12th Aug 2019, 07:16
Plastic use is prevalent now, even with Japanese engines. But when anything new is done for the first time, you can expect problems. As you mentioned - BMW plastic impeller water pumps. But once sussed out they can do well. My last car, a 4-cylinder 2001 BMW hatch, which I kept until it was 19 years old, had a water pump with impeller made not of plastic, but an improved composite material. I have been waiting for it to leak for years, never did. Finally as a precaution, I replaced it together with the thermostat (and plastic housing), upper and lower hoses - all original from the factory. The water pump and its composite impeller were still in very good shape.
As for aluminum alloy engines - you are correct, they are very sensitive to overheating. But you can't use iron anymore - modern engines need to be run hot enough to comply with emission laws, and alloys are better at handling heat, but they need to be proactively maintained with the right coolant, and making sure that the owner keeps tabs on any leaks (in hoses, for example). Plastics are unfortunately here to stay, but periodically it doesn't hurt to replace some of these parts, like elbows for coolant hoses, during routine maintenance.
10th Aug 2019, 21:32
What if you go proactive and simply replace with all new plastic parts as a precaution at say 50.000 miles. That’s if the car is not a rust bucket (a concern on the east coast). It’s easy enough to go on Amazon, Rock Auto or a similar site. I bought a new Delco wiring harness for my Chevelle. Next day it was at my front door. Had 3 of my car friends over and knocked it out. You don’t even need to leave your home anymore. Not sure if these parts are that easy to find, but I give all the information and most always hear back shortly. I don’t buy parts that are of questionable origin as quality is always a concern. With YouTube videos and a free weekend, there’s many topics to try yourself. I replace parts that are still functional, but my concern is age. In the case of the new wiring harness, it was fire. If you have a review car like this but very low miles, it’s usually a smart move. My car was built in Van Nuys California and spent most of its life on the west coast, so didn’t have to deal with rust. If yours is, try to find a better one.