Background:
'87 Ford Bronco II, 4x4, 4WD, not extended cab, 2.9L
Distributor ignition, multi-port injectors
2 fuel pumps (pre-pump in tank, main in frame, both are new)
New ignition module (2nd new one)
Car runs for 30 - 58 seconds and then looses computer pulse to injectors, does not lose ignition spark or fuel pressure
Any ideas what is causing the loss of computer pulse?
Additional details:
Cold start requires priming of engine
main power relay is new (brown)2nd new one
fuel pump relay is new (green)2nd new one
While engine runs, just before stalling, if we turn the key off and then on again, the engine will continue to run, so long as the key is constantly cycled
Power is provided to the injectors from a common source and the injectors are grounded through the ECM, which makes and breaks the ground to operate the injectors.
If power to the injectors is constant and not being lost (not likely), then my guess is the problem is the ignition module or a wire harness problem between the module and ECM.
This vehicle uses the infamous TFI-IV module which is prone to failure and this module is what provides a signal to the ECM, which then triggers the injectors.
Heat failure is what kills them, but you might consider pulling the module and having someone like AutoZone hook it up on their tester. The usual failures may not show up on a test but in your particular case a problem may actually appear.
You can also try pulling the codes as I think there are a few codes that may be retained in memory for the PIP and SPOUT signals which the module provides to the ECM. An inexpensive code reader or VOM can be used for this. Hope that helps.
Check the ECM ground. Behind the right kick pannel next to the ECM. Also try bypassing the inertia switch located on the right side of the trans tunnel under the edge of the carpet.
~Michael