Wanted: Motronic Guru

[BOBINYELM@xxxxxxx]

I recently acquired an '84 733i that has a strange gremlin in the F.I. If you
rev the engine (in neutral), it accelerates fine, but if you let off the
acclerator at all, the engine dies (fuel injectors lose their grounding signal
from the computer as verified by noid lights plugged into the injector
harness) until the rpms decrease to 1500 rpm or less.

A relay mounted externally on the fuse box (the most forward one with
green/yellow, green/brown wires on it) can be felt (if one keeps his fingers
on it) to switch when the accelerator is relaxed and to switch again when the
revs decay to below 1500 rpm, corresponding to the situation mentioned in the
first paragraph. The same relay can be felt to switch on/off right at the
point the throttle is first opened gently the slightest bit. To eliminate the
idle/full throttle switch from the equation, I unplugged the switch on the
throttle body (which senses idle), and it made no difference to the relay's
switching. If the same relay is unplugged from its socket the engine dies when
it hits 1500 rpm (again the injectors lose their ground signal), but catches
again as soon as the rpms decay below 1500 rpm. It is interesting to note that
the first two accelerations after the engine is started are without this
problem, but then it comes back. During all of these situations the spark
keeps firing and the computer is still receiving power.

I have checked the pulse and speed sensors (as well as switched them from one
function to the other), the temp sensor, confirmed the harness continuity, and
swapped the computer and air flow meter with my '84 633CSi with no effect.

My manual is the 5 series Bently, which is great, but the 6/7 series have
different fuse/relay configurations which makes troubleshooting difficult. On
these 6/7 models the F.I. power relay is mounted on the right firewall, the
fuel pump relay is mounted aft on the fuse box leaving two mystery relays
forward of the fuel pump relay. The center one is a rectangular low profile
"potted" unit with no markings, and the forward one is the standard four prong
square type (this is the one I feel switching). Anyone with suggestions would
be most welcome to offer same before the car mysteriously catches fire :<).

Thanks,
Bob

'84 633CSi
'84 733i (living on borrowed time)
'82 B9 Alpina

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