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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 severe.

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 engine redline.

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 acceleration problem.

Another victory for lab scope testing!
By Dave Tidaback, autoline European import specialist (23 years experience)

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