Dear oh dear.
1. Anyone who believes anything they see on on the fuel gauges of a 20 year old aircraft with bladder tanks as being anything other than a rough estimation and especially the Heath Robinson fuel system of the PA31 is, one day, going to get a surprise.
2. EGT probes whether they indicate some "mythical" temperature are calibrated or not, old or new can only indicate THE AVERAGE of the exhaust temps from all of the cylinders.
I gaurantee you that the actuals at each exhaust port will be different and can be some miles apart.
Unless of course you have an exhaust temp analyser for each, which again is only interesting as you usually have to cycle through each to find out whats going on and who has the time for that on T/O.
So the only indication that actually means anything is "the peak" and which side of it you are, given that the individual cylinders do not have a seriously deviant one amongst them.
Where is the hot one and there will one or more that are causing you to run richer than you might?
And whilst I am on that subject I have seen more than a few engines with no less than 3 different size injector nozzles on them. When was the last time your fuel distribution system was on the test bench?
Given the instrument inaccuracies, age and calibration status of the probes and having regard to the fact that it is an average, peak is the only measurement for any particular condition that is absolute.
Fuel flow gauges by the nature of their design can be reasonably accurate but in any event event aren't nor should the primary means setting FF, it comes back to the EGT, where the peak is and how rich of peak you mean to operate. Lean of peak may be fine for an engine system in and kept in absolutely tip top condition and constantly monitored in flight for changes in engine and ambient temperatures.
The fuel flow will then be whatever it becomes.
Which brings us to the nub of the problem.
If the operator has to operate the engines to six decimal places of fuel flow to make a profit or just break even, then he shouldn't be there.
The 5, 6 or even 10 litres ph = maybe $10,000 he thinks he may save, over the life of the engine, say $3,000 per year is, if I recall correctly, less than one cylinder overhaul and if you don't do it that way there will be many more than one.
Everybody chases the EGT end, how many chase the fuel distribution system? The EGT end only tells you what's already happened.?
I guess what I'm trying to say is it's never one thing on it's own and I am not at all comfortable with the recieved wisdon from the ATSB on this Whyalla thing, it's much too neat.