Propane fumigation systems have been used to convert EFI engines but fumigation conversions work best on carbureted systems. We have propane injection conversions for TBI engines and these systems work very well. They do not generate CELs and propane operation is invisible to the driver.
Fumigation conversions were also used on MPFI engines before propane injection systems were readily available. However, they never worked very well and were often prone to intake manifold backfires. Impco-based fumigation conversions also relied on Dual Curve for "fixes" that allowed the vehicle to run properly and not have CELs but their products are now obsolete. Although propane injection conversions are slightly more expensive than fumigation conversions were, injection conversions offer better fuel economy and reduced maintenance, which have a faster payback in the long-run. Fumigation systems also had difficulty meeting emission regulations.