There are several good examples in the posts why your vehicle is getting poor economy. Other examples are: slipping flywheel lockup in top gear. At around 65mph, the transmissions final drive, the valve body is supposed to lock the torque converter 1:1. If you have VCDS and measure the top gear slippage at 65 mph, and there is more than 30 rpm, your going to toast the tranny and it will stop working altogether.
The Thermostat is a sensitive issue. We always check thermostats prior to install and most important, we make sure it's a good make. Recently, we rebuilt an overheated BRM. The #1 culprit was a Chinese thermostat that didn't open completely until the temp was 212 degrees. All thermostats should be checked prior to install to make sure that on closing, it is cracked open just a smidge at 185 degrees.
The single most likely contributing factor are injector nozzles that are properly set. As we have been accustomed to saying about gasser engines, "No matter how well you build an engine, it won't run without spark plugs." A diesel engine will not run well unless injectors are properly flow rated.
If you don't have injectors that are set well, overfueling will ruin economy and also ruin pistons. Unlike the gassers, too much fuel means pistons that can get melting hot or in performance engines, crack.
We check injectors against VCDS. First, we like the injection timing in the middle of the graph. All fans, A/C, radio and lights should be off for all settings.
We take timed runs; 3rd gear, starting at 35 mph, going to WOT (wide open throttle). When the speedometer hits 40mph, count 3 seconds (stop watch is on almost every cell phone) and make note of speedometer reading. If you start with the I.Q (injection quantity) on a low enough number that the injectors smoke, then raise the I.Q. by doing a 'Hammer Mod', raising the I.Q. point by point until you lose performance. I.E., if raising the IQ too much creates a reduction in speed at 3 second count, revert to the previous I.Q. setting. That will optimize your fueling.
Then, with timing set and IQ set, you can examine the VCDS/ engine module/ Block 13 Idle balance, and Block 15 liter per hour fuel economy (FE).
At idle, the Idle balance numbers should be within .5 mg/ str, or better, .2mg/ str. A good FE number would be between .2 lph to .4 lph(Liter Per Hour). Record your VCDS results. Then, raise the throttle position to 1575 rpm. In order to maintain engine speed, place a proper length stick between the fuel pedal and front of the seat to precisely set rpm. Please lock e-brake.
Now, record the block 13 and block 15 readings. Most commonly, the idle balance will vary substantially from idle, if injectors have not been set for flow rate. FE in block 15 should be on average, 1.8-2.4. Since the second stage on the injectors is now required in order to make the engine run at that speed, (At 1600rpm, the idle balance program is shut down in the ECU. The idle balance numbers will freeze), You can get the flow rating between injectors at this higher engine speed. Since this is where the engine spends almost all its time, getting the block 13 and 15 numbers optimized at 1575rpm, creates best driving results.
At 1575rpm, a negative number for any cylinder in idle balance means less fuel in entering the combustion chamber. Conversely, a positive number means that cylinder is receiving more fuel. The objective it to get each cylinder to get the same fueling by altering the shimming for the top stage. Performing the proper flow per cylinder not only creates overall power improvements, but also FE improvements.
This method of vehicle-tested nozzles is how we set flow rate for our injectors. Instead of emulations, we get actual operation parameters. To give you some idea, for my own nozzles in a 2002 ALH, our most recent tank of fuel, running P764 nozzles(.230um), with Stage II cam, ported head and Stage 4 Rocketchip tune, and driven an average 75-80 mph, we achieved 46 mpg. Yes, we could get better FE if we could just slow down...oh well.