Compression/Leak-down testing procedure?

[Keifersmith]

I may try to compression/leak-down test the miata this weekend. I’ve got the general idea but tried to look online to get more detailed “step-by-step” instructions. Didn’t have much luck finding concise instructions. If anyone knows of a good reference could you please point me to it?

In general, this is my take…

First run the engine to normal operating temperature. Then do the following tests.

For leak-down test, use the following: http://www.harborfreight.com/cylinde...ter-94190.html

Remove all the spark plugs.
Put car in neutral to allow crank to rotate freely.
Rotate #1 piston to TDC of the compression stroke.
Put the car in gear to prevent the compressed air from rotating the crankshaft.
Connect leak-down tester to cylinder. Apply 100 psi to the regulator side of the tester. Read the % leakage on the second gauge of the tester.
Repeat above for each cylinder rotating each to TDC of the compression strike.

Easy enough to stick a long wire in the spark plug hole to verify that the piston is at the top of the stroke but since the piston will be at the top of the stroke twice per cycle, how do I know when its at the top of the compression stroke?
Does it matter how or which way I rotate the crankshaft?

For compression test, use the following: http://www.harborfreight.com/flex-dr...ter-92697.html

Spark Plugs still removed from previous leak-down test.
Connect compression tester to #1 cylinder.
Press the gas petal all the way down and crank the engine at least eight revolutions.
Observe and record the maximum reading from the tester gauge while cranking the engine.
Release pressure from the tester/cylinder and remove the tester from the cylinder.
Repeat above for each of the remaining cylinders.

I think I read somewhere that I need to disconnect the coil or it’ll get damaged by cranking the engine without being connected to plugs. Is this correct and/or is there an easier method?

TIA for any suggestions/advice.

[Rob®]

You should disconnect the coil when doing a compression test. I always cut fuel as well - either pull a fuse or disconnect the harness to the injectors. Make sure your battery is 100% charged and ready to go.

You can buy the leakdown tester or make your own for like $20. Leakdown tests can be done on a cold motor, usually is actually since it's done to check motors before/just after you bought them to put into a vehicle. In fact, I don't know that I've ever done one on a motor in a car before..If you're not TDC on compression, the piston/crank will move or the air will rush out of an open valve, so it's pretty obvious you didn't have it just right. Just rotate the crank and try again.

[Keifersmith]

Thanks for the info Rob. Seems like when I swapped the coil a year or so ago it was a royal pain to disconnect. If I pull the fuse for the injectors maybe I could just leave the plug connected to the spark plug wire and ground it to the block instead for the cylinder being tested? This assumes that the I leave the plugs/wires installed for the cylinders not being tested. Can't see how that would damage anything but maybe I'm missing another point. Agree that if I can disconnect the coil easily that's probably the better way to go.

One other question. In what little info that I dug up it says that when doing the compression test and cranking the engine you should press the gas petal all the way to the floor. I guess the point to this is to open the throttle body to maximize the volume of air that can be pulled into the cylinder? Can't think of any other reason for this.

[black roadster]

Keifersmith,

Give me a call and I can explain how I do it. I don't feel like typing it all out.

tew juan for - se7ev O se7en - 911 too

[shortshift]

Kiefersmith - You are correct regarding the need to depress the gas pedal during the compression test. You're measuring the degree that the engine is compressing air, and the source for that air is through an open throttle. Also, you'll want to write down the compression reading for each cylinder. Not only are you interested in the absolute compression reading for each cylinder, you'll be looking for comparitive readings as weel. A ten psi swing from one cylinder to another is no big deal, but a 165 psi cylinder compared to a 110 psi cylinder might suggest a problem.