Hey all,
My 89 740 n/t just melted it's pistons and I just pulled the engine and put in one from a 91 740. I could get the old engine to run with really bad compression before I pulled it. The 91' engine turns over fine but won't start. The plugs are firing correctly. The injectors are also firing. Not sure what is the deal. Does anybody know by chance any compatibilty problems or timing or sensors different between the two engines. I kept all the 91' engine sensors on it. I am using though the 89 wiring harness. It looks as though the same b230f engine and I am sure it has to be something electrical. The newer engine might have been sitting for a while though ~8 months. Every once and a while it sounds like it fires but can't get it going for more than a second.
Thanks in advance
Benajmin

I have done exactly the same installation, installing a 90 B230FT into a 1987 745T. If the plugs are firing, you did correctly use the early distributor from the LH 2.2 engine. I also assume that you correctly used the Idle Air Controller from the 1989 car. You also need to use the throttle body from the 1989 car. The LH 2.4 throttle body from the 1990 has no idle speed adjustment screw, but that should not prevent the car from starting. The Coolant Temperature Sensor from the LH 2.4 car substitutes just fine on the LH 2.2 system, but not vice-versa. Did you check the cam timing? How about induction leaks? You did use the original Air Mass Meter, right? You might want to check the resistance of the Coolant Temperature Sensor at the ECU.

Most importantly make sure that you don't have the wiring connectors for the knock sensor and the coolant temperature sensor exchanged. This mistake is easy to make on an engine conversion and the car will usually not start, and for sure won't start when warm. Knock sensor wires are green and the Coolant Temperature Sensor wires are gray and black. Also check the air control valve and throttle switch connectors. Air control valve wires are green, yellow/red, and gray/red. Throttle switch connector wires are gray and brown.