I wouldn't drive it. The engine is telling you it has problems and continued operation will inevitably lead to further damage.
The poor fuel economy happens with a bad/failing injector. Get a code reader and find out the codes your engine is throwing. With the problems you're describing I suspect more than one injector and the FICM will show up. The FICM could be bad and giving a false report on the injectors perhaps but you have a historic injector issue. Swamps Diesel has a rebuild program (Swamp's Webstore - Custom Electronics
) for $570 which is better than the $700+ from Ford. You can check the voltage on the FICM with a multimeter - you don't need an AE or other code reader. There are quite a few threads on FICM voltage checking but here is the Swamps' site how-to:
In re-reading this I also wonder about your turbo after such a long period of no usage. Have you been watching the boost gauge?
The stock gauges are really poor quality too. You should get an OBDII tool (like the AE if you can stand having a laptop connected full time or an Edge Insight or equivalent otherwise) to monitor:
Engine Coolant Temperature (ECT)
Engine Oil Temperature (EOT)
Transmission Fluid Temperature (TFT)
The stock boost gauge is fine but the stock coolant and transmission temperature gauges are not accurate enough and there is NO oil temperature just a pressure gauge.
You also need a Fuel Pressure (FP) gauge...not available through the OBDII tools without a sending unit installation.
A gauge for Exhaust Gas Temperature (EGT) is helpful but requires the installation of a thermocouple to read this value - easy enough install and the OBDII tools may read this input like the FP sending unit.
You should get a coolant filter too and change out the coolant with Extended Life Coolant (ELC) rated EC-1 or better. Both of these modifications are something that International (Navistar) did for their VT365 engine which is almost exactly the same as the Ford 6.0L which International also built.
I know that with repairs looming it hurts to hear that you need to make additional purchases but the gauges, coolant filter, and EC-1 coolant will prevent most future problems.
The fuel pressure gauge especially in your case is relevant as running out of fuel often contributes to fuel injector failure as does a FICM with low voltage which often follows from old worn out batteries.
The 6.0L is a hard-working engine and very respectable but without these modifications they just aren't forgiving if the previously mentioned symptoms manifest themselves.
1. Read codes from engine using an OBDII tool (AE is a great tool) and address the faults reported...FICM replacement from Swamp's Diesel Performance
if necessary and new injectors (not rebuilds from the least expensive site you can find). Use reputable vendors or a trusted Ford dealer for assured quality parts.
2. Order gauges (like Insight or equivalent...not to tune though...OR analog gauges for EOT/ECT/TFT/FP/EGT monitoring)
3. Install coolant filter (www.DieselSite.com
has a great one with all parts and easy instructions) http://www.dieselsite.com/index.asp?...ROD&ProdID=324
4. Flush and exchange your coolant with EC-1 quality ELC
I suspect you are looking at some expenses but this is still less expensive than an engine rebuild. With these modifications and repairs you can have more confidence in the continued operation of your Ford 6.0L PSD.