open end of the Tee if you are done.

Step 2c. Solenoid Functionality – Tools & Equipment
• Vacuum gauge
• 6’ of 5/32” vacuum hose
• 5/32” Tee ( if you didn’t take my advice in 2b)

Install 5/32” Tee into vacuum hose you cut in 2b.
Connect 6’ hose to Tee & gauge
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Route gauge thru drivers window & into car
Getter warmed up, cruse at about 60 mph in 3rd,
Go WOT, w/ a Tune the gauge should spike then hold at about 20” while under boost.

If the 30FF is not gone then there’s only 2 more options:
1. Wastegate rod adjustment back to OEM spec
2. New Turbos


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