Q: I have a 2002 Ford Ranger with 218,000 miles. Lately, after my truck is warmed up and I shut it off, it builds pressure in the overflow reservoir and bubbles out of the lid. At first, I thought it was a bad lid, so I got a replacement, and it still did the same thing. It's not overheating or doing this when the vehicle is running; it's just after I shut it off. I figured maybe the thermostat was bad, so I replaced that, and two months prior, I also replaced the radiator and water pump. I have to fill it up with water every time before I go anywhere. I could really use some help.
I'm assuming the reservoir itself is not cracked and leaking. Otherwise, going by the pressure coming out in the expansion tank (reservoir), and the high mileage, the first thing to check is the cylinder head gasket. There are a couple of ways to do this. On a severe head gasket leak, the simplest is to take a good whiff of the coolant in the expansion tank. If you get a strong smell of hydrocarbons (exhaust), the head gasket is blown and is injecting combustion pressure into the cooling system. The same thing can be accomplished with an exhaust-gas analyzer. Stick the probe in the tank, and look for a notable hydrocarbon reading.
But the most conclusive test, which I've modified over the years, is pressure testing each cylinder. This involves topping off the radiator with coolant and removing all spark plugs, and one at a time positioning each cylinder's piston at top dead center of the compression stroke. Then compressed air is applied to each cylinder with a hose mounted into the spark plug hole. Shop compressed air is about 150 psi, and cranking cylinder pressure is about the same. At TDC, the valves are closed, the cylinder is sealed, and the air pressure will penetrate any small openings in the head gasket into the cooling system. At this point, take a look at the topped-off radiator. If the coolant level rises at all, or you see bubbles coming up, the head gasket is blown at that specific cylinder.
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