On 4/8/2013, Jim Marino in Baltimore, MD, USA wrote:
"I took my MGA out yesterday, and everything was running great until the car died a few miles from destination. Cooling system was relatively empty and coolant was all over engine bay. When I filled up the system with water, it was just streaming out the water pump weep hole. Seems the seal completely failed. There were no signs of water dripping when I started my journey, and I was trying to find out if these seals go that bad that quickly. I'd like to know if something else was the cause. I also hope I didn't damage the engine, though it seemed to run fine for the short trip home after it had cooled down".

Most likely there is no damage to your engine. Just install a new water pump and drive on. These old cast iron lumps are pretty robust. If it was steaming when you shut it off, that means there was still some water in it. The water pump takes water from bottom of radiator and returns it to top of radiator. As long as the engine is running at road speed, the water circulates with the cooling jacket full of water.

If it leaks, the upper part of the radiator may have no water. As water level drops the coolant temperature may be expected to rise. As long as it has the pressure cap and doesn't boil, this can go on until water level in the radiator is quite low, even below the level of the water pump, and it still circulates with the engine cooling jacket full of water. When water level is that low, and you switch off or come to a dead idle, water can drain backward from the block back into the lower radiator. Then the cooling jacket may be suddenly full of air, and the water pump impeller may be running in air with no possible circulation, and you have almost instant overheating of the engine (the cylinder head in particular).

As long as you shut it down immediately on first indication of steam it will be okay. You should let it cool for 10 to 20 minutes before adding cold water. If it boils and steams while adding water you haven't let it cool sufficiently yet. Cylinder head temperature around the exhaust valves should be below 212dF before introducing cool water. If you recklessly pour cold water into a scalding hot engine you run the risk of cracking the cylinder head. If you do not want to wait to refill, then just trickle the water in slowly, and wait patiently while it steams and boils off and stops boiling before you add a little more water. You would be trickling a little water into the cylinder head to gradually cool it down until it will be below 212dF before you pour in the bulk of new cool water.

When you get a new water pump installed and get it running again, keep your eye on the temperature gauge for a while. Overheating the thermostat in a hot cylinder head can render the thermostat non-functional. If it fails in the open condition the engine will run too cool. If it fails in the close condition (much more common) the engine will overheat as soon as it is up to running temperature. A thermostat is cheap, so you may want to buy a spare when you are ordering the new water pump. Otherwise it is a standard thermostat available at any local auto parts store (think small block Chevy V8).

Water pumps have a couple different failure modes. Most commonly the bearings wear out first, and you can wiggle the fan blade and shaft while it still does not leak. It may make bearing noises for a long time before catastrophic bearing failure. There is usually plenty of warning to get you to change the water pump before it loses the coolant.

Sometimes the seal fails first. In this case the water should leak out of the weep hole which is behind the shaft bearings. It is good if you can notice a drip on the floor when hot or lowering of coolant level in the radiator needing a periodic refill. This usually comes on slow and gets gradually worse, and you should change the water pump before it loses enough water to overheat. I think it is rare for the seal to fail suddenly.

However, if it has been leaking for a while without noticing it, and you are driving with low water level, the overheating condition might come on suddenly, especially when you slow down form road speed to engine idle speed. When it does start blowing steam it will be at the pressure limit of the radiator pressure relief cap (most likely 7 PSI). This much pressure will push the fluid out of any leak location, so leaking at the water pump weep hole is to be expected if the seal failed. Perhaps you were lucky, and the leaking fluid sprayed the ignition system to kill the engine before it was too badly overheated.

By the way, 160dF is too low for normal engine running temperature. At that temperature the fuel doesn't vaporize well or burn completely, so it needs slightly richer fuel mixture. Aside from lower fuel economy and the heater not working in cold weather, the unburned fuel washes oil off the cylinder wall to accelerate wear of the cylinder wall and piston rings. The excess fuel can also sneak past the piston rings to dilute the engine oil, which may eventually cause crankshaft bearing and camshaft failure. I recommend installing a 180dF thermostat (minimum) to control minimum running temperature.

As a side note, MGA with original cell core style radiator might run 160dF with normal mild touring (and a low temperature thermostat or no thermostat). Many replacement radiators with VT (vertical tube) cores will not do such a good job of cooling, and may be expected to run about 190dF even with the thermostat wide open with unregulated flow.