Mitsubishi Eclipse 3G Club banner

1 - 2 of 2 Posts

17,852 Posts
Discussion Starter · #1 ·

Boost Leak Theory
Boost Leaks, while easy to diagnose and fix, are often over-looked by turbo car owners for one reason or another. Whether they are overlooked because of laziness or ignorance, they rob power from your turbo car and cause it to perform at much less than its potential. In this article we will discuss not only why and how boost leaks could be holding your car back but also how to make and use your own boost leak tester.

Any air leaking out of the plumbing system from the compressor side of the turbo to where the intake manifold meets the head is considered a boost leak. In a vaccume condition like at idle, air would be being sucked in through a boost leak, and in a boosted condition like full throttle air would be blowing out of a boost leak.

Most turbo cars use an air flow sensor to meter air as it is brought into the turbo. As air passes through the air flow meter, it is counted and the ecu assigns a certain amount of fuel for that amount of air. Lets consider a hypothetical situation of a particular boost leak. Air passes through the air flow sensor, is measured and counted, and the ecu assigns a certain amount of fuel for that particular amount of air. As the air passes through the turbo and begins its path to the head, some of it escapes through a boost leak, (in this hypothetical situation we will use a bad throttle body gasket as an example of a boost leak). The air escaping through the leak was measured and considered in calculations when the ecu assigned the correct amount of fuel for the mixture, but the ecu does not know that some of the air is leaking out through the damaged gasket, and does not adjust for it. In this situation less air will actually be reaching the engine than was metered, and the fuel will remain unchanged, throwing off the tune and causing a rich condition. This rich condition will cause your car to run at less than its potential.

Not only will this rich condition cause your car to run at less than its potential, you may notice that your car has trouble holding boost through the gears at wide open throttle. Throwing off your tune, and not being able to hold boost is enough to make you lose a great deal of power that your car should be making.

Boost Leaks arent only harmfull to cars with airflow meters though. Any turbo car could potentially be harmed from boost leaks. Boost leaks make the turbo work harder to do the same amount of work, and could lead to a turbo failure because of it. To demonstrate this, imagine building pressure in a baloon by blowing air into it with your mouth. Then make a tiny hole in the baloon and try to blow it up again. You will notice you are doing more work than you did the first time to inflate the baloon to the same size. Hopefully this section has motivated you enough to check your car for boost leaks. Now on to how you can make a Boost Leak Tester with parts readily available at your local hardware store.

Parts Required

3" to 2" Coupler 2" PVC Cap Air Line Fitting Teflon Tape

Parts Required:
1 (3" to 2") Rubber Coupler w/ Clamps
1 (2") PVC Cap
1 (1/4") Air Hose Fitting
Teflon Tape

Boost Leak Tester Assembly

Assembly Notes: This boost leak tester was designed to fit over a 3inch turbo inlet. If your turbo inlet size is different than 3", you will have to purchase a different coupler that will fit over your turbo inlet, and possibly a different PVC Cap depending on what size the opposite side of the coupler is. The assembly process is the same though, as long as one side of the coupler can fit over your turbo inlet, and the other side ends up with the PVC Cap in it.

Step 1:
Drill a hole in the top of the 2" PVC Cap.
Step 2:
Wrap 1/4" Air Hose Fitting Threads in Teflon Tape.
Step 3:
Screw 1/4" Air Hose Fitting into 2" PVC Cap.
Step 4:
Place 2" PVC Cap in the 2" side of the coupler and tighten the clamp.
Pressure Regulation Note: This boost leak tester was designed to be used with an air compressor that has a built in pressure regulator. If your compressor is not able to control the amount of pressure it puts out. You can purchase an in-line pressure control regulator and gauge from the air compressor section at the hardware store, and set the target pressure right at the tester.

Boost Leak Test Procedure
This section will describe to you in detail, how to perform a boost leak test on your single turbo setup. If you are using a twin turbo setup, an alternate method will be described after this one.

Single Turbo Setups:

Slip BoostLeak Tester over the compressor inlet of your turbo.
Tighten Clamp
Set pressure on air compressor to desired amount of boost. (It would be a good idea to set the pressure on the air compressor to the amount of boost you would like to be running in the car. For example if you are planning to run 30psi, set the compressor to at least 30psi.)
Attach the air hose from the compressor to the quick disconnect fitting on the boost leak tester
At this point you can take the time to listen for air leaking out of the system. The system will be pressurized so any air leaking out should be easily audible. If you are having trouble finding a leak, feel around the general area and feel for air on your hand.
Once you've located the leak, now go through and fix it

Twin Turbo Setups:

If you are using a twin turbo setup, the procedure and assembly change a little bit. Instead of using a boost leak tester like we have described, you will make two smaller boost leak testers that do not have fittings tapped into them. They will essentially act as plugs to block off the intake inlets on each compressor housing. Then you will use a small barb fitting on the air blower attachment for the air compressor to feed boost into a vacuum line. Once both turbos are blocked off, you can unplug a vaccum line from the blow off valve, or wastegate, and feed air into the system through that line.

Testing Tips:

If you are using a bleeder type boost controller, air will leak from it durring the boost leak test. Either take it off and replace it with a hose or try holding your finger over the bleeder hole to keep it quiet so you can hear actual leaks.

When performing a boost leak test, if the system is pressurized and you arent hearing any leaks, try opening the throttle body.

Have someone sitting inside the car keeping an eye on the car's boost gauge to see if it is reading the full amount of pressure being fed in through the compressor. This will give you an idea of if there are any leaks you might be missing.

Write up Courtesy of

1,429 Posts
Nice. I actually just ordered a boost leak tester from It had a gauge built into it and was roughly the same price as the DIY parts.

The DSM guys are very into boost leak tests, for good reason. I am going to to try to include a boost leak test w/every oil change.

I will also add that a soapy water mix will make any leaks plainly visable.

Thanks Russ!
1 - 2 of 2 Posts