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I recently bought a non starting 1990 740 with Regina system and now, following the FAQ and some good help from this board, it works just fine.
Start by removing the fuel injection relay behind the ashtray. Remove the little pocket above the ashtray first. This rectangular relay is on the left and mine was whitish in color. Pry it open with a penknife blade and you will see 2 relays soldered onto a board. Re-solder all the joints by melting them with a soldering iron. A little more solder wont hurt. Make sure it ends up making concave joints not convex joints.
Remove the two relays on the radiator reservoir bottle. Open them and re-solder them too.
One of them runs the a/c external fan and the other provides the injectors with their 12 volt supply. In my case, both were unreliable and had visible solder cracks.
If the car will not start then suspect the crank shaft position sensor. Measure the resistance between the center pin and the two outside pins at the connection at the firewall. If you have open circuit on both sides it probably wont work at all. Mine was open circuit on one side and it still worked. The other side showed 180 ohms.
You will probably also have to clean the throttle body, clean the flame trap and all its hoses, clean out the breather body assembly, replace the small vacuum hoses on the inlet manifold. Check the hose leading to the fuel pressure regulator. If it smells strongly of gas the regulator is suspect.
With the engine running, removing this hose will result in a slight idle increase. Pay particular attention to the vacuum hose going to the MAP sensor attached to the strut behind the driver side shock tower. Make sure the vacuum elbows are not leaking. This is very important!! Its what maked the mixture right while the engine is running by sensing manifold pressure and reduction.
After doing all of these things, as well as applying Deoxit to all the connectors, I now have an engine that starts instantly and reliably and idles cleanly.
Before I started, it would start maybe, then refuse for hours. When I was able to get it started the idle would hunt horribly and cause stalling.
All of that is now gone. Total cost was about $10 for cleaning products etc but it would not hurt to replace old relays and replace the crankshaft position sensor. They will give you grief !!
good luck,
Steve
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