Repair or replace: when is it time to get a "new" car?

The Prufrock household has four cars: two 1997 Honda Accords, and two 240 Volvos—a 1990 and a 1991. One of the Hondas was purchased new, and has 134,000 miles on it; the other has 97,000 miles on it and was purchased from a neighbor for $3500. The two Volvos were purchased on eBay; we bought the 1990 four years ago for $2,500 and the 1991 18 months ago for $1,500. Both have about 250,000 miles on them. Last year, I spent $1,800 putting the neighbor purchased Honda into shape for my daughter—timing belt service, new tires, oil pan gasket, distributor, spark plug wires. I also had a major service on my Honda, to the tune of about $2,300 worth of maintenance. The 1990 needed a new alternator and hoses, to the tune of $800, and the 1991 had fuel pump issues, and a few other things, amounting to about $900 in repairs. All told, we spent about $5,800 in maintenance on 4 cars for 4 drivers.

The cars are older, but the mileage on them is relatively low or reasonable for their age. I drove the 1991 Volvo yesterday, and as I was parking it, the “check engine” light came on. I promised my husband I would take it to get the oil changed, since he neglects such things, and I’m afraid to go out and start the thing because I’m thinking “big car repair bill” like yet another timing belt service.

My question is, how do you know when to give up and replace a car? I can’t afford to buy a newer car, but on the other hand, I feel like these old ones are bleeding us dry. I know the clutch on my Honda is going to go out soon, and I want to make sure I can replace it, so I know I’m looking at $800 in repairs this year just for that.

When does it make more sense to get rid of a car than to fix it?