Too much or too little detergent to avoid washing machine smells?

Well, There is an write-up called ” smelly washing machine” somewhere on this site. But I will tell you what I do and a few ins and outs which I will keep simple ( because if you delved into all this it really isn`t! ). My machine is a Hotpoint WM56 which is second hand and is over 20 years old and has gone wrong twice – since I installed it new for the previous owner, who passed away, all those years ago I now its history 100{e5d1b7155a01ef1f3b9c9968eaba33524ee81600d00d4be2b4d93ac2e58cec2d}. There are two issues 1) lime / snowy white chalk which may cause problems but smelling is not one of them and 2) bacteria ridden sludge which is the issue with causing smells. My washing gets mega dirty crawling around repairing w/machines every day so I overdose the machine using Surf powder. It does seem that bio liquid causes a fair few problems reference smells but it has more to do with, as with the powder version, the low temperatures recommended as per daft TV commercials based on the rather idiotic idea that you save money which you do not especially if it nukes your machine prematurally.
“I believe liquid detergents are the worst offenders, so presumably it’s better not to use too much liquid detergent -or- would that fail to dissolve fatty dirt and actually create residues?” So, you can say yes you are right if the detergent is enzyme based meant for low temperature liquid but that might not be true if its a liquid old fashioned ( but good ) non bio liquid with a bleach based detergent of a decent make. It is a combination of heat ( or the lack of it ) and detergent that disperses the fat / dirt deposits. Actually, in reality, the wash cycle loosens the offending dirt / grease and the series of rinsing actually washes the clothes, not necessarily what you may have thought. So, for coloureds most synthetics are washed at 40 degrees using a bio powder and cotton white towels and tea towels at 60 degrees or 60 degrees + using a bleach based powder detergent. All I can and will say is 1) all my work clobber is washed on programme F which is a 50 degree cycle that takes 1 hour finishing off with a 900 short spin. 2) Anything white in the bed linen department that my lodger uses, and this is where things get a bit complicated, she sets on Programme D which is hot fill only with the economy button IN = the machine will wash at 60, 1100 spin OR she sometimes uses D with the the ecomomy button OUT = the machine still fills up with hot water but washes with what it gets given = no heating, 1100 spin. It all depends whether the boilers been on that particular day or not as the case may be. Either way all the washing always comes out just dandy using Surf powder whether at 50 or a possible 60. We do not faff about using non bio higher temperature powder for the whites, where she sometimes adds 2 cap fulls of bleach ( which is theory should kill the enzyms but I can tell you that it all seems to work out very well ).
“Does the same apply to washing powder? I have the impression some of the ingredients in a powder actually dissolves residues (as used in a maintenence wash) but would slightly too much end up contributing to residues?” Does not seem to make much odds but a good point since a Maintenance wash ( m/wash ) is done without clothes in the machine. And, there is m/washes which are a specific programme on some machines that tumble at 80 rpm inside of normal tumble speed that is about 55 rpm what machines do and don`t no idea without looking it all up!

The bottom line: This is what I have done on my machine for the last 14 years since I got my paws on it to prevent smells. I have hard water so every 8 weeks this is what I do: I set the machine on D for 60 degrees I use 1 box ( 250 grams ) of Aqua Softna ( trade name ) in the soap draw with 3 CAP ( NOT cups!!! ) fulls of liquid lemon Flash ( 2 quid a large bottle that lasts a year ) and start the machine. 4 years ago the bearing oil seal sounded like a mouse – a precurser of oil seal failure = then bearing failure so decided to dismantle the machine to change offending seal so avoiding any future trouble, point is the machine was and still is spotless except the spider lattice which did have a bit of residue powder but it is no big deal. So, machine is 22 years old, more or less internally spotless, gone ” wrong ” twice, oil seal, and the usually WM failure point: the curse of the top tank springs. I finally have to day that when I first got the machine I carried out 2 modications. A change to a more reliable motor Fhp motor and a tweek to the water level pressure switch to increase the water level on wash up by 6 liters so it washes better, indeed it does!

To add on a re-read. And if, by how you`ve written it, you are using 400 grams of washing soda crystals instead of detergent….don`t. Also Soda crystals acts as a water softenering effect which is why you are getting so much froth and adding Flash is going to be asking for trouble combined so in your case don`t do that either. I do wish people would tell the full story at the very beginning, so full of surprises. All the best to you.