First, check that the air was bled out of your system when they replaced the water pump and radiator.
Then as your car warms up, check the temp of your upper and lower radiator hoses. Both should be hot. If one is and the other is not, then your thermostat is stuck closed.
If that checks out good, then I'd say you have a blown head gasket. Sometimes you'll see signs of water mixing with your oil and leaking out of your tail pipe, sometimes you won't.
What will happen is, exhaust gas is pushing its way past a small leak in the gasket and into your coolant. The hot bubbles eventually get trapped in the highest points in the heater core and radiator, which creates a lock that your water pump can't push past.
A quick way to test for this is to remove the radiator cap (or overflow tank cap if your radiator doesn't have one), and while someone revs your engine to at least 1,500 rpm, look at the coolant. If theres large bubbles (same time as engine revs), then that is the exhaust gas.
DO NOT have this gasket replaced. That would be like giving a liver transplant to a drug addict.
Buy a rebuilt or used motor with under 60,000 miles on it. You'll only spend a few hundred to a thousand more than you'd spend on labor anyway. And this motor will last many years as long as you take care of it.
Friday, September 10th, 2010 AT 5:52 AM