From Dsmwiki
A boost leak is a leak in the portion of the intake system that is boosted by the turbocharger's compressor. The compressor pressurizes intake air, and this air is eventually fed to the combustion chambers, so every intake component between the compressor and the combustion chamber is exposed to boost pressure and must be able to withstand that pressure without leaking.
Boost leaks are a problem for several reasons:
- In the stock configuration, the MAF or MAS is before the compressor, and this is what tells the engine computer (ECU) how many molecules of air are entering the cylinders during their intake strokes. The ECU needs to know this for a couple of reasons, but the most important one is that it needs to know how much fuel to add to that air in order to get the desired air-fuel ratio. If some of that air leaks out of the system, the ECU does not know this and still injects fuel based on what it was told by the MAS. In other words, you're leaking "metered" air. Under boost, this means that the ECU thinks there is more air than there really is, so it injects too much fuel, the car runs rich, fuel economy goes down, and the car is less powerful.
- If the boost leak is big enough, the amount of air being pulled through the MAF/MAS will be much higher than the amount of air actually making it into the cylinders. Eventually, it hits the ECU's "safety limit" and the ECU cuts fuel altogether (called "fuel cut").
- If you have upgraded to an aftermarket boost controller and boost gauge, the boost gauge is usually tapped in somewhere off of the intake manifold. So the boost gauge is reading what the boost pressure is in the intake manifold, not at the compressor outlet. If your boost controller is properly installed, it is reading a boost signal off of the compressor outlet. If you have a major boost leak, the pressure at the compressor outlet may be MUCH higher than the pressure in the intake manifold (and on your boost gauge). So if you are attempting to adjust your boost controller with a large boost leak, you can drive your turbocharger to higher boost levels than you intended. This can harm the turbocharger, increase knock, hit fuel cut, etc.
In a stock vehicle in perfect working order, the following components will have to withstand boost:
- Compressor housing
- Compressor/j-pipe gasket (1g)
- J-pipe (1g)
- J-pipe/Lower intercooler pipe (LICP) coupler (1g)
- Compressor/LICP coupler (2g)
- Boost controller vaccuum lines (all of them)
- Boost controller tee fitting
- Boost control solenoid (BCS)
- LICP
- LICP/Intercooler coupler
- Intercooler
- Intercooler/Upper intercooler pipe (UICP) coupler
- UICP (both sections)
- Coupler between UICP sections
- TBV/BOV gasket (1g)/coupler (2g)
- BOV valve
- Upper and center chamber of the TBV/BOV
- TBV/BOV vaccuum line
- UICP/throttle body elbow coupler
- Throttle body (TB) elbow
- TB elbow/TB gasket
- TB
- BISS o-ring
- TB throttle shaft seals
- TB/intake manifold (IM) gasket
- IM
- FPR vaccuum line
- PCV valve vaccuum line
- PCV valve (intake manifold end of the valve)
- (other TB vaccuum lines...someone fill this in)
- IM/Head gasket
- Intake ports (in the head)
- Injector seals/insulators
- Intake valve stem seals
- Intake valves
- Head gasket
- Cylinders
- Piston rings
- Piston crowns
- Spark plug head
- Spark plug crush washer
- Exhaust valves
If anything before "Intake valves" above is leaking, that's a boost leak. But the remaining items might appear as leaks if you are testing for boost leaks. Intake valve stem seals are a grey area for me, because they are exposed to air-fuel mix, but I'm not sure how well it's mixed at that point...if the top of the port where the valve stem passes through the port is seeing mostly pure air, then it would leak mostly air, and you'd run richer, but if it sees mostly fuel, it'd leak air-fuel mix, and if it saw mostly mixed air-fuel, it probably wouldn't affect AFR in the combustion chamber. Maybe not a point worth bickering about, but I point it out nevertheless ;).
That's enough for me for now.
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