TechTips is a monthly feature in AutoInc. Each month,
specialists who field calls for autoline Telediagnosis., a
technical hot line service, share some of the most common
vehicle problems and their solutions.
Motronic engine with flat spot in acceleration Vehicle:
1983-1987 BMW with Motronic engine management problem:
hesitation or "flat spot" on acceleration.
The Motronic engine management system integrates both
ignition and fuel control functions into a single control unit. It
contains a multidimensional "map" of timing advance and
injector duration values based upon engine speed, load,
temperature and throttle position signals. If any of these
inputs is inaccurate or missing, the effect on driveability can be
A case in point is that of a 1986 BMW 325e with a very bad "flat spot"
in mid- range acceleration. This car would
accelerate normally to 3500 rpm in any gear, then simply refuse to go
any faster. Routine testing of inputs from throttle
switch, air flow meter, coolant temperature sensor and engine speed
sensor showed no abnormalities. Fuel pressure
and delivery volume were adequate. If the engine could be coaxed past
the flat spot between approximately 3500 and
3800 rpm by feathering the throttle or by downshifting to a lower gear,
the car would then accelerate normally to
When I questioned the technician working on the car in order to
double-check his test results, I found that he had
checked engine speed sensor output by testing AC voltage at the
disconnected sensor plug, and had approximately
1.5 VAC at a fast cranking speed. When I asked if he had a lab
scope available to test the speed sensor, and when it
was found that he had, I examined the output waveform he faxed
to me. It showed a nearly normal sine wave at
cranking speed, with an occasional "jag" in an otherwise smooth
pattern. As speed was increased, the pattern
remained essentially the same until 3500 rpm, when it dissolved
into an almost unrecognizable "hash" of jagged peaks.
Checking service records showed that the car had a starter
replaced for poor engagement and grinding at about the
same time that the driveability problem was first noted.
Examination of the flywheel showed that the ends of three ring
gear teeth were ground off by the poor starter engagement.
Replacement of the flywheel completely cured the midrange
Another victory for lab scope testing!
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By Dave Tidaback, autoline European import specialist (23 years experience)