Well, as this question was originally asked in reference to an entirely different type of car, I have reviewed it and changed a few names to protect the innocent, so now it is very generic and should apply to nearly any engine (at least piston engines), including our beloved MGs. The original inquiry also related to finding water in the oil, and because the answer has a lot to do with checking for a leaky head gasket it is included here as well.

A little water in the oil is common for an engine that has had the thermostat removed, or runs only short trips and does not get up to normal running temperature. This usually takes the form of a creamy emulsion on the inside of the valve cover, but can get bad enough to discolor the oil in the pan and on the dipstick as well. It doesn't necessarily indicate any damage to the engine, although a blown head gasket is also a common cause when there is a lot of water in the oil.

If you suspect a blown head gasket (or a crack in the cooling jacket wall) do a pressure test on the cooling system. I bought a radiator pressure tester at NAPA some years back for about $60, but any good service shop or any radiator shop should be able to do it for you. The device is a hand pump (looks like a short basketball pump), with a pressure gauge and a hose connecting to a fitting that goes on the radiator in place of the radiator cap. Fill the radiator with water completely to the top, leaving as little air space as possible. Attach the tester and pump up the pressure to about 10 psi. Then let it sit for a while and watch the gauge. If it holds the pressure you do not have a blown head gasket, at least not into the water jacket area.

The pressure may drop a couple of PSI to start with, and then stabilize. This can happen when the rubber hoses stretch a bit under pressure, but the reinforcing cords in the hoses should limit the expansion, and the pressure should stop dropping. Give it a few minutes and pump the pressure back up to 10 psi again.

If it will not hold the pressure, the water is leaking out somewhere. First place to check is the front of the water pump, just behind the belt pulley. Then check the water valve for the heater (if it has one) and all of the hose connections. If the water leaks out on the floor the source of the leak may be obvious. If it loses a substantial amount of water without making the floor wet, chances are that you will be finding it in the oil sump. The most common cause for this is a blown head gasket, but there is a remote chance that it could be a crack in the head or in a cylinder wall.

For an internal leak you can do a pressure test on each cylinder. You need a fitting that will screw into a spark plug hole and to which you can attach an air line, and you will need a pressure source, so an air compressor would be nice. You can make the fitting from an old spark plug, break off the ceramic, punch out the material in the center hole, and weld or braze an air fitting to the remaining steel part. For each cylinder in turn, rotate the engine to top dead center on the compression stroke for the cylinder being tested, apply about 20 psi pressure, and look for bubbles in the radiator. You might also listen for leakage into the crankcase, but any sound there may be masked by other (air leakage) noises. If you drain the oil and leave the drain plug out you might find water dribbling out the oil drain (not a good thing).

If you have gotten this far, congratulations, you would be smack in the middle of doing a blow down test on the cylinders, so listen up and check a few other things while you're at it. If you hear hissing at the carburetor you have a leaky inlet valve. If you hear hissing out the tail pipe you have a leaky exhaust valve. If you hear hissing at the oil filler cap you have leaky rings and blow by into the crankcase. If you have pressure on one cylinder and hear hissing at the adjacent spark plug hole, you may have a blown head gasket between those two cylinders. This is fairly common between #2 and #3 where the two adjacent exhaust valves create a hot spot. With a bad inlet valve you can get a hiss between #1 and #2, or between #3 and #4, where two cylinders share a port in the intake manifold. With a bad exhaust valve you may get a hiss between #2 and #3 where there's a shared port in the exhaust manifold.

Another tool that should be in everyone's tool box is a compression tester (about $20 at your local cheap parts store). This is just a hand held pressure gauge and a pipe with a rubber bung on the end. You hold this tightly in the spark plug hole while you crank the engine over for several revolutions to allow the pressure to build up and the gauge reading to stabilize. You can then add a couple squirts of oil in the cylinder and repeat the pressure check. If the addition of oil to the cylinder increases pressure by about 20 psi, the piston rings are probably not sealing well. Ideally you would like all cylinders to have pressure readings within a range of 10% or 10 psi, but the absolute reading is not terribly important unless it's down around 100 psi or less. 130-140 psi in a good stock MG engine. 175 psi is a good high compression engine. Less than 100 psi makes it hard to start. Greater than 10 psi variation between cylinders gives it a rough idle.

And now on to the problem of starting a long dormant engine. Please turn the page.