if you haven't already then t’s 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.
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 and its best to have the boost pressure measured actually measured with a boost gauge to check that the turbo is healthy and that the sensor is reading correctly
If its a variable vane turbo (they’ll be an actuator on the turbo body - but not a wastegate) then check its vacuum pipework as above and check the connections to the diaphragm / solenoid valve
Also worth considering adding a bottle of injector cleaner into the tank as a clogged injector nozzle will reduce power and give poor combustion - the next stage on from this is to remove all the injectors and have them ultrasonically cleaned and flow checked
Also 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.
also check the connections to the EGR valve, if either the vacuum pipe of electrical connection are damaged / corroded then the EGR can be on all time which certainly will hurt performance. Its also possible that its a fault with the EGR valve so its worth removing the valve and cleaning it out with brake cleaner, if it looks particularly clogged then replace it.
it might also be worth measuring fuel pressure at the injector rail as this will show if the main pump is in good condition
in that case its going to be a lack of fuel so either the ECU isn't opening the injectors enough or there's not enough fuel pressure ( hence my suggestion of measuring it at the injector rail)
This could be an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for 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 around each joint in turn. If the engine rev's up you've found your leak.
Now you mightthink that spraying lighter gas around a hot engine isn’t wise, however the flash /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 oflighter gas to provide a spraying 'wand' and allow a direct blast of gas intoeach area, especially those difficult to reach with large implements.
Might also be worth checking the wiring and connector to the airflow meter for any signs ofcorrosion or damage. you can do a quick fault find if you unplug the meter andrun the engine without it.
if the engine condition is the same then chances are the meter or the connection to it isfaulty
It’s also worth cleaning the MAF sensor wires, as they can get coated with dirt over time which then offsets the reading
Use some contact cleaner or brake cleaner to spray onto the wires to remove the dirt – on no account touch the wires with anything as they are very fragile
just get back to me on this post
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1 other thing to try is some fresh fuel and also with the fuel filler cap loosened off as if the cap 1 way valve is faulty it can generate a vacuum inside the tank and stall the pump
you should get a slight hiss but nothing huge
if this doesn't help then I'd next check that the air filter is in good condition and that there's no leaks on the pipe running to the turbo , post MAF meter
I'd next suggest to blank off the EGR valve with a thin sheet of aluminium / spacer as this still sounds like you're getting exhaust into the inlet
leave the valve connected electrically though
Ok I'd next consider removing the O2 sensor in the exhaust to check that the cat condition is OK
the other thing to check is if the throttle / air valve on the inlet manifold is staying open as if its clogged up they can snap shut and choke the engine
so its still possible that the cataylst is blocked and limiting power I'd say the next step is to drop the exhaust off the turbo so you can peer inside and check that the cat brick isn't broken up
Ok I'm wondering if the boost pressure sensor ( MAP) may be faulty and over-reading? as the ECU will limit the amount of boost pressure to protect the engine
ideally you would plug in a scan tool and look at the monitor values to see that its not exceeeding the limit ( which I think is 1.2bar of boost)