Need diagnostic help

91 Isuzu MU 2.8TD. Two issues not sure if they are related or not. The truck is blowing white/blue smoke when at low revs, also reved and surged on me today. Other thing is when we turn the key off there is a delay before it cuts out. Hubby does most the work on the truck himself. We are thinking turbo. Are we likely to be on the right track! Or any other ideas!

blue smoke is oil and white is unburned . i would check the turbo soonest as it can run away on you when it starts feeding it self with engine oil. dont even need to leak both sides fuel is injected normally but when it sucks it like a carburettor then it wont burn right hencewhite gray smoke also from oxigen starvation and extra fuel.

once turbo is found to be rooted and repaired and it still wont cut off when you turn it off then you have to replace the fuel-cutoff switch on your injector pump also they cost 100 to 160$ or 25 pound from UK plus shipping or cheaper

Thanks all for your comments

I think it's possibly nothing to do with the turbo, but air in the injector lines.Check for leaks including back to the seals on the fuel filter.1991 is getting old, fuel hoses are probably getting stuffed.You could pipe in a bit of clear hose (temporarily) between filter and pump, at the pump end, to see if any bubbles come through when it's running, or check by cleaning all the hoses down thoroughly, pumping the primer, and checking to see if there's even the slightest oozing anywhere in the fuel lines to the pump, or banjo connector on the pump.
The running on is caused by pressurised bubbles in the injector lines, expanding and holding some pressure after the fuel is cut, so it will tick over until the pressure drops to below cracking pressure for the injectors. Might be a bit harder to start than usual, but that depends on state of the injectors, pump, and how fast the starter cranks it over.
White smoke, erratic running, and particularly surging are typical symptoms of air in the lines.Blue smoke could be the turbo, but that doesn't explain the white smoke.
Bleeding the system takes time, you've got to pump quite a lot of diesel through to get the system clear, 1/2 litre or even a litre.

Thanks mm, will pass this on. This is the 4jb1 which is a very reliable engine, pretty sure there were no recalls. I think the recalls were with the later model 3.1L engines.

Drain the fuel filter into a clean container, check for water in the fuel. Best places to start are the free places!

On that line had a Medical callout to a tractor operator whose old POS tractor took off at high rpm whilst lawn mowing and tried to climb a tree all because the blow by was fueling the engine.even the fire service guys didn't know how to kill it. Just pulled off the breather hose!

That won't work if turbo seals are leaking, but yes, it's a good trick on a n/a diesel.

Nope, that was the later 4JX1 diesel engine series.

plus 1 definately 4jx1