A. When it did start; Did it start quickly and idle perfectly OR did it stumble, miss, blubber and then finally idle smooth?

B. Did that mechanic wiggle or move any wiring before or while he was installing the fuel pressure gauge? [note: a 3406E is getting a little old, now and wiring looms can get worn thru, look very closely at areas where the loom touches ANYTHING; example, look at loom between ECM(computer) and as it goes under the cylinder head, it attaches to "ladder brackets" with plastic ties=they can rub there. The loom also goes behind air compressor and it can rub on water line/hose and/or fuel lines and have bare spots rubbed thru it there. They also get rubbed going up and over the front cover(in front of the 1st valve cover) on the "ladder bracket" there, too.]

C. The previous recommendation to reset the depth of the front rpm/timing sensor is a good idea, too.

D. Do you have a fuel hand priming pump on this engine? If not, I highly recommend installing one. It helps in troubleshooting along with filling/bleeding fuel filters, too.

E. You didn't give me a year or serial number, so I will tell you that the book spec for a 1999 engine 3406E (s/n 2WS prefix) says 78 psi fuel pressure at idle, and 93 psi fuel pressure at HIGH idle. {BIG note: There are two styles of transfer pump= early and late; early =all cast housing and has lower fuel pressure=probably only see 62-65 psi at idle WITH its old, original style "fuel return check valve". The late style= higher pressures and it has an aluminum end plate on it(=where the fittings attach to it), this style can use either an old, style "fuel return check valve" OR the newer, higher pressure "fuel return check valve". Special note: Just don't use an old pump, with the newer,high pressure "fuel return check valve". }

Either way 50 psi sounds a little low for idle, even with early style transfer pump and earlier, style fuel return valve, but IF engine started very quickly and sounded smooth immediately, then I would be looking at electrical condition as much as fuel pressure condition.(so if you had 35-50 psi cranking and no start and no smoke, that should tell you that fuel pressure isn't your main culprit, even though it is a little low: BUT you have now changed fuel filters, SO maybe it will improve; who knows?)

Now, The old style transfer pumps "can" suck air, when they are old and the lip-seals start to fail, but they don't actually leak fuel externally, either. Also, there high pressure relief valve,internally, can get stuck in one position and causes low fuel pressure while cranking, low fuel pressure at idle, but the extra high fuel pressure at high idle(MAX rpms). I need more fuel pressure readings to determine that.

Really need the answer to question #A first.