Leak Down

Driving it regularly, I've become aware of the Miata's drinking problem. It goes through over a quart in 1000 miles. I broke the handle off of the dipstick which was always awkward to read, so I bought a new one. It sucks to read too. After 500 I went to add 1/2 quart and added the second 1/2 too. It was too much and the car was smoking more than usual. That got me thinking, so I bought a leak down tester.

I have done a compression test in the past. If I recall correctly, I was getting 165psi for each cylinder with no more than 5, maybe 10psi difference. A compression tester is a device that takes the place of a spark plug. It won't light the fuel mixture (it's best to disconnect the fuel pump and ignition before testing), but it will measure how much pressure the engine can create when cranking. 165 isn't bad. I've read that when the compression is as low as 130 it's time to rebuild. Its the oil burning that bothers me.

It shouldn't. The car still passes emissions, and I drive a long enough drive that the catalytic converter heats up and must be able to keep itself clean. Catastrophic failure isn't at the top of my list of fears. The "cat" failing from getting gunked up from oil is.

This is a compression tester hanging off the top of the engine.

The compression tester just gives you a general picture of the engine's health. It doesn't distinguish between bad rings and leaky values. That's what a leakdown tester is for. In combination with a compressor, you pump air into the cylinder and find out where it leaks out. You expect a bit to come out the crankcase as it blows past the rings, but just a bit. In older engines  you might get some to come out of the air filter or exhaust pipe if the respective valves aren't doing so well. If the head gasket is going, you might get some bubbles from the radiator cap. I could hear some air coming out of the oil cap.

Here's pictures of what I got from three of the cylinders. You can see the tester here. One hose goes to the compressor (the left one). The hose on the right goes to a cylinder. I set the input pressure to 90 on the left gauge and then measure what the cylinder holds. The first one has me concerned. 90 pounds in and only 75 out. The other two look more like 90 in and 85 out.

Setting this up is a bit more involved. The engine isn't turning on a leak-down, like it is for a compression test. You have to turn the engine to Top Dead Center (TDC) for the measured cylinder so that the valves are closed. In the back of the bottom picture you can see a wooden dowell in one of the cylinders. It makes it easier to see the position of the engine. A common mistake is to get the wrong TDC, since there are two. The wrong one has the valves open and will show awful numbers.

For now, I wonder if the values weren't perfectly closed on the bad cylinder above, so I intend to retest. If it's as good as the others, I might just go for a roll bar and sway bars this winter. If I measured correctly, I will probably go for an engine rebuild.

The good news from the three that tested well (I didn't upload the reading of 83 I got), included silence from the air filter and exhaust pipe: good valves.