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i have VW jetta 2.5 (manual) when i go uphill slowly in to second gear or in reverse, without throttle. due to very low engine speed (probably 150-350 rpm). i get weird knocking/pinging sound. is it detonation? could damage engine by this way ?
You shouldn't be letting your engine labor at such a low RPM.
Keep the revs above idle, probably > 1000 RPM.
If you need to drive that slowly, drop down a gear.
Yes, you could be causing damage to the pistons, conrods and crank bearings.