Fluctuating Boost and EGT Fix for Vacuum Actuated VNT/VGT (Variable Geometry Turbo)

Categories: Chip Tuning

This is to help solve the boost drop off seen in the factory setup. This will also remedy any power surges & EGT spikes when vehicle is fitted with a Performance Diesel Chip that is a result of the Diesel Chip reading a boost increase/decrease and therefore varying the fuel volume.  I found in my own Mitsubishi NS Pajero that whilst at cruise on the freeway the boost would fluctuate by 3-7 psi.  When the boost decreased the EGTs increased to around 600deg Cel.  This was traced to the factory vacuum solenoid or the factory emissions control (not sure which one) bringing down boost to increase the exhaust temps.  This is NOT desirable for engine longevity.  Fitting a Dawes Valve ($69 see online store) will smooth out this issue.  In the mean time I removed the factory VNT turbo and fitted out aftermarket stage 3 turbo now and this has the more reliable mechanically operated wastegate.
No two vehicles are exactly the same and circumstances change for each vehicles however this is what I have found not only in CRD Nissan Patrols but now also in other late model Variable Vane Turbo vehicles that have the vacuum solenoids operating the turbo vanes. So to cover all bases regarding i recommend the following.
You will piggy back (or in some cases disconnect) the factory boost controller (vacuum solenoid) and install the Chip Tuning Dawes Valve.  This “assisting” of the factory boost control allows a linear boost curve.
If your ZD30 is an early pre 2005 DI then you should bypass the ECU controlled Vac solenoid, if it’s a much later model (common rail diesel post 2006) we still recommend the procedure to settle the issue once and for all.
Early ZD30s have their turbo boost drop off over 3000 RPMs in 5th gear which makes the EGTs spike badly.  Thus attracting the commonly used term for these engines “hand grenades”.
The details below illustrates how we fit the Dawes boost controller on vehicles with Electronically Controlled Turbo or VVT such as Nissan Patrols CRD, Mitsubishi MN Triton or NS (or later)  Pajero etc.
The Dawes Valve setup works by simply feeding positive boost into the vacuum side of the VGT vacuum actuator to achieve its set tension.
The Dawes Valve System works in a similar way to a Ball Valve, but without the vent hole to release pressure for the waste-gate.   Manual boost control is achieved by bypassing the ECU controlled Solenoid Valve to have total control via a Needle Valve from inside the vehicle (if so desired).   This can be fitted to all cars with Vacuum Controlled Nozzle Actuators
The Needle Valve controls the rate of turbo spool and the Dawes Valve controls maximum boost (to a moderate 18psi at full throttle and 4000rpm or as desired)  This system eliminates boost spikes to over 25psi as well as boost fluctuations for EGR, which helps to maintain lower EGT’s by having constant boost levels at lower pressures. It creates a linear boost curve that is far more predictable than the factory curve.
Result:  The boost becomes more linear with the Chip Tuning Dawes Valve fitted and the Vac switch by-passed.  The bleed valve gives you the ability to change the boost settling.  Max safe boost level for these vehicles would be 18PSI.
Vehicle Tuning
Vehicle Tuning

See links below for power and turbo performance both with and without a Dawes Valve fitted to an NS Pajero
ns pajero boost issues1 (boost issue seen here)
ns pajero boost issues2 (boost issue seen here under light and full throttle)
ns pajero boost issues3 (boost issue corrected)
The third link above shows the benefit of fitting a Chip Tuning Dawes Valve while retaining the VNT control solenoid and use the Dawes as an adjustable limiter to reduce boost mis management. This option works well if retaining the EGR.  It still allows for boost to fall away under ECU control.  (See schematic below for fitment guide)


Dawes Valve

Single Stage Dawes Manual Control System (Maintaining the factory solenoid)

Boost Control

Single Stage Dawes Manual Control System (Stand alone)

