Since I spent a lot of time writing this, and not everyone checks both boards:

There are many things that can cause a "LOW BOOST" Condition.

To physically check a turbo for operation, you can:

A: Take the air inlet to the turbo off and look and see if its spinning while the engine is idling. Be careful not to let anything enter this area as even small soft things like bugs could cause a problem.

B: If its spinning, then it is at least working. Now you can check to see how much wear it has. Turn the engine off and let the turbo stop spinning. Grasp the nut on the end of the wheel and try to move it in and out of the turbo. If there is any more than a slight (.002 in IMO) of movement the thrust bearing in the turbo is wearing and will soon allow the compressor to suck itself into the housing. There is probably .010 or less of clearance between the wheel and the housing.

C: Pushing the wheel up and down, try to spin the wheel slightly. If you hear any noise (metal on metal) the wheel may hit the housing during operation.

D: Oil leaks will normally be really bad on the turbine (exhaust) side of the turbo since the heat is harder on the seal, and often the seal gets coked up and wears away. A mild oil leak will eventually cause smoke out the exhaust in the back. A severe leak will actually drip out between exhaust joints or cause smoke under the hood. Compressor oil leaks can happen, and usually are because of a bad oil drain. However, since the engine is sucking air through the compressor most of the time, oil leaks here will be visible in the compressor outlet.


Checking Boost.

For a turbo to make boost, there must be a differntial across the turbine wheel.  The more boost a turbo can make with a smaller differential the more efficient the turbine wheel is.

Things that can affect boost.

A: Restriction. Either before the compressor inlet or after the turbine out let. The harder the turbo has to work to suck air in, the less work it can do pushing air out and will make less boost.

Likewise, the harder the turbo has to work pushing exhaust out, the less work it can transfer to the compressor wheel to make boost.

Possible intake restrictions include: Airfilter, air tubing, collapsed tubing
Possible exhaust restricitons include: BAD CAT, Bad Cat, bad cat.
also: collapsed exhaust, clogged muffler with BAD CAT chunks, potatoe stuck in tailpipe.

B: Exhaust Bypass. In order for the turbine to do any work, the exhaust must pass over it, not around it. The same thing that keeps your boost in safe levels, can keep it from reaching FUN levels. The wastegate operates by opening  a hole before the turbine that allows exhaust into the downpipe without going over the turbine. There is a valve that blocks the hole. This valve is connected to the wastegate actuator externally (gold can on turbo). The WG actuator has a diaphram with a spring pushing the rod into the actuator. On the other side of the diaphram, boost pressure pushes against the spring, moving the rod and opening the valve.

B1: Solenoid. In the simplest setup, boost pressure from the turbo outlet is directed to the WG Actuator. However, this can create inconsistent boost levels, and there is no control over boost other than tightening the spring on the actuator. So on many cars including the TGP, an electronic solenoid is placed inline with the boost signal.

By keeping the boost signal from reachjing the actuator, it will not get pushed open and therefore not allow exhaust to bypass the turbine. The solenoid vents the boost signal to the atmoshphere to keep it from reaching the actuator. So if the solenoid isn't working, you will see very little boost as all the boost signal will get to the actuator. Similarly, if a line is broken or missing, you will get ALOT of boost since no boost signal will reach the actuator. In this situation, exhaust backpressure will actually push against the wastegate valve causing it to open and limit the boost.

B2: Actuator. As mentioned the actuator has a spring and a diaphram. If the diaphram is ripped, you will get high boost since the boost signal will not be able to push against the spring. However, if the spring is broken (typcially from rust), the boost signal will easily push open the wastegate. Similarly, exhaust backpressure can blow the WG valve open with a broken spring. Easiest way to check an actuator is with a hand held vacuum pressure pump (Mighty Vac). It should start to move around 4.5 PSI.

B3: Exhaust leak in the crossover. A serious leak before the turbo will keep the exhaust from reaching the turbine and reducing boost levels. However, it has to be a BAD leak and will be quite obvious.


C: Boost Bypass. This is pretty simple. Allowing boost to escape before it reaches the motor. Usually intercooler hose connections are loose or blown off either because of loose fittings or oily fittings.

D: Engine Problems. Typcial N/A engine performance issues can really affect boost such as cam timing, valve sealing, crankcase sealing etc. But there are usually indicators to point in the right direction.

Hope this helps

Chris

Looks awesome man! Now if we could just get people to use the search function!!  :wink:

excellent chris, if only people would look at the sticky's.  if it's a sticky it's usually relevent!!!!!!!

easy to understand for sure GREAT job