Vw polo 1 litre has idling problems it keeps reving to around 800 rpm then droping to around 400/500 all the time engine being cold or warm makes no difference. It's a w reg with 50000 miles

run the fault codes
turn off the ac compressor
idle should be around 950 for auto

if it is a turbo diesel (TDI) I suspect that the injectors need attention
if they are squirting , dribbling or are not atomizing as required then the diesel may be igniting before ignition time causing piston slap (the rattle)
Take it to a diesel specialist shop for a diagnosis and quote

you may have to remove the + side of the battery and leave it off for about a minute or so and replace to resetting the computer to the new parts you put on it. Believe me it makes all the differents in the cars nowadays Also remove the cables when ever doing that kind of work on your car because you could fry the new part just doing it with the battery still hooked up...just letting you know that is all

The first thing you need down is to have the engine scanned for fault codes stored in the control computer, do an active scan engine running and a passive scan.

You need to check the cable the runs from the gas pedal to the engine, it might be rusty and need some drops of penetrating oil at the point it meets with the engine accelerator valve.

try cleaning your battery terminals and tightening them. if that doesnt solve the problem your battery or alternator may be dying, some auto parts stores will do a charging system check for free that will tell you if one or the other is bad.

wear r the sheer bolts on a vw polo 1;0 2001

you have a bad or dirty idle air control valve...It controls the air flow to the engine when at idle or when you release the accelerator pedal... when you press the pedal down it closes and when you release the pedal it opens and it is not opening fast enough to allow the car to breath causing it to stall

100% sure it shows one of your cylinder compression is weak, please do a compression test to isolate the problem and get the head/pistons checked . This is a known problem with VW/Skoda 1.2 3 cylinder mills..

Hello, VW polos 1.0, 1.4. 1.6, suffer from carbon bulid up in the throttle bodys causing bad idling, esp the Bosch Mono-Motronic (Throttle Body Injection systems), a good clean with carb cleaner should repair the problem, also engine temp sensor replacement and idle speed control valve replacement can help idle problems.