There is an engine which is a bit underpowered when idling, it can not even keep the car moving on a flat road when in gear and idling at 700..800 RPM - see link.
What can be done to improve its torque at low RPMs?
I am looking for a list of things to check, each of which can help a tiny bit, and maybe together they help enough. For example things like cleaning/changing the air filter? What maintenance/repair can be done to the engine itself (besides changing oil) that might improve the torque at low RPM?
The engine in question is an old Mercedes-Benz OM601.940 4-cylinder 58 kW (79 hp) 2299 dm3 diesel engine from year 1992. It's not a turbodiesel! - it's a classic plain naturally-aspirated diesel engine (neither turbocharged nor supercharged).
I am not looking for a way to make a huge improvement, just a checklist of maintenance things which if not done can cause the engine deliver less torque then when it was new.