V-Belt Drive Wobble: Causes & Solutions | Posts 16+

Hello,

After I replaced my timing belt, I noticed the following:
Before the timing belt replacement, the ribbed pulley/vibration damper element on the crankshaft was not perfectly centered. The fit used falls into the category 'interference fit'.
With the air conditioning turned on, there were slight vibrations in the steering wheel at idle, and the values of the idle speed control fluctuated by up to +/- 0.6 mg/stroke.
The powerbox/fuel injection pump is adjusted to the point of stalling.

Well, here's the bombshell:
After reducing the fit of the ribbed belt pulley using spray paint so that it can be mounted on the timing belt sprocket without any play, i.e., perfectly centered, the following effects are observed:
- Idle fuel consumption values are 0.0 mg/stroke for all cylinders except one, which shows 0.1 mg/stroke.
- Absolutely no more noticeable tendency to idle roughly!

Coincidence? Is it conceivable that a malfunctioning auxiliary power unit could amplify the oscillation of the EDC's control system? Or is it possible that the engine, with its new timing belt and tensioners, is simply running "heavier" and masking the effects mentioned?

Best regards, Rainer.

Hi Rainer,

Rainer K. wrote:

...Is it conceivable that an auxiliary propulsion system that produces eggs could, in fact, stimulate the control system of the EDC to oscillate more strongly?

"Yes, that's also a conceivable cause! I had previously suspected that it might be a "rolling wave" phenomenon (in the EDC), which is triggered by various factors. However, since even certified (Bosch) workshops are unable to determine the cause, it remains just a hypothesis."

Quote:

Or does the engine, with new timing belts and tensioners, simply run a little "heavier" and thereby mask the effects mentioned?

It's possible that this is intentional, and the electronic vibration suppression system is designed to compensate for it. If this additional resistor is missing, it can lead to unwanted oscillations (jerking).

I recently browsed through my collection of 'ams' magazines and found an article from 1982 featuring a fuel efficiency test of an Audi 80 D. The car was in its standard condition and managed to travel from Hamburg to Sicily and beyond on a single tank of fuel, maintaining a constant speed of 60 km/h, resulting in an average fuel consumption of approximately 2.1 liters per 100 km. Not bad, right? My old Golf 1 diesel also used to manage 4-5 liters in mixed driving conditions, with a forward-looking, low-rev driving style, but not slowly.
The Audi 80, back then, had a curb weight of approximately 860 kg, offering the same interior space as a current A4, which can weigh close to 1800 kg (especially with a 2.5 TDI quattro engine and full equipment), and likely consumes more than double the amount of fuel under similar driving conditions.

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