if you haven't already then its worth considering fitting a new fuel filter as if only slightly blocked this can lower fuel supply pressure and choke the main pump, the pump will be more sensitive to this when its cold as diesel thickens considerably with low temperatures.
also some models have an electric lift pump in the tank and its worth checking that this electric pump that feeds the main fuel pump is working correctly, it should run for a few seconds then switch off when you first turn the ignition on. If the connections to this are corroded or damaged then the pump could stop at any time. Check that its relay switches in and out and the relay contacts are clean and bright – replace the relay if in any doubt and do the same for the fuse, ideallymeasure the voltage at the pump and check its getting 12V
in that case I'd suggest to check the condition of the cam position sensor as this is responsible for the injection timign and if out will limit power
and check all the high pressure hoses post turbo/s for any split hoses or cracked metal pipes, also check the ends of the intercooler as its not unheard of to pop the end caps off under high boost.
If this is OK then check the small vacuum lines to the turbo and its connected control solenoid on the bulkhead, any cracks or leaks can give turbo issues
Also have a feel around with your bare hands with the engine running to see if you can feel any escaping gas under your touch. Another method is to use a soap and water solution sprayed onto any suspect areas - if there's a leak it'll bubble up with the engine running
Also check that the vacuum hose running to the dump / turbo bypass valve is intact with no leaks as if the dump valve is opening prematurely you'll be losing all your boost
If this is OK then remove the high pressure intake hose after the turbo and have a feel of the turbine wheel - there should be no play radially at all - but a little bit of axial / end to end float is normal
one other possibility is that the wastegate actuator spring has gone soft so allowing the wastegate to open prematurely - if your actuator has a threaded end you can increase the preload on it to see if this improves things otherwise these units are quite cheap compared to a new turbo so it may be worth trying a replacement
just get back to me on this post
I'm assuming that you don't have any helpful error codes that will point to the issue?
if not then you have fuel pressure ( has this been measured) , you have boost pressure ( has this been measured)
then it may be wise to do a compression test across all cylinders to confirm that they are all within 10-15 psi of each other
OK thanks for that
to get a low MAF reading then it could be a bad vacuum leak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and can lean the engine out and can also allow the engine to rev up when not desired causing rough running.
As its a mechanical fault it tends not to turn on the fault light and you can sometimes hear a 'hissing' noise with the engine running.
Check the hoseclips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.
The best way to locate a leak is to have the engine running and warm and then spray lighter gas/propane or brake cleaner around each joint in turn. If the engine rev's up you've found your leak.
Now you might think that spraying lighter gas around a hot engine isn’t wise, however theflash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.
Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.
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