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3-Series (E21, E30) Chat relating to the BMW 3-Series from 1975-1983 and 1984-1991 line. Specific models: BMW 315, BMW 316, BMW 318, BMW 318i, BMW 320/4, BMW 320i, BMW 320/6, BMW 323i, BMW 320i. E30 Family models include: BMW 325e, BMW 325i, BMW 325is, BMW 325ix.

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Old 01-08-2009, 11:09 PM   #1 (permalink)
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E30 Sudden drop in tachometer, loss of power

Coincidentally i was searching and another guy names alex had a similar problem with his '89 325i...

"Hi all,
On my '89 325i I am experiencing following problem: usually while accelerating, the engine would lose power suddenly for a fraction of a second and then recover. At the same time, tach swiftly drops to zero and immediately recovers. Lights, speedometer, radio, etc work w/o any interuptions. This is certainly annoying if not dangerous.


Now, since the tach behaves this way (drops to zero and recovers), while the car is moving with clutch engaged, I am guessing that the problem isnt with fuel or ignition system components, but instead I am experiencing a computer
glitch. What is the name for the computer that controls the ignition (it ain't OBC, right?)

Any thoughts?

Thanks in advance,
Alex "


anyway... It doesnt happen all of the time, mainly at freeway cruising speeds (75-80mph) usually at steady and constant engine speeds, is it a problem with the ECU? Ignition?
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Old 01-08-2009, 11:44 PM   #2 (permalink)
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Loosely, computers either fail or do not. I'd have a look at the fuel system; filter, and water issues may cause that sort of performance problem.
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Old 01-09-2009, 04:54 AM   #3 (permalink)
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I would check the ground for your ECU or any other ground found on that circuit.
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Old 01-09-2009, 11:47 AM   #4 (permalink)
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i had a the exact same problem with my company work truck (01 chevy s10 auto trans.). driving along at 70 or so the truck would intermittently drop the power, the tach would go screwey and then it'd be fine

a tune up consisting of new wires, plugs, dist. cap, ign. rotor, and an oil change fixed it.

not sure if that will help but that did it for the chevy.
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Old 01-09-2009, 09:16 PM   #5 (permalink)
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Sounds like a bad AFM. Check to make sure your AFM vane opens and closes freely and that the connections to it are good and withour corrosion. When I switched from an m20 to m30 AFM which I later found out to be bad, it exhibited similar problems.
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