A two stage boost controller can be used which utilises 2x Dawes Valves and an additional Electric Solenoid Valve to achieve two selectable boost levels. We set one valve to 10psi for economy and engine preservation and the other to 15psi, which helps to control EGT’s and give additional power when it’s needed.  High boost is activated via the TPS and a voltage switch.
Boost Control

Dual Stage TDi Dawes Manual Control System

Boost Electronic Controls
Vehicle Tuning

Fitting a Dawes Valve is relatively easy, however a boost gauge and intake manifold pressure supply is needed to set up, monitor and control boost. A pressure supply from the intake ducting (post turbo) is routed to the Dawes Valve and used to lift the ball off of its seat and release a small amount of pressure to cancel out or reduce the vacuum signal to the turbo actuator which in standard trim is used to reduce boost.  To do this, the top of the valve is plumbed to the turbo actuator and the vacuum supply pump.
A needle valve is necessary to reduce the rate of spool up if the ECU control solenoid is bypassed. It can be fitted as a “set and forget” option or in cab to make adjustments on the go. In some cases it may be omitted if the VNT actuator stop limiting screw is adjusted accordingly on the turbocharger.
A clean air supply is needed to reduce the vacuum signal and is taken from the Air Filter Resonator, which is the same source as the ECU Solenoid Valve uses from “Port C”.  See image below.
Dawes Valve
Setting up will depend on the system being used, but some basic principles should apply. For full manual control, the Dawes Valve should be set to around 10psi at 2000rpm and the Needle Valve adjusted to give no more than 6psi at 1500rpm. These are safe conservative limits. As RPM and load increases, the Turbocharger will overcome the Dawes Valve and boost levels will rise beyond the initial setting.  Ideally, boost should peak to 18psi at 4000rpm and an ideal setting of 16psi at 3600rpm should be achievable.
To set up or adjust a manual controller, close the needle valve and start the car (warm). At idle, begin to open the needle valve until the turbo actuator arm drops away from its stop screw position.  Then just close it slightly until you see the arm lift and touch the stop screw.  Lock it there.  Then simply adjust your Dawes Valve to the amount of boost that you wish to run on.  Suggested is around 15psi at about 1/2 throttle with a slight load.   You should see boost climb higher with more revs and load, and provided it doesn’t exceed 15psi below 3000rpm and 18psi at 4000rpm, it should be good.  Any other minor adjustments can be done with the limiting screw or the needle valve, but shouldn’t make much difference to the setup apart from altering the spool rate slightly.
We also play with Dual Dawes.  With a dual boost Dawes system, a boost solenoid (just a simple on off air valve) simply cuts the positive boost pressure from a low pressure DV set at (wx) so a high pressure DV activates at its set pressure of (xy.)  Solenoid OPEN, means VGT boost checks at (wx) and solenoid CLOSED means boost checks at (xy).
(wx) may be say 18psi, (xy) may be say 25psi.
Depending on the customers set up, ( in some cases dual chips to run separate systems like petrol injection ) one may either use a normal 12v switch, use a dual map switch with a chip or use a channel on our standard DPS chip via a relay.   Obviously we need a voltage clamp on the MAP sensor beyond 22psi otherwise you get a CEL here because of excess flow.   Chip Tuning has these in stock.
The in-cabin bleed is a needle valve that controls turbo ramp up with the vacuum solenoid bypassed (or just removed running the Dawes Valve).  It works by balancing the available vacuum at given RPMs against the actuator arm.  A very popular and good quality German made Flutec DV-06 oil valve can be used. There are other needle valves on the market you can use.  These valves need to be very accurate so a fish tank valve won’t cut it.)  Chip Tuning has good quality Italian Needle Valves in stock.

Following pics are the Setup for Fitment on a MN Triton

Dawes Install
Dawes Valve

Italian Needle Valves

Chip Tuning Needle Valve

We also sell Fuel Cut Defenders (Voltage Clamps)

Boost Cut Defender

For more info on the FCD see here:  Fuel Cut Defender

Special thanks to Chaz and Tony James