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Car Won’t Build Boost (Beginners Guide With Pictures)


I fell in love with Turbochargers when my father bought a Saab Turbo in the early ’80s, I’ was just a kid, but I was hooked on that Turbo Kick.

A Turbo may fail to boost for several reasons, but here are the 8 most common causes:

  1. Blocked air intake
  2. Sensor fault
  3. Charged air pipe fault
  4. Waste-gate fault
  5. Waste-gate Actuator fault
  6. Waste-gate Solenoid fault
  7. Blow off valve fault
  8. Turbo failure

I’m a mechanic for twenty years and turbos have made big strides in power and reliability. In this post, we’ll look at all the most common issues around a non-boosting turbo, strap yourself in!

What Is A Turbo?

Turbo charger

Turbos are back! Once only high-end sporting models were graced with a Turbocharger, now manufacturers are fitting them to even small engine, base models.

But what is a turbo? A turbo is an assembly that is bolted to the exterior of your engine. It uses exhaust pressure to spin a compressor wheel, which forces fresh air into the engine. The forced air increases the power output of the engine by about 25%, (depending on turbo size).

So why do you care? Turbos are kinder to the environment, they make any engine more efficient and that’s why they’re making a comeback. Manufacturers are under ever-increasing pressure to produce more efficient engines and while the world seems to be moving towards electric cars, we’re not quite there yet.

How Does A Turbo Work?

I understand that you may not want a complete lesson on how a turbo works, so I’ll make this explanation broad, short, and in plain English.

Turbo charger

More fuel/air mix equals more power, simple right? A turbo causes your engine to be more powerful, it does this as you know by forcing air into the engine.

Turbochargers have gotten more sophisticated in recent years, however, the basic turbo is still the most common type fitted today. Some cars and trucks will have two-stage turbo’s and the latest technology is variable geometry turbos.

A turbocharger is like an air pump that’s powered by wasted engine exhaust gases.

Turbo charger and manifold

Although the turbo looks complicated it’s not really. Think about it in two halves, the turbine side (Exhaust) generates the power to drive the compressor side (Intake). It’s like an air pump, stuffing air into the engine.

It’s a very clever idea. Exhaust gasses that otherwise would go out the tailpipe are redirected at a turbine wheel inside the turbo. As the exhaust gases are moving quickly and the turbine wheel is small, it causes it to spin, really quickly, in excess (150,000 rpm).

Attached to the other side of the turbine wheel axle is the compressor wheel, it spins at whatever speed the turbine spins. The compressor wheel sits inside the compressor housing known as the snail or front cover.

Air from the air filter is vacuumed through the inlet of the housing by the compressor wheel and is then forced through the outlet of the housing to the engine, via an inter-cooler. The problem with compressed air (known as charged air), it’s hot, and hot air carries less oxygen.

An engine wants as much oxygen as possible. So the air is fed through the intercooler to cool it, before reaching its final destination, the engine.

The inter-cooler sits at the front of your car and resembles a car radiator, but it’s an air to air cooler. That simply means air is being used to cool air. That’s a very broad outline of how a turbo works, there are however other components and sensors needed to maintain the turbo and control the charged air.

I won’t cover them here since I’ll be covering them below in the likely causes of no boost.

Likely Causes Of No Boost Pressure

Scan tool

This section will cover the most common causes of no boost pressure but it won’t cover every possibility.

Modern cars as you know are run by computers and the great advantage to that is the computer’s ability to self-diagnose. A technician will always start by reading and noting all the codes present, if you can, have the codes read before removing any sensors as this will lead to spurious fault codes.

All cars built in the last twenty years will be OBD 2 compliant, which means a simple handheld device known as a code reader can be plugged into the vehicle and any fault codes read.

Most of the fault codes are standardized and a quick google search of your code(s) will give you the heads up. When scanning for codes, you’ll likely have live codes and stored codes, it’s not unusual for a car to have lots of codes stored, live codes carry the most weight when diagnosing.


A turbo will cause detonation (pre igniting) in an engine, and that’s not a problem in stock form. Knock sensors are constantly listening and the ECM is constantly adjusting the timing.

Knock sensor

But if your car’s turbo has been modified (Bigger) and the ECM hasn’t been reprogrammed, it’s possible the turbo is causing too much detonation and the engine timing is being backed off.

Similarly, if your ECM has been reprogrammed and the injector flow rate or timing doesn’t match the turbo specs, the higher temperatures may cause the turbo to fail prematurely.

A cracked header or blocked catalytic converter will also cause low or no boost pressure, it’s not very common but worth checking.

