1976 VW 2000cc bus, fuel injected, automatic single loud backfire, then dead! Pulling out of a parking lot at around 25 mph, I had a single very loud backfire, then the engine died. The motor will crank / turn over, but will not start. Did a recent complete tune-up and timing adjustment, but didn't adjust valves yet. It's been idling rough due to a vacuum leak I've yet to identify, and cylinder number 3 had low compression. It doesn't burn oil, but there has been a fair amount of oil on the motor recently. Any thoughts?

no country told nor year car told wow what a challenge.
I am USA so no books here on your car for there (UK you)
always cranks fast , and stalls driven
and fails until parked cold. sure.
a bad fuel pump can that
lets now define what cut out means. ok,
EFI tech,
EFI has spark and fuel injection.
if spark cuts out does so like INSTANT DEAD, RPM drops like a rock.
if spark misfires you feel it and all scan tools here set it as P03xx errors. DTC.
now if fuel cuts out it is slow, like watching desk fan slow down turned off, due to slow flagging fuel pressure.
stalls are the most hard to diagnose,
vast causes. (3 in macro form)

1. bad spark (warning if PCM sees this bad cuts fuel)
2. bad fueling
3. bad engine ( I'd say not in this case)but must be listed.

and a load of tests and tools needed and skills to find it
if my car stalled in my drive way, it be all over spark when I found it dead parked, hot.
if dead NOW cures are 10x more easy, to find.
or when dead do the super double secret un-flood test on any car.
dead engine hot
key on
depress throttle (foot right to the floor) called WOT
and crank, this magic cuts all fuel now, and unfloods the engine so it can start, but only i spark is good,

Every one of these I work on usually has a bad coolant or air temp sensor.

check around gear box repair shops to see if there is a replacement box that has an overdrive gear (5th)
if it is an automatic , the problem may be that overdrive is not coming in and that will drop the rpm 250 rpm and increase the speed up to the 75 mph mark
there is nothing wrong in the box as the rpm and the speed are about right in relation to the diff ratio(not mentioned )
3200 rpm with a overdrive manual runs around 74 mph or 120 km/hr
another method is to put a diff in with a taller ratio
example --current 4.1 back to 3.2

If it runs when you pour gas into the intake then stops you have a bad fuel pump. In 1976 most used car orators. This being a German auto at that time... Maybe had the early fuel injection using the lamda (sp?) Mechanical system... If it does then you need a mechanic with experience on mechanical fuel injection.

Improper Ignition Timing

Defective Ignition Components
(ignition coil/s, distributor*, distributor cap*, distributor rotor*, ignition wires, and spark plugs - NOTE:*Distributor-less engines do not have these components)

Lean Fuel Mixture

Defective Catalytic Converter
(Some symptoms of a bad catalytic converter:
Major loss of power over 15-25 mph.(does this speed sound familiar to you?)
Strong sulfer or rotten egg smell emitting from the exhaust.
Loud rattle being heard from inside the catalytic converter.

Defective Air Injection Components

Faulty smog pump if fitted.

Low Cylinder Compression.

100 psi per cylinder is way too low--should be in the vicinity of 150-165 per cylinder
back firing indicates wrong firing order and as you have replaced the plugs etc check that you have the leads on the correct plugs in the correct order
the timing for the engine should be around the 10 degree btdc and not tdc

here is a page of everything you need to know about a vw http://www.ratwell.com/ has all parts listingd

it sounds like the your electronic fuel injection system is pulling to many amps and is throwing a relay maybe a loose wire grounding out? you have to go through the whole electronic fuel injection system and check each wire for breaks and proper voltage

nope, distributor plugs and wires, change those and the pcv valve. Run you total no more than $100 for parts if you shop around a bit, since parts prices vary greatly. the spark is not entering the cylinders correctly to burn the fuel properly, replace those items and you will not have this issue unless you have some rare deadly engine disease- :-) Hope it helps and have a nice day!

From minor problem to major problem. Try to replace your spark plug wires, rotor cap and the spark plugs, also it can be the injectors, plus if possible, try to replace the fuel filter which is located low under your EFI unit. If problem not solved, then it will probably be your A/T trans.