compression test

[deoxford]

Ok I did do a search first, I can't find how to do the compression test
Can anyone help me out?

Also I got the tools in to do the CMA, and I have read it over a few times.

Do you really need to be driving to get the CO reading at 2400?

And as always thanks for the help



Derek

[Jeff Alton]

Compression test is pretty simple. The basics are: remove sparkplugs, insert tester into one cylinder, fully open throttle, and crank engine through at least 4 compression strokes, jot down the reading. Repeat 5 more times. You may also want to disable your fuel system so that you aren't filling other cylinders with fuel. Also due this test on a warm engine. I also like to remove the coil wire.

Hope this helps, Jeff

[Grady Clay]

Derek,

Get the kind of cranking compression tester that screw into the sparkplug hole. It is easier to use and more consistent.

Pelican Parts, Harbor Freight, Craftsman, SnapOn and more all sell them.

Cranking compression is a dynamic measurement and the cylinder leak test is a static (at TDC compression) measurement. Each has their own benefits. The cylinder leak test gives you a quantative reading for how well the cylinder is sealed at TDC. If there is leakage, you can usually isolate the problem to the rings, valves, or head-to-cylinder seal. The cranking compression gives you indication of the dynamic functioning of the rings, cams and valves.
I always do both.

The cranking compression test is done with the ignition and fuel off or disconnected at the source and all six sparkplugs removed. It is also done with the throttle mostly wide open. You should crank the engine approximately the same numbers of turns for each cylinder to where the gauge is maxed-out. The battery bust be in good shape and will benefit from being on a low-amp charger.

The procedure I like to use is to first do some spirited driving, a DE event, or a road trip. Then go to a high pressure car wash and remove all six plug connectors. Blast away around the sparkplugs with the engine still warm (not hot.) Reinstall the connectors and drive home.

The tests are best done at room temperature although there are rare occasions to do it with a very warm engine. Pull the connectors and squirt compressed air around the plugs to make sure there isn’t some lingering piece of sand waiting to fall in the cylinder. A flashlight and mirror work well for inspection.

Find TDC compression for #1 cylinder. That is when the crank pulley is at “OT” and the distributor rotor points to #1 terminal on the cap. I then remove #1 sparkplug (only) and measure the cylinder leakage. Continue in the firing order turning the crank 120 degrees each time (2 rotations total for six cylinders.) After all six are measured and the plugs now all out, then I measure the cranking compression. It doesn’t matter which order you use but you must pair the compression and leak readings for each cylinder, the measurements complement each other.

The reason for the above effort is that when a sparkplug is removed and the exhaust valve is open, a spec of carbon can land on the exhaust valve seat. This can fool you into thinking there is a failing exhaust valve.

If in doubt, repeat the tests. If the tests come up questionable, go out and drive on the highway some more. Generally speaking it is best to do these tests on a regular basis, say just after a DE weekend or road trip. You want to establish a base-line so you can spot a trend or a problem. With multiple tests you can then apply statistical techniques.



Some of the troubles you can get into are:

The screw-on spark plug terminal can not have been tight and falls alongside the sparkplug making it very difficult to get the plug wrench on. Same is true for other errant hardware or a broken piece of a plug connector.

The plug connector can be damaged or broken. Always inspect for cracks or carbon tracks.

There has been much discussion on the Forum about the best spark plug wrenches.

When unscrewing the sparkplug, you will develop a feel