Date: Mon Jan 24 20:19:24 1994
Subject: Engine cuts out momentarily
I have an intermittent problem that someone may be able to help out on:
I have a '72 240Z, triple Weber DCOE40 carbs, stock mechanical fuel pump,
Allison electronic ignition (LED w/ optical pickup), stock distributor. This
set-up has been running well for quite awhile.
Here is the problem: When letting off the gas, the engine frequently cuts out.
It will immediately restart just from the engine turning over due to the car
rolling. The cut outs typically last less than a second. The car idles and
accelerates good. It has only cut out when decelerating. There seems to be
some correlation between the cut outs and the rate at which I let off the gas.
If I ease off slow enough, it will not cut out. Engine speed does not seem to
matter as it will cut out at any RPM.
The problem appears to be electrical since the tach will also drop to zero when
the engine dies telling me that I am not getting any spark. I am suspecting
that there must be some connection between the cut outs and the vacuum advance.
Vacuum is the only thing I have thought of that changes significantly when
letting off the gas. However, I don't understand how this could cause me to
loose spark. I also examined the distributor cap and rotor for damage. None
was apparrent. I replaced the rotor with a spare but the problem persists.
I'm not totally ruling out anything yet (electrical or fuel).
Thanks for any help you can provide,
Steve Walters IZCC #122
[ Vacuum advance is there at part throttle cruise so if it were related
to that, you'd have cutout at steady state cruise too.
Check the grounding of the engine and the ignition system. Hook a
(preferably analog or peak-holding digital) voltmeter between the engine
and the (-) terminal of the battery. Cause the condition and watch
the voltage. Do the same with the ignition grounding lead. Bet you'll
see the ground go away. If not, check the (+) voltage at the ignition
module the same way.
What I suspect is that engine motion is causing an intermittent ground.
Had that happen on my BMW once. The exhaust and engine/tranny assembly
is all rubber mounted. The ground lead between the engine block and
battery had corroded in two. The exhaust system just happened to be
touching the chassis. Going around a right hand turn, the contact would
be lost and the engine would die for just a moment until the centrifugal
forces reduced enough for contact to be restored. This one drove me
batshit! I found it using the above method.