Difficult start. Leaky injectors?
OK so here's the symptoms. 127k 300GES truck.
If I leave the truck for a week between starts, the cold start is terrible - takes about 10 seconds of cranking and then will struggle to catch.
After this - hot or cold (as long as just overnight) it literally fires on what seems like a quarter revolution of the crank - i.e. perfectly.
The fact that it starts well cold if only left overnight makes me think the cold start side of the system isn't at fault - I've read that injectors can leak down a little if left for long periods and this might be the cause if left for a week or more.
Also I have noticed a slight but noticeable petrol odor in the cabin when left for a long period.
Any thoughts? New set of Bosch injectors from IAP is £88+VAT....
Chris
The instant start is achieved through a fuel accumulator with a one-way valve. This prevents the fuel in the injection system from draining all the way back to the tank, when the engine is switched-off. The accumulator is housed with the fuel-pump and filter under the rear left wheel arch. If the accumulator's diaphragm is perished it will not hold pressure and the fuel-pump(s) will appear more noisy, but the most important part is the one way valve and these have a tendency to stick, and may in fact be difficult to open, causing your long-start issues. Also, the one-way valve can fail completely or stick in the open position also causing long-start problems, as the fuel takes a while to flow back up from the tank. I would start with these parts first.
if you can attach a fuel-pressure gauge to the fuel distributor you can check fuel pressure on cranking.
The injectors themselves are passive spray nozzles with no moving parts.
The other possibility is that your fuel-pump relay, which detects engine cranking to maintain fuel-pump operation is on its way out, so one other test may be to check the fuel-pump voltage when cranking.
As for the fuel smell, at 20 + years old, it may be a good idea to change/replace all the rubber fuel lines.. Some of the small ones to the tank, particularly on the return-line, can start to disintegrate with age and the spring-clips that hold them can rust away. These little hoses are quite expensive, but you need to get them from MB, as they are shaped and some even have different inlet and outlet diameters. There are others in the engine compartment.
it's a small device on the end of the fuel-pump, before the accumulator (under the rear wheel-arch). Usually when you purchase a new Bosch fuel-pump, it's included. You can also buy the valve separately.
Search Google for ' Fuel-pump non-return-valve'
http://www.bosch-automotive-catalog.com/en/internet/ECAT-RS/vehicle-info/-/vehicle/equip/MB/2280/
use this link to the Bosch Catalogue to find the Bosch part numbers for any required parts:
http://www.bosch-automotive-catalog.com/en/internet/ECAT-RS/vehicle-info/-/vehicle/equip/MB/2280/
click on 
then, copy the part number and search for the part number you need in Google and you will then find a number of motor factors offering the part at reasonable prices compared to the MB parts dept.
eg fuel pump
https://www.amazon.co.uk/Bosch-0580254911-Fuel-Pump-580254911/dp/B00DGYYCZG
or non-return valve:
just found this, which may also be useful.
http://www.w124-sg.com/2013/05/cranking-issues-with-w124s.html
New symptom: When I tried to start the vehicle today after the "long crank" it started up and the fuel pump was noisy for about 15s then ran quiet again.
I'm starting to think this is the accumulator - which is annoying as it's the most expensive part.
Disconnect small hose from rear of the accumulator, if it's faulty, you will find fuel in this hose, as it means there is a leak in the diaphragm.
But you should put a pressure gauge on the fuel system for proper diagnosis rather than guessing, or it will get expensive.
As temporary measure before actually starting switch on and then off your ignition several times to prime fuel pressure into your system and then start. Hopefully it should start as instantly as you were used to confirming fuel pressure loss internally as you have already suggested.
Thanks, great tip... Will report back.
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Thanks for this very useful.
One question on the one-way valve - I can't work out from the diagrams on the web where it is located - in fact I can't see it at all in any of the pictures of the pump/filter/accumulator setup nor is it in the workshop manual I have...
Chris