Blocked Air Intake

Air filter

As you know the compressor side of the turbo sucks air through the ducting and the air filter. Any restriction in the airbox, filter, or ducting will obviously block airflow to the engine. Lack of air equals a lack of boost. I found several mouse nests and I’ve found cleaning rags in the airbox creating a low boost issue, it does happen.

Things you can check:

  • Check compressor intake pipes for blockages and kinks
  • Check condition of the air filter

Sensor Fault

Boost pressure sensor

Your car, as you’re well aware is run by the ECM (Engine Control Module). It relies on various sensors for information and makes fuelling decisions based on those sensor readings.

Sensors are pretty durable but can give bad readings when they’re failing. Bad wiring is a common problem, sensors and ECM’s are very sensitive to resistance since that’s how they communicate.

All ECM’s are pre-programmed with a default band. Meaning, if sensors feed the ECM bad information, it may not carry out a particular function, such as activate the turbocharger. Reading fault codes should point you in the direction of your fault, but all too often it doesn’t.

The main sensors that affect turbo activation, include:

  • Boost pressure
  • MAP (Manifold Absolute Pressure)
  • MAF (Mass Air Flow)
  • IAT (Intake Air Temperature)
  • ECT (Engine Coolant Temperature)
  • O2 (Oxygen Sensor)
  • Oil temperature (Some models)
  • Oil pressure (Some models)

Charged Air Pipe Fault

Chargerd air pipe

The air that leaves the outlet of the turbocharger compressor housing is pressured air (charged air). It travels to an inter-cooler and onto the engine intake manifold through a collection of metal, plastic, and rubber hoses.

Because the air is pressured, it’s not uncommon for the pipes to comes loose at their connection points. This will of course allow the charged air to escape and not reach the engine and therefore not create any boost.

Most turbocharged cars will have a sensor that picks up on a low boost condition, however, you may need to scan the onboard computer to check.

Turbo boost pipe

Most turbo-charged cars are fitted with an intercooler. They’ve been known to crack and as they’re usually fitted low down at the front of the car often meet with accidents.

Refitting or replacing charge hoses and pipes is pretty simple to repair. It’s also common for hoses, to kink and simply cracks. Symptoms of a loose pipe or cracked inter-cooler include a lack of power, a whoosh sound, and possible engine light on.

Things you can check:

  • Check all hoses are secure at their connector points
  • Check for split pipes or kinks in the pipework
  • Check the inter-cooler for damage

Turbo Waste-gate

A challenge for turbochargers is what to do with unwanted exhaust pressure. Take for example driving uphill with the gas pedal down, the exhaust side turbine wheel is working really hard and building lots of pressure. Now imagine you take your foot off the gas. (Overrun)

Turbo waste gate

All that pressure is now backed up in the turbine housing, putting a huge strain on the turbine wheel.

The solution is the waste-gate. It’s a valve (Door) inside the turbine housing that opens and releases the unwanted exhaust pressure and also diverts incoming exhausts gasses away from the turbine. The valve closes as you hit the gas pedal again and now the gases are once again directed at the turbine wheel.

The wastegate valve is a very simple mechanical door, however, they do give trouble. Waste-gates can crack, stick, seize closed or open, or can be just stiff to move. Carbon trapped in the valve seat can also cause it to stick open.

An open or cracked wastegate won’t allow the turbo to spool and so you won’t have boost.

Things you can check:

  • Check the waste-gate is free to move by hand. You’ll need to wait until the exhaust is cool.
  • Have a helper rev the engine while you watch the waste-gate lever open and close. A working waste-gate, actuator and solenoid (more on these later) will allow the Waste-gate to open and close when the engine is revved.

Waste-Gate Actuator Fault


A waste-gate is useless without being able to control it. The waste-gate actuator is bolted to the body of the turbo and is tasked with opening and closing the gate. The actuator is a simple piece of kit, it’s a sealed container (Can) that contains a spring and a diaphragm.

Actuator lever

At the bottom of the can is a lever arm that connects to the wastegate, and at the top, is a hose pipe that connects to a solenoid (more on the solenoid later).

When pressure is applied to the actuator, it moves the actuator lever arm which moves the waste-gate open. When pressure is removed, the spring inside the can forces the wastegate closed. May also have a vacuum supply hose to speed up closing the wastegate.

Motorised turbo waste gate actuator

Note, some modern turbos may have a motorized actuator. If you’ve got wiring on the end of the actuator instead of vacuum hoses, yours is motorized…. aren’t you lucky!

If the actuator or solenoid fails, the spring inside the wastegate actuator (can) is designed to be overcome by the sheer force of exhaust pressure inside the turbine housing. This allows the waste gate to open, relieves pressure, and protects the turbo from over-boost.

Different size springs inside the can will change boost characteristics.

Things you can check:

  • Have a helper rev the engine while you watch the actuator. A working waste-gate, actuator and solenoid will allow movement at the actuator lever arm. As per the waste-gate test.
  • Move the actuator arm, over and back by hand. There’ll be resistance, but it should move.
  • Check pipes are connected and free from splits, blockages or kinks
  • Apply pressure to the actuator and check for movement.

Waste-Gate Solenoid Fault

Turbo waste gate solenoid

A waste-gate solenoid is an electric valve that regulates vacuum and air pressure to the wastegate actuator. The actuator as you know operates the waste-gate lever arm and applying air pressure moves the arm.

If your turbo is modern and you can’t find any vacuum hoses on the actuator, then it’s likely it’s motorized and so you won’t have a wastegate solenoid.

It’s the solenoid’s job to route air to the actuator, and when the waste-gate needs to close it can supply a vacuum too. The solenoid itself isn’t clever enough to do this, instead, the ECM (Engine Control Module) controls its function.

You can find the solenoid, by following the vacuum pipes from the turbocharger actuator.

Things you can check:

  • Check all pipework is connected and without cracks, kinks or blockages.
  • Check that the solenoid pipe connector is free from debris.
  • Check the electrical connectors are secure and without damage.
  • Using a voltmeter, have a helper rev the engine while you use the volt meter to check voltage across the connector.
  • Use the volt meter to run a resistance across the solenoid.

Blow-Off Valve Fault

Turbo blow off valve

The blow-off valve is positioned on the compressor housing side of the turbo. Visually, it looks like an actuator without the lever arm.

Some modern turbos, may not have a blow-off valve fitted. It’s basically the same as a waste-gate but fitted on the compressor side of the turbo. Its function is to protect the compressor wheel from over-boosting.

Blow off valve

It has a hosepipe fitted on top of the valve, which is connected to the engine manifold vacuum. Inside is a rubber diaphragm and when maximum engine vacuum is applied (idling engine) the valve opens and releases compressor side pressure.

Things you can check:

Apply vacuum to the blow-off valve and check operation.
Remove and check if the diaphragm is damaged or stuck open.

Turbo Failure

Turbo charger and manifold

Turbo failure is common in older cars, no surprise there! Turbos are under tremendous stress, they fail because of:

  • Poor oil quality or low oil level
  • Ingesting foreign object
  • Excessive heat build-up

The center housing of a turbo is called the Cartridge. It houses the wheel shaft and bearings, it’s fitted with an oil and coolant supply to help lubricate and cool the bearings.

When a turbo shuts down, the temperatures actually rise for while, that’s when poor quality oil gets fried and cokes the bearings inside the Cartridge, this can also lead to oil blockages.

Blockages in oil supply lead to bearings seizing or ovalizing the bearings which cause the compressor wheel to impact the housing and disintegrate.

Turbo wheel

Regular oil changes and maintaining coolant and oil levels will prolong the life of a turbo.

Common turbo failures include:

  • Shaft bearings seized
  • Damaged cracked or broken turbine/compressor wheel
  • Off balance shaft
  • Wheel to housing contact damage
  • Excessive shaft axial or end play
  • Seal failure causing oil consumption (oil inside the the compressor or turbine housing)
  • Oil level check

As you know, the main cause of turbo failure is poor oil quality or no oil. But low engine oil level can be caused by the turbo too. It is common for turbo seals to leak and allow the compressor to suck up the engine oil and send it into the engine. The result is a blown turbo and engine.

Before replacing a turbo, it’s important to know why the old turbo failed. If for example, your turbo seized, fitting a new turbo won’t solve the problem if the original problem was a blocked turbo oil feed.

It’s also worth checking all pipes, exhaust, and intercooler for debris.

Things you can check:

  • Remove intake pipe from compressor housing and check for free rotational movement of the compressor wheel.
  • Check also for damaged or missing compressor wheel blades.
  • With the intake pipe removed, check for excessive compressor wheel end play (in & out).
  • With the intake pipe removed, check for excessive shaft axial play (up & down), which can cause compressor housing, compressor wheel contact.
  • Monitor oil consumption, and with the intake pipe removed, check for excessive oil in compressor housing.

Replacing a turbo on some rear-drive cars is a simple enough job, but on transverse front-drive cars, it can be a bit of a challenge and will likely require a shoplift. Check out this post where I outline the process of swapping out a turbocharger.

If you need a new turbocharger check the prices on the Amazon link below.

Amazon Turbochargers

Related Questions

What happens if the turbo’s not working? The car/truck will feel sluggish and in some cases, it may not even start. Fuel consumption will be high and the vehicle may go into limp mode. A check engine light should be illuminated in the dashboard.