clarification
Alouette3,
Maybe I can help clarify a few things to help you out. I work with the Arriel engines everyday and I am very familiar with the coking issues. First of all as you stated there are two types of coking, oil and fuel. It seems you are concerned with the fuel coking so we will address that one first.
Since the Arriel has a centrifugal injection type fuel system we have a injection wheel that rotates and sprays fuel into the combustion chamber. In order to get the fuel into this rotating wheel we have a stationary manifold with labrynth seals that mates up to it and supplys the fuel. In between the lips of the labrynth seals is where fuel can coke up and cause a drag on the engine. Typically this doesn't cause any significant issues in flight but it can lock up the engine after shut down making a restart difficult if not impossible. So far this phenomenon is very isolated to a few areas of the world. Out of thousands of Arriels operating it seems only ones in certain areas experience it.
There was a time early on that it was suspected that prist may help the situation however I was never able to get any definite clarification from the operators who tried it as to if it worked or not. We also fond that it tends to happen more during the winter season (if memory serves me correctly) and less in the summer. This may have been were the prist solution came from. If they started using it in spring and the occurances slowed or stopped that could make one think they have cured the problem only to find it comes back when the temps change. I would suggest at this point that prist is used for it's intended purpose only and not for fuel manifold coking prevention.
There was also another additive that was tested and approved specifically for coking prevention and that is Turboline plus 100. It did seem to have an effect at one time but I don't know if it ever actually cured the issue. There were events of coking while using the additive but I don't think it was confirmed that the operator used it 100% of the time.
There was another bout of coking in Hawaii a year or so ago and it seemed to be a fuel quality issue of some sort. Only the operators on one island seemed to have it and there was some issues with the fuel delivery system on that particular air field. I won't comment any more on this as I was not part of that investigation first hand (the others I was).
Now just to clarify the oil coking issue while we are here. The Arriel 1 engines have had issues in the past with gas generator rear bearing coking hence the requirement to clean the rear bearing periodically. There were a few modifications that seemed to all but do away with this issue in conjunction with the use of an HTS gen III oil such as Mobil 254. This phenomenon stems not quite from the reduction of scavenge oil flow at low speeds but the lack of cooling air around the bearing area. At ground idle the temps tend to decrease for the first 30 seconds or so but afterwards they can creep back up. If the bearing temps are high and the engine is shut down you can see whare this would creat more propensity for coking in that area. The modifications I mentioned earlier (TU274,281,283,and 284) help increase the volume of cooling air in the rear bearing area among other things, which significantly helps. Nowadays if you have these mods you don't have to clean the rear bearing unless there is an issue and only the oil tubes leading to the bearing area have to be checked for coking periodically.
To summarize:
Fuel coking
Forget about prist as it is not proven for this purpose. If you are having fuel coking issues contact TM to find out the latest fix. They may recommend Turboline plus 100 additive and will most likely want to test your fuel supply.
Oil coking
Stick to 30 second cool down period. If you need to ground idle for longer periods go back to flight idle and stabilize before performing a normal shut down sequence. Use and HTS oil if possible (although with the new mods I have a few operators that are having good luck with older generation oil, even aeroshell 560!).
If you have any further questions feel free to send me a PM and I can help you out as best I can.
Max