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2.0 TDI engine resonance

Regarding the 2009 vs 2010 Jetta TDI, keep in mind that VW changed the MT gearing in 2010—made it a lot taller in every gear except first. VW was on a quest to try to up the EPA test numbers (they got one whole additional MPG on the highway cycle!), and they may have changed other engine control parameters as well, as evidenced by increased problems with stalling on launch, oddball random power drops, etc.

I have a 2010 Jetta TDI Sedan, manual transmission, manufactured 8/10. About 2500 miles on it. The engine has a resonance starting at about 1050 and through to about 1300 rpm. This appears to be a true resonance; it is revealed through proper and gradual acceleration through the gears. I have not noticed any other resonances up through rpm, which is as fast as I have spun it to date—and steady cruise on the highway, at that. I rarely need to get it over 1800 or so in normal acceleration. The engine revs smoothly from about 1400 rpm or so, up.

If you drive this car for maximum fuel economy, you will inevitably find yourself in city traffic at steady cruise right in the resonance band. You can downshift to raise rpm out of this resonance point, but even after carefully modulating the throttle, you will be using 10-15% more fuel in the lower gear--most of the time. As is true of most resonance bands, increasing engine loading in the band increases the audible volume of the resonance; i.e., trying to modestly accelerate out of it without downshifting. And, when the engine and hence the engine mounts get warm, the resonance is worse since warm mounts transmit more of the vibration into the body.

There is a stretch of road I drive regularly, 45mph speed limit strictly enforced, with a slight grade for a mile. In 6th, I'm right on the resonance peak at 1200 rpm. If I downshift to 5th, I'm out of the resonance but my fuel economy drops from 45 to 38, even after carefully feathering the throttle post downshift.

Now, there are occasions where downshifting into 5th from 6th to avoid the resonance point will yield BETTER economy, but that is usually when you are below 1000 rpm in 6th. By and large with this engine you invoke significantly more fuel use at steady speed/light throttle in the lower gear at 1400+ rpm, then in 6th at 1100-1200 rpm.

Even my wife, who drives more 'normally' than I, complains of hitting the resonance too often in city traffic, and having to shift around it.

Ironically, this 2.0 TDI engine reminds me of the old 1.6 engine (c.1979 to 1992), which had no balance shafts. The 1.6 had a resonance around 1500 rpm that made 5th gear at a steady 35mph very unpleasant, even though the engine otherwise was efficient at that speed, and could even accelerate modestly from there without protest, without downshifting.

With the next diesel after the 1.6, the first 'TDI', the 1.9 VE engine, VW eliminated all resonance points through at least 3500rpm (I've never driven my 02 Golf beyond that). No resonance, none, nada.

But, VW has managed to reintroduce resonance with the 2.0 TDI.

I test drove a 2011 Golf TDI recently, and it had the same resonance, though it seemed very slightly more muted in the Golf.

I’m not even going to approach the dealer with the problem in my Jetta; I know he will say it is normal and blow me off. Been there, done that with other issues on VW cars, too many times. That’s why US VW service ranks at the very bottom of the barrel. And anyway, I suspect it IS normal.

We can only hope that the next 4-cylinder TDI, purportedly in development to meet the coming super stringent Euro emissions, will once again be resonance free. It can be done...the very first TDI is 'living' proof!

In the meantime, I believe we are stuck with a lapse in VW/Audi engineering of the first order--a resonance in a balance-shafted engine that thwarts running the diesel at optimum economy in light-load conditions—and makes acceleration through the gears for the ‘normal’ drivers less than completely joyful—as it could be otherwise.
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