Kanga
Thanks for that explanation, have read that before today. I realize the fuel is metered to the High pressure element of the pump.
However, all fuel delivered to the high pressure area through the suction valve and all normal cooling flow past and around the pump is done by a suction pump which draws fuel from the tank and through whatever filters are there.
If this pump can't get enough to maintain fuel pressure to the suction valve and deliver that controlled amount to the HP pump, it is the suction pump which runs with less than a normal full fuel flow. The flow goes to the HP and and any not used ie (normal flow return rate) goes back to tank. If there is enough to run but no return flow, that is not desirable.
Minimal cooling or no cooling on the HP pump (although it may be pumping some to the engine), which creates heat, but there may be no return flow to tank happening.
This may go on for a while as the engine is running. It may low or show signs of not running properly.
If the finer filter is between the OE filter and the engine, in the event it does restrict and cause the above situation, the OE warning of negative pressure which normally occur across the OE filter won't be happening across the filter of the OE unit because there is no pressure drop across it.
So the engine can be running, close to or no return flow can be happening and the warning won't activate.
That is how I see it and have worked out the operation with the aux filter in that position.
Hopefully the OE always will protect the finer aux one and with proper service intervals it should never be a problem.
I am surprised the Diesel Care people fit them like that with the possibility of the negative pressure sensor not being able to do any warning of the downstream restriction of fuel flow at the finer filter. Finer is good I agree.
I notice in their explanation they do mention the restriction sensor still works normally, it does, but only for that OE filter and not if the aux filter restricts the whole system it is there to protect.
Some (Toyota Dealers spring to mind) tell there customers, (in book too I believe) when a restriction occurs, ie Light on, to drive to the dealer. Some of the V8's have wrecked the system because it was operated to get to the dealers.
All the filters are good to have and more protection is insurance but when any filter blocks, immediate action seems to be needed.
If your finer filter is restricting and the OE sensor can't tell you how do you know apart from the uncharacteristic running?
I haven't yet seen any wording from anyone, which covers all working aspects of the system so as to ensure all is allowed for.
Maybe I am missing something. Ideally, perhaps the factory should be the 2 micron with a pre filter of 5 or so.
I noticed the company mentioned 30 micron as an option for personal choice. Hmmm. Not sure what that means, as 30 micron and CRD aren't really a combination for good filtering. 10 or 5 maybe as a pre but not 30, unless purposely used with yet another filter to add a graduating effect to the system.
All good discussion, and thanks for the reply.
Cheers
mydmax