Although I have not done a TBI I have done a few dozen conversions over the 4 years I worked at a conversion shop. I do not know what the laws are where you are, but here the conversion must be done at a licensed conversion shop by a licensed conversion tech. Also here any conversion now must use a dedicated ECM just for the propane. I was not involved in conversions at the time this requirement came into effect here.
From what I know about conversions going dual fuel as you suggest, you are creating some very significant compromises. For example, propane like a cooler thermostat, much more initial timing advance but the same total advance, much higher higher compression (around 13.0), and a cold intake manifold.
Using the dedicated propane ECM will take care of the timing, but everything else is a compromise. So do you want the best performance and gas mileage out of gasoline, or propane. With a compromise you will get neither. A dedicated propane engine will get about 10% less MPG than it did on gasoline, but it will produce about the same power. A dual fuel conversion running on propane will get about 20% less MPG and make less power.
Combining 2 fuel systems means maintaining 2 fuel systems. If you need to pass emmissions this can be a challenge as well as more cost.
Things are also now much more complicated as far as reliability and maintenance goes.
You are also now carrying more weight. Propane weighs 1 lb per liter. A 100 liter tank, about all that will fit under the truck, will weigh about 160 lbs when full.
On the topic of fitting tanks, all compressed gas fuel tanks must be cylindrical, not the most space efficient. Remember also that you can only fill the tank to a maximum of 80% capacity.
I know it sounds good that if you run out of propane you can just keep going on gasoline, but besides the above mentioned compromises you also need to know that fuel injectors are cooled by the gasoline running through them. While running on propane you are cooking the gas in the injectors, be prepared to service them often. Also if you are running mostly on propane your gasoline is getting stale, read less MPG and power and increased injector service again.
Long post but hopefully I've answered most of the questions you would be asking.
Thanks for your comments, sure is useful talking with a guy with experience.
Ok, in regards to legality (emissions) I don't think its an issue here (State of New Mexico). What I don't think you need is an ECM to make it efficient. Of course there's always a state-of -the-art way of doing things but...older carburated cars didn't need it and most Propane vehicles still don't use it, here's why, because LPG is a clean fuel by nature and has clean combustion itself and the few adjustments you can do on the evaporator can fine tune the carburation (or combustion mix). If you measure emissions on a badly carburated Propane engine, I'm sure you won't get even 20% of a what you get with a gasoline operated car, no matter how good this gasoline system is.
So, I know the government is always trying to get more money and keeps inventing ways to charge more on everything and creates more and more regulations but just tell me, what's the case of penalizing those who operate cleaner burning fuels by imposing more fees and requirements? A propane car is already a virtual zero-polluting vehicle and the Gov should give stimulus instead of obstacles. Okay but thats just my rant on the issue.
Now in what refers to timing, I haven't really looked into it, good that you point it out, I don't know what you mean by "initial" timing advance, do you mean at cold start? or "initial" meaning at slow speed (even when hot). If the latter is true, it could be an issue. My idea is that the current MAP will keep on doing the job (timing advance) since I don't see a reason to change it. The MAP works with the manifold pressure and that determines the advance.
What I think could help is changing the performance chip (if that is possible) to a custom programmed chip suitable for LPG. If I can't get it, then its still ok, as far as I can see, the current ECM with current chip should do the job not too bad.
All the other sensors don't have to be a problem but would need to experiment or get some advice (IAC, temp, Oxigen). In regards to the TPS, since there will be no gasoline feed but still as the throttle opens more fuel will feed, (vacuum increases) it should act similar as before. (Am I ok on this?).
Thermostat, can use a colder one, shouldn't affect gasoline performance much anyway.
TBI injectors cooked...
In regards to the injectors (TBI, remember) they are way up there, LPG will not pass through them (they are designed for liquid not gas) and still being dry most of the time is not very good but I don't think that will affect too much. Will have to experiment and see. Now I would love to hear if someone has experimented using the actual TBI injectors to inject LPG, but thats another issue.
Clarifying, I didn't plan to use the actual TBI injectors with LPG but rather use the LPG system as a parallel system in the truck and only using what can be used as common components.
What I'm planning is to inject the LPG directly into the intake Mani or it could be with a carb adapter. . .actually this issue is what mostly I need help with and would like your opinions.
If everything else fails...
If I can't get it to work satisfactorily with Propane and the current ECM system I can look forward to replace the TBI with a mechanical carburator and change the distributor to a vacuum one...yes, go back a bit to the stone age but, keeping in mind that I will keep it in good tunned shape and use it occasionally, but I'm hoping I won't have to go that far...
keep up the good feedback, Thanks