Hi everyone, I have taken it upon myself to start a new topic for this as will hopefully help people in the future should this issue arise.
This problem is rare, but does occur with a small percentage of electronic solenoid over vacuum variable geometry turbo systems such as the 2.5HP MN Triton.
I have personally come across a few people (including some dealers) that are at a loss with this over boost condition that some of these Variable Geometry Turbo equipped vehicles can experience so want to bring the use of a pneumatic control valve to light as many of the Nissan ZD30 hand grenade engine owners have in place now. They tend to blow up at a sniff of boost over 18psi as in detonate.
Also many other reasons, such as EGR blanking which raises the EGT's with the vehicles ECU software as it now cant draw the air though the EGR valve and EGR cooler. The air fuel ratio drops to dangerous levels. This happens with the MN's too, only the engine is tough enough to tolerate it for a long time.
Back on topic, the overboost issue often crops up with a mix of stock vehicles, big bore exhaust systems, EGR blanking and ramping up the fuel by chip or tune with no map sensor clamping. On that topic, I have found it is not possible to totally clamp the MAP sensor voltage below 22psi on the MN's as this may cause the turbo to remain in attack and run away before the ECU can catch it.
The symptoms are bought on by a very long and hard higher RPM run (over eight seconds) usually towing or when overtaking then the vehicle will go into limp mode showing a CEL. Most scan tools wont read or clear this particular fault. Code is MN triton specific P1298 (VGT system over pressure.)
What we are seeing is a large boost spike as the turbo spools up much quicker than the ECU can control the actuator via the electronic over vacuum solenoid. The said solenoids main purpose is to control ramp up of the turbo and assist with new strict emission laws by dropping the pressure whilst the EGR is active and TB is partially closed etc..
This modification I'm showing in this particular thread has no effect on the way the electronic solenoid works or emission control
except simply pushes positive boost into the vacuum side of the VGT actuator should it ever over boost hence brings the boost back to target till the electronic over vacuum solenoid catches up.
The said modification is fitting a "Dawes" type valve to the vacuum line between the electronic over vacuum solenoid (after the damper) and the VGT actuator and feeding positive boost pressure to the valve from a tee in the MAP sensor line. (Closer to the inlet manifold the better)
The valve is specific for this application as has a tee on the bleed side and single barb on the boost side, it works with a ball and spring as some other pneumatic boost controllers only has no air bleed holes
in the valve body on the turbo actuator side. They wont work in this application as will allow the vacuum to escape. This is a closed system, not vented. (we do not use a bleed when backing up the electronic solenoid.)
I hope this diagram makes sense.
What you can see is, it runs stock until the valve bleeds pressure in to prevent boost spikes or turbo runaway at the users setting. This mod works on stock, chipped and tuned vehicles.
These valves will soon be available though Chipit NTN amongst other items on special request.
I will add, this valve can be used with out the Electronic over vacuum solenoid with many advantages if you wish, but requires a damper and bleed valve to work nicely. There is a bit in the platinum area on this.
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