Nissan patrol 4.2 engine .

Ifit runs ok at higher revs it is very unlikely to be a restricted exhaust!

Axle. Yes, I agree it is a possible , ie, connection issue at certain frequency of vibration.

Hi guys .
The problem has been going for a couple of years ... very slight and intermittent at first and now more constant , hence the pump rebuild . Guys that did the pump have a very good name around my area . The pump rebuild fixed the smoke at start-up and improved fuel economy. I have worked through a few of your suggestions and am thinking it's a fuel supply issue . Coming down a long gentle hill at 100 - 110 ks and feathering accelerator to keep speed the same ... it " coughs and farts " and once you touch the accelerator, it evens out .
I'll replace fuel lines ... check the fuel primer for cracks and check out the electrics on the fuel pump . Might be worth a visit to the pump guys and talk to them as well .
It's fantastic how much help people volunteer on this site .

JOHNNY
It isn't a COMMON RAIL DIESEL engine! EGR is NOT an AIR supply, it is HOT EXHAUST GAS.
Why will the intake be sludged up? with WHAT? CLEAN AIR?

Every engine is required to direct any crankcase blowby into the engine intake manifold where it is burnt, instead of just venting it into the atmosphere.
Blowby is usually an oily mist which creates an oily film inside all of the air intake system, this in itself is not an issue.
Most engines also employ exhaust gas recirculation where, under certain operating conditions, exhaust gas is injected into the intake manifold to mix with intake air, the purpose is to reduce combustion temperatures and thereby reduce nitrous oxide emissions from the exhaust.
This is fine in theory but in practice the soot in the exhaust gas combines with the oil mist from the PCV and forms a thick sticky sludge in the inlet manifold and EGR valve, eventually this sludge hardens and can cause a restriction or blockage, this is a serious side effect of these two environmental control systems and the reason various EGR delete mods.
Normally the EGR valve is closed at idle and high power and only opens at low to medium power, if the ECU is programmed to run with EGR at these medium powers and there is none due to a blockage it can cause rough running, Boodahs Issue appears to be in this range hence my suggestion, especially as he said it is a high time engine.
Not too sure what RMD is on about, but I don’t think he’s a diesel mechanic.

JoHNNO
You said "some engines rely on the EGR AIR SUPPLY to run Properly. You Can't be a DIESEL MECHANIC, as ALL mechanics know there is NO AIR passing through the EGR valve on any ENGINE.
You also said 'Normally the EGR valve is closed at idle and high power and only opens at low to medium power, if the ECU is programmed to run with EGR at these medium powers and there is none due to a blockage it can cause rough running," How? explain How!
Diluting the incoming air charge with gas would cause less smooth running than MORE Air, ie normal, to more efficiently burn the injected fuel!
Was that from a GOOGLE SEARCH? It reads like it is!
What sensor is then sending the wrong signal to the ECU. it doesn't just act by itself!

Allan.
I agree about a decent assessment. However, some possibles mentioned are very unlikely. Time does degrade components but the diesel men to rebuild a pump and/or injectors are not overall diagnostic inclined.
Now realizing, after someone above mentioned timing control, that then means a TPS is present, and if over 400,000 km in much the same position of lighter load/cruise, the carbon track WILL BE WORN and likely not tracking as designed. The TPS would be the FIRST ITEM TO CHECK, or even simply replace because it cannot be very good and likely worn. That item will immediately correct the injection as it's Wiper clears the worn area as pedal is depressed or to idle.