3 litre Diesel EGR valve block off

[YNOT]

Having read lots on the forums about all these topics, one question that I keep coming back to is this. In the effort to control overboost after bocking EGR and fitting a 2.5' mandrel exhaust should/can I:
1) Fit Dawes valve
2) Fit Dawes valve plus needle valve
3) Fit needle valve only
4) Adjust VNT screw
5) All of the above?

My GU is a 2008 CRD (with gas) and despite many folks thinking that these are less prone to erratic boosting mine is now regularly hitting 20. This isn't enough to put up a code but I believe it is too much? How much is too much? How worried should I be?

I gather I should be aiming for <15psi consistently?

Also my EGT is tipping 700 at times on the open road. Hopefully the drop in boost I seek will also help this?

Thanks knowledgable ones!

Harry

The first thing I will say is I have never worked on a ZD30 CRD so this information is assuming the VNT turbo setup is the same as the DI motors.

There is a couple of different ways to control the boost
1) Adjust the boost limiting grub screw. By my way of thinking not ideal as it may also slow the rate of spool.

2) Fitting a dawes valve on its own in conjuction with the factory boost control. This still gives the engine computer control of the boost but eliminates overboosting. This is a good method if you want to keep a functioning EGR system as it lets the computer reduce boost to allow the EGR to work.
Dawes boost limiting.jpg

3) Fitting a single stage dawes and needle set up. This gives full manual boost control, the computer is no longer in control of boost. The dawes sets the maximum boost and the needle valve contols the rate of spool.
Dawes.needle stage 1.jpg

If your post turbo EGT's are hitting 700 you need to be lifting off the accelerator, those temps are WAY to high. Don't let the EGT's get over 550.
Reducing boost will make the problem worse, diesels run hot when they are rich - the opposite to petrol engines. You need to be aiming for a constant 17Psi boost not the erratic boost you are currently experiencing.

Tony

Thanks to CHAZ for the images.