If it won't start it's most likely electrical or electronic and that's what you must rule out first, followed by fuel delivery problems; these are my top tips:
Batteries
The starter/vehicle batteries MUST be in tip-top condition and delivering at least 2 x 575 CCA - the factory spec for a P30 model is : 2 x type 097 12v 55AH 575 CCA The nominal size is: 242l x 175w x 190h 12v 55AH 575 CCA. Each battery should have its own earth cable not one chained through to the other. If fitted, switching in the leisure batteries during the start process can really help because the engine will not start if the ECU input voltage drops below about 10 volts which can easily happen during cranking. You need a proper switch or solenoid unit - jump leads are no substitute. A sign of weak batteries can be that sometimes the engine fires up as soon as the key is released from the cranking position.
Electrics
The PMD (Pump Mounted Driver) unit which sits on top of the main mechanical pump is a bugger and overheats; it is the single biggest cause of failures on these engines. All new kits come with an extension lead and a heatsink so you can mount it in the airflow under the bonnet - the cruise control not working is an early symptom of PMD failure. There is no need to remove the old PMD, it can just stay there. About £100 for a Chinese copy or £350 for one from the USA (which is probably the same!!)
Glow plugs AC Delco 60G - they must be in good condition and working - it's easy for the electrical connections to corrode off. These are about £15 to £20 each and are easily available on Ebay - easy to test using a 12v car battery, they must glow near white very brightly and you must individually test all 8
Glow plug relay - works or doesn't - check glow plug voltage and current draw - pretty cheap at about £80
Damage in the loom - really common - once the engine is running give it a good pull and wiggle and see if you can stop the motor running. I have also seen loom voltage induction issues where a simply opening out the loom wiring and re-wrapping it fixes an issue.
Crankshaft position sensor - rarely fails because it a simple hall effect unit that sits in the front left lower part of the engine and read the crankshaft damper position. It's a pain to change because you have to slack off the serpentine belt and move the power steering pump out of the way. Should throw a fault code if it fails but the engine should still run because it then uses a default value and the timing signal from the main injection pump. About £75
Injection pump timing
The injectors are timed mechanically and the pump sits on top of the vee and is mechanically driven from the front of the engine. The pump timing has to be set correctly - three pinch bolts (15mm I think) alter the pump position to advance or retard the injection. The precise timing is then modified by the ECU using a stepper motor which moves an optical eye. The pump drives a small chopper disc which gives the eye its signal. This is all inside the top of the pump and is easy to get to. The mechanical timing must be reasonably close and then the stepper adjusts the injection timing correctly as the engine starts, runs and is under load etc. You cannot measure the injection timing offset manually or with a normal code reader, however if it's wrong it will throw a fault code - the injection timing self-corrects but it must be within an acceptable tolerance to start with.
P0216- injection timing control circuit
This code sets when there's a 5 degree or more difference between Actual injection timing and Desired injection timing. The timing stepper motor on the passenger (right) side of the fuel injection pump extends or retracts, which rotates the cam ring that holds the optical eye inside the injection pump to alter timing. This code sets when the ECU can't get the timing right. Stepper motors rarely fail and it's more likely to be a wiring problem if it's not moving. Cheap and easy to swap if it has failed however.
P1214- injection pump timing offset
The ECU monitors crankshaft position through the Crankshaft Position Sensor and that is used to determine the amount of injection timing offset necessary for proper operation. This code will set if the TDC offset is greater than +2.46 degrees or less than -2.46 degrees.
First step is to perform a TDC Offset Learn procedure - normally this will resolve your issue. Use the procedure listed below:
1. Ensure all immobilisers are off
2. Install suitable scan tool and read fault codes
3. Using the scan tool, clear any fault codes present - this is critical
4. Start the engine and bring to operating temperature - the offset relearn will only work on a fully warmed engine. Turn off engine.
5. Turn ignition switch to the ON position, DO NOT START THE ENGINE.
6. Fully depress throttle for at least 45 seconds.
7. Turn ignition switch to the OFF position for 30 seconds.
8. Restart engine. Using scan tool, verify that TDC offset fault has been cleared to zero.
9. Start engine and bring to operating temperate, engine RPM less than 1500.
10. Using scan tool (Autoenginuity only), verify that TDC offset is between negative .25 to negative .75. If the TDC offset is within specs, relearn procedure is complete and also the fault won't recur.
11. If TDC is not within specs try it a few times - if that gets you nowhere then work out what you did to cause it! It may be necessary to reset the injector pump timing.
Dirt in the optical eye chamber can cause this issue but if you remove the eye to clean it, it MUST go back in exactly the same place...if you get it wrong the engine may not start. The eye is not just bolted in one place, it's on a sliding mount and where you tighten the eye/bolt makes a difference. If you think there is dirt, it's better to suck all the diesel out with a syringe and clean it with brake cleaner and a low pressure airline. Only remove the eye as a last resort and be super-careful to put it back in exactly the same place.
Top Tip - do not alter the mechanical timing or the optical eye position unless you really know what you are doing (and ideally have a GM Autoenginuity setup) or are happy to mess about with this stuff for hours and hours.
Mechanical fuel delivery
Engine sluggish to start but then runs fine - normally the small lift pump that draws fuel from the main tank to prime the large filter unit. The main injection pump is more than capable of pulling fuel from the tank and running the engine once it has started, the lift pump primarily aids starting. A failed lift pump does not always cause a fault code. You can use a simple 12v fuel pump if yours has failed and just fit it inline where the original GM pump was. Ensure it's actually getting 12v!
Fuel filter blocked - older vehicles are full of bits in the tank especially those fitted with metal tanks - some are plastic. Make sure the fuel is flowing by momentarily slightly releasing the black bleed top on the filter - there should be a good flow from this bleed outlet. A new filter element is about £30.
Air lock in lines - there is a small tee at the front of the engine that allows you to bleed air from the system.
Air/fuel leak in the rubber leak off pipes - these 3.5mm braided hoses run between the injectors and return unused fuel to the main pump (not to the tank like on most vehicles) - they can go brittle with age and leak as well as let in air.
Injectors - should be good for 100k miles plus - about £250 for new set or £140+ to have yours ultra-sonically cleaned. There is more than one type so you'll need to get one out and have a look - you do have to remove the turbo and exhaust down pipe to change all the injectors on the right bank.
Turbo - it's only a light pressure unit that operates on the right hand exhaust manifold and should go on for years - loss of power occurs when the waste gate jams open (lack of use) but is easy to free off or if the plastic vacuum pipes fracture - it is purely mechanical so no fault code will show (see more below).
Code reader
A basic GM code reader will give you the basic fault codes and data streams (like air and water temperatures and RPM etc.) but not the detailed ECU readings and not the ability to read and/or alter any settings such as injection pump timing. It will also allow you to clear fault codes.
Turbo boost control and wastegate actuation
It's pretty basic on these vehicles. Under normal operation, the wastegate is pulled and held closed by a vacuum unit. If the vacuum pipe is not connected or vacuum is low, the exhaust pressure will blow the gate open meaning very low (negligible) turbo boost. The black vacuum pipe goes from the wastegate actuator to an ECU controlled pneumatic valve that opens when the engine is running thereby closing the gate (by sucking it closed) and creating turbo boost. The actual vacuum is generated by a vacuum pump on the front of the engine which is connected to the vacuum switch by a hard white or yellow plastic pipe that can become brittle and fracture. It's actually pretty simple to replace the plastic pipe with a carefully trimmed and bent length of 3/16" copper brake line. See this for more details 6.5L GM Diesel Wastegate Solenoid Troubleshooting & Replacement Procedures (dieselhub.com)
The wastegate can also jam open or closed - as part of regular maintenance, ensure it's moving freely - obviously when it's cold! It should move easily by hand and stay where you move it to if the engine is not running - there is no spring in the wastegate actuator.
Low power can also be a sign of a failed or sub-optimal vacuum pump which is mounted on the front of the engine and driven by the serpentine belt. It needs to deliver minimum 15 inches of vacuum and ideally 20-25 inches at the wastegate valve. About £160 for a replacement pump unit. A new wastegate actuator is about £40.
Plenty of people have replaced the vacuum system with a mechanical spring that holds the gate closed but it's trial and error about how stiff the spring needs to be and it's basically a bodge - personally I would always try to fix the vacuum system because that's how it was designed to work.
Exhaust manifolds and bolts
The 6.2 and 6.5 normally aspirated and turbo engines did not have exhaust manifold gaskets fitted as standard, however, due to the large number of engines that develop gas leaks between the head face and the manifold, Fel-Pro produce a metal/fibre gasket kit (part number MS91587) that can be fitted. It's possible to fit each gasket without removing the manifold completely by removing the glow plug heat shield tubes (when fitted) then the 8 M10 x 1.5 bolts (using a 15mm 6 face socket) and all the glow plugs, then carefully pulling the manifold away from the head with a pry bar and/or ratchet strap - it only needs a 2 to 3 mm gap to slide the gasket in from the front or rear provided your vehicle has enough space (mine did) - be patient and do not try to rush this otherwise you could damage the gasket. The Turbo side also needs removal of the turbo hose on the pressure side of the turbocharger.
The manifold bolts should all come out cleanly - if they do snap then unless you have excellent access it's probably cylinder head-off time! If you need new ones they are not expensive and are basically M10 x 1.5 flange bolts; they do not need to be super hi-tensile, 8.8 is more than enough.
The GM 4L80E transmission (gearbox)
3 Speed with overdrive ( so a 4 speed )
Here is a superb video which explains which type of GM 4L80E you have in your vehicle 4L80E Differences between Early and Late models - YouTube