Vauxhall Astra 2000W......I suspect thermostat stuck open or non-existent. Slow to heat up and goes cold quickly. Temp gauge hardly moves above lowest indication. Is it easy enough to check?
It is almost certainly (99.9%) a stuck open thermostat. If the car heats up to normal on idle then, after going for a run cools down (due to cold air cooling the radiator), it's a stuck open thermostat.
I've had this on four/five cars. Twice in the past two years.
easy to check.
start the engine, feel the top hose and the engine . the top hose shouldn't get hot untill the termostat opens, if there's no termostat (or stuck) the top hose will start to warm up quickly if there is one fitted and working, the engine will get hot before the thermo opens and then the top hose warms up.
easy peasy.
doesnt matter what car or engine type (except electric cars !!! )
Bet you get sludgy oil in the winter as well. Check under oil filler.
Temporary cure if you only do short journeys is to partially block off the radiator with cardboard. Be careful though in case it overheats, and be ready for the barrage of people to tell you not to do it. Keep an eye on the guage and remove it quickly if the weather warms up
It works though, as my thermostat has done the same. I'll have it replaced eventually, but I can't afford it at the moment.