When you start the engine The engines tandem pump produces the vacuum (sucks).

This vacuum (suction) is used to control four systems.

1. Brake servo

2. EGR valve N18

3. Intake manifold flap valve N239

4. MAP regulator valve N75

Vacuum passes through a one way valve to the brake servo to give assisted brakes.

Vacuum passes to the EGR valve The EGR valve controls the EGR actuator which moves a flap between the exhaust and intake manifolds and holds it open. The EGR valve can bleed vacuum to the air filter which lets it shut. An electrical impulse from the ECU also controls the bleed off and therefore the opening and closing.

Vacuum passes to the Intake manifold flap valve. Vacuum in the intake manifold flap valve regulator, acts on an actuator to pull the intake manifold flap shut for 3 seconds when the engine is switched off. Vacuum is then lost and the flap valve springs open again.

Vacuum passes to the Map regulator N75. This allows vacuum to pass to the turbo actuator and pulls the actuator to actuate the turbo. (In the old days, more vacuum meant more turbo, this is not the case with variable vane turbo’s.)

The Map regulator N75 can bleed vacuum to the air filter. The Map regulator is electronically operated by the Map sensor via the ECU. The Map sensor detects excess boost pressure in the intake. An excessive boost pressure signal causes the ECU to send a signal to the Map N75 valve which opens and bleeds vacuum to the air filter. This reduced vacuum reduces the pull on the turbo actuator and cuts turbo pressure.

The vacuum system contains THREE one way valves to help prevent failure of all four systems if a vacuum pipe leaks.


Tandem pump failure. All vacuum lost, brake assist lost. Engine will run rough or may not start at all.

Vacuum lost to EGR valve. EGR valve will stay closed. Engine will start and run fine. EGR valves can become stuck open. Engine will run rough, produce excessive smoke. Valve can be cleaned and unstuck. As a temporary measure valve can be pushed shut, vacuum pipe disconnected and blocked.

Vacuum lost to Intake manifold flap valve. This may not show, or it will show as a shuddering engine on switch off.

Vacuum lost to MAP regulator valve N75. This will mean no vacuum to turbo actuator and so turbo will not work. Engine will start and run fine but have no power to accelerate or climb hills.

The vacuum system contains a sensor which measures air temperature and pressure. This is the MAP sensor. This sensor feeds signals to the engines ECU. Using these signals the ECU can determine three things.

  1. Manifold pressure within specification

  2. Manifold pressure too high

  3. Manifold pressure too low.

If the ECU receives a signal for too high manifold pressure the MAP N75 valve is opened and vacuum bleeds to the air filter. This loss of vacuum cuts vacuum to the turbo actuator and the turbo stops working. The engine will loose power suddenly and refuse to rev, climb hills etc. This is called LIMP mode. The ECU should now store a fault code but not necessarily show a fault light. Switching ignition off and on should reset the limp mode (not the code) and the engine should run normally. As soon as the manifold pressure goes too high, it will drop back into limp mode.

So the vacuum system is made much more complicated because of the signals sent to the ECU.

If the engine drops into limp mode you have to get a VCDS or Dealer code read. (Avoid cheap code readers as these often give daft results)

It is even better to get a VCDS scan of engine measuring block 011 whilst the engine is accelerated from about 1000 rpm to 4000 rpm and then using excel to plot a turbo spec and turbo actual graph against rpm. (How to do this is explained in another section)

Code 17964. Boost pressure too low.

Vacuum pipe damaged

Map N75 valve failed, bleeding vacuum to air filter

Turbo actuator failed, not pulling on turbo

Map sensor failed, reading too low.

Intercooler pipe split/joint leaking.

Code 17965. Boost pressure too high.


Vacuum pipes wrongly connected

Map N75 valve failed, cannot bleed vacuum to air filter

Map sensor failed, reading too high

Turbo vanes stuck in max position

In theory both codes can be caused by a faulty turbo (sticking vanes). In practice the vanes stick in the max pressure position so cause boost pressure too high, not too low. As a turbo fault is the most expensive option, it’s best to check everything else first.

What to do with Code 17964.

Start the car and feel round the big air pipes from the turbo to the intercooler and back to the inlet manifold. You should feel air blowing out of a split pipe or bad connector especially if the engine is speeded up.

Ask someone to start the car while you watch the turbo actuator.

It should move 15 – 20mm as vacuum acts on it and stay moved.

If it doesn’t move the actuator is faulty or you have no vacuum in the pipe leading to the actuator

Pull the pipe off the actuator, attach a spare bit of pipe and suck hard.

The actuator should move 10 – 15mm and stay moved as long as you suck.

If the actuator doesn’t move it is faulty.

If the actuator moves and stays moved the fault is in the vacuum pipes or MAP valve (N75).

Testing the vacuum pipes.

Pull the turbo actuator pipe off the actuator and blow into the pipe.(You can use a bicycle or football pump to apply pressure but DO NOT pump it up.)

It should not be possible to blow/pump into the pipe.

If you can blow into the pipe there is a vacuum leak further down the system.

If you cannot blow the vacuum pipes are ok.

(You must be very sure of this as a slight leak will cause intermittent turbo loss.)

It is often easier to replace all the vacuum pipes one at a time than it is to find a leak.

If you have replaced all of the vacuum pipes and can still blow into the actuator pipe you probably have a faulty MAP valve (N75).

If the N75 valve is jammed open, too much vacuum will be lost to the air filter and stop the turbo actuator working.


To be sure, try blowing directly into the N75 valve using a spare piece of hose whilst blocking the IN pipe to the valve. (A pump helps but don’t pump it up.)

Any leak means the valve is probably faulty.

Another simple test is, engine running, pull the vacuum pipe off the N75 valve that leads to the turbo actuator. The actuator should immediately drop back.

Replace the pipe, watch the actuator pull across and then pull the pipe off that runs into the N75 valve the turbo actuator should drop back again.

If any of these tests suggest that the N75 valve is leaking you need to replace it.

The MAP (N75) valves are quite robust and don't often fail.

No vacuum leaks found at all. This suggests the MAP sensor is faulty. The simplest way to check this is to replace it.

If the MAP sensor is ok and the vacuum system is air tight and the turbo actuator is moving and staying moved then the fault is probably the turbo.

It is possible for the whole system to be working ok and the turbo actuator works BUT it is not pulling the turbo vanes open. This means that the turbo runs fine but generates less pressure than it should. (This is however unlikely because the vanes usually jam open.)


Clean the turbo with turbo cleaner (This rarely works, long term)

Strip the turbo and clean properly

Replace the turbo with new or reconditioned unit

What to do with Code 17965

Ask someone to start car while you watch the turbo actuator.

It will move 15 – 20mm and stay moved.

It must do this for you to register “pressure too high”

If it doesn’t move or stay moved the system is lying to you and the MAP sensor is probably faulty so replace that first.

It is just about possible to have over boost and a vacuum leak but this is very unlikely so you should be able to ignore the vacuum pipes.

With the engine and ignition off the N75 vacuum valve is SHUT. This means that no vacuum will go to the turbo actuator. So if you pull the turbo actuator pipe off and blow into it (or suck on it), nothing will happen because you are connected directly to the pipe going to the air filter, which is open to "air".

As soon as the engine is started you should feel "suction" if you put a finger over the end of the pipe that goes to the turbo actuator.

Always make sure the bleed pipe to the air filter is clear by pulling it off the N75 valve and blowing into it. It must NOT be blocked.

A failed N75 valve should not cause over boost. It is only open to vacuum and pulling on the turbo actuator when it is energised by the signal from the ECU.

If the valve fails or the ECU signal fails it will stop vacuum to the turbo actuator and "bleed vacuum" to the air filter. This stops boost and is a fail safe.

If it should stick in the open "vacuum" position it will cause overboost. (I have never heard of this happening).

N75 valves are reliable and don't often fail.

If the MAP sensor and the N75 valve are not faulty or have been replaced then the fault is probably the turbo.

The turbo vanes have been pulled open by the actuator and are sticking open causing over boost.

This isn’t caused by a faulty actuator. It is usually caused by the actuator linkage being jammed.


Clean the turbo with turbo cleaner. (This rarely works long term)

Strip the turbo and clean properly

Replace the turbo with new or reconditioned unit

A faulty MAF (AMM) should not give either under boost or over boost codes.

If an engine goes into limp but recovers when you switch off and on. It cannot be a MAF fault.

A faulty EGR valve should not give either under boost or over boost codes BUT

A vacuum leak in any vacuum pipe can cause problems especially if a one way valve is not working.

To ensure that the EGR valve is not adding to your problems:

Disconnect the big air pipe so that you can press the internal valve shut.

Pull the vacuum pipe off the EGR valve and plug the vacuum pipe (not the pipe on the EGR) with a bolt or similar. (It must be air tight). The EGR valve will now stay shut. The engine will run fine like this but it might flag a fault code.