Wastegates

A wastegate is a valve that lets some of the exhaust go round a turbo’s turbine instead of through it. How a turbocharger works showed why every turbo petrol engine needs one: a turbo big enough to make good boost at low revs would make far too much at high revs. This lesson looks at how the wastegate is worked, and how the ECU uses it to set the boost.

There are three parts to it:

• The gate itself: usually a small flap inside the turbine housing that covers a hole, or port, leading round the turbine.
• The actuator: a canister bolted beside the turbo, with a rod to the gate. Inside, a rubber diaphragm is held back by a spring.
• The control: on older and simpler cars, a hose straight from the compressor; on most modern ones, a solenoid valve or an electric motor run by the ECU.

The demo shows a turbo with the throttle floored. Exhaust comes in on the right and goes either down through the turbine, or through the wastegate and round it. A hose takes boost from the compressor side up to the actuator. The slider is engine speed, and the buttons change how the wastegate is controlled.

It starts on Spring only, at 4,000 rpm. The boost in the pipes pushes on the actuator’s diaphragm, and the spring pushes back. This spring is set to 0.6 bar (8.7 psi), so:

1. Below about 2,200 rpm the boost is lower than that, the spring wins, and the gate stays shut. All the exhaust drives the turbine, so the turbo builds boost as fast as it can.
2. At about 2,200 rpm boost reaches 0.6 bar (8.7 psi), beats the spring, and the gate cracks open.
3. Higher up the gate opens further to let the extra exhaust round. Boost doesn’t stay quite flat: it creeps up to about 0.8 bar (12 psi) by 6,000 rpm, because a spring pushes back harder the more it’s squashed.

Now press ECU control. A solenoid valve sits in the hose, and the ECU pulses it open to bleed some of the actuator’s air back into the intake. The actuator now feels less than the real boost, so the gate stays shut for longer, and boost climbs to the 1 bar (15 psi) the ECU wants. From about 2,700 rpm it holds there, with the actuator feeling only 0.6 bar (8.7 psi) to 0.8 bar (12 psi). A boost sensor tells the ECU how close it is, and it adjusts the bleed many times a second.

Finally, press Stuck shut and slide up the revs. Below 2,700 rpm nothing seems wrong. Above it, boost runs on past 1 bar (15 psi), and by about 3,000 rpm it would pass the ECU’s limit, so the ECU cuts the power.

The spring sets the lowest boost the turbo will run, often called base boost or wastegate pressure. If the solenoid, its wiring or the ECU fails, the actuator feels the full boost again, the gate opens at the spring’s setting, and the car still drives, just with less power. That makes the spring a safe fallback.

The ECU can only raise boost above the spring’s setting, never lower it, because all the solenoid can do is take air away from the actuator. So engineers choose a spring a little below the boost they want, and let the ECU make up the rest.

There’s one more force on the gate: the exhaust pressure on the flap itself, trying to push it open. At high boost that pressure can beat a standard spring and force the gate open too early, which is why tuners sometimes fit a stiffer actuator.

Many modern turbos, such as those on the 2.0-litre TSI engines in the Mk7 VW Golf GTI and many other VW Group cars, replace the canister with a small electric motor. A sensor tells the ECU exactly where the gate is, so it can set it to any position directly, without waiting for boost to build.

That lets the ECU do something a spring can’t: hold the wastegate open when you’re cruising. With the gate open, the exhaust escapes more easily and the engine wastes less effort pushing it out, so it uses a little less fuel. The moment you press the accelerator, the ECU snaps the gate shut so the turbo spools up as fast as it can.

Diesels mostly don’t have a wastegate at all. Their variable geometry turbos control boost with the ring of vanes round the turbine instead, usually worked by a vacuum canister or an electric motor.

Almost every road car has an internal wastegate: the flap and port are cast into the turbine housing. It’s compact, cheap and quiet, because the gas it lets round goes straight into the same exhaust.

An external wastegate is a separate valve, bolted to the exhaust manifold before the turbo, with its own diaphragm and spring in one unit. It can have a much bigger port, and its shape lets the gas through more smoothly, so it controls boost better at high power. The gas it lets out goes either back into the exhaust further down, or out of its own short pipe, a screamer pipe, which is very loud. External gates are common on race cars and heavily tuned road cars.

Two things can stop the wastegate holding boost where it’s set.

• Boost creep happens when even a fully open wastegate can’t let enough exhaust round. An internal gate’s port is only so big; at high revs on a powerful engine, more gas can arrive than it can carry, and boost keeps climbing. It’s most common after fitting a freer exhaust or a bigger downpipe, because the gas can then rush through the turbine more easily too. The fixes are a bigger port, a turbine housing with a bigger gate, or an external wastegate.
• A boost spike is a short overshoot when you floor it. The turbo spools up so fast that boost passes its target before the gate has opened far enough, then settles back. The ECU learns to start opening the gate a little early to stop it.

• A split or loose hose. On a wastegate pushed open by boost, a hose that splits or slips off means no boost reaches the actuator, so the gate never opens. Boost shoots up, the ECU logs fault code P0234 (overboost) and cuts the power. A split hose elsewhere in the solenoid’s plumbing can do the opposite, letting the gate open too early, so boost is low.
• A split diaphragm. Old rubber in the actuator cracks, the boost leaks out of it and the gate can’t open: overboost again.
• A sticking or faulty solenoid. Stuck shut, it can’t bleed the actuator, so boost stays at the spring’s setting and the car feels flat. Stuck open, or with a broken wire, it can make boost erratic. Faults are logged as P0243 and similar codes.
• A rattling gate. The pivot and linkage wear, and the flap rattles in its seat, a metallic buzz most noticeable when cold, idling or lifting off. It’s well known on electric wastegates on many VW Group engines, and a revised actuator or turbo is the usual fix.
• A seized gate. Rust and carbon can stick the flap. Stuck shut, the car overboosts (P0234); stuck open, it never makes full boost (P0299, underboost). A wastegate that hardly moves, on a car used for short trips, is the most likely to seize.

You can check a pressure-operated wastegate with the engine off: the rod should move smoothly when you push it (wear gloves; it gets very hot), and a hand pump on the actuator should open it at about the spring’s setting and hold pressure.

Boost is the easiest thing to change on a turbo car, and the wastegate is how it’s done.

• A remap raises the boost target in the ECU, and the ECU drives the solenoid or electric wastegate to reach it. This is the best way, because the fuel and spark are changed to match.
• A manual boost controller is a small adjustable valve in the actuator’s hose, often a ball held against a seat by a spring. It keeps boost away from the actuator until a set pressure, so the gate opens later and boost runs higher. It’s cheap, but the ECU doesn’t know about it, so on a modern car it often logs overboost or knocks the timing back.
• An electronic boost controller is an aftermarket solenoid and control box that does the same thing more precisely, with settings you can choose from the driver’s seat.
• A stiffer or adjustable actuator raises the spring’s setting, so the gate holds shut against more exhaust pressure. Shortening the rod to preload a standard spring does a similar job, but too much stops the gate shutting properly at all.
• A bigger wastegate port, or an external wastegate, cures boost creep on a heavily tuned car.

Every one of these raises boost, and more boost means more heat, more load on the engine and clutch, and more risk of knock. Change the fuel and spark to match, or the engine will pay for it.