I reckon you get the tensioner sorted out first. Whilst waiting on that, put the battery on a trickle charge so it has a full charge when ready. Then with the battery installed, have it on the trickle charger, scan the car, saved the codes, clear the codes, start it and take for a drive. Scan it again when you get back to see what resurfaces.
They may be old, but there is still a lot of electrics to drain the battery. Fingers crossed that it's only due to a low battery that you have the misfires, maybe due to fuel pumps not running at full power.
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2001 A8 D2 FL 3.7 Q - LPG Conversion, Engine: AKC, Gearbox:FBB, Colour:LY5X Aqua Blue pearl effect, Interior:WJ light beige/dark beige valcona leather with burr walnut inserts
My DIY jobs - Replaced alarm horn, Replaced Coolant Temp sensor, Updated RNS-D firmware, Installed Reversing Camera and Digital TV, Dashcam Installation, Retrofitted Rear Blinds, Auto-Dimming Rear View Mirror, Chrome Boot Struts
Sold!  2008 S8 D3 5.2 V10 - Engine: BSM, Gearbox:KLW, Colour:LY7F Suzuka Grey Pearl Effect, Interior:Black/Black with Silver/Black valcona leather with carbon atlas inserts, Bang & Olufsen, Heated windscreen, Soft-close doors, TV, DAB, AMI, Quad Zone, Auto Boot, Auto dim Mirrors, Keyless entry, PDC with rearview camera, AFS II Headlights
My DIY jobs - Fan Jet Windscreen Washers, Retrofitted Electric Side Blinds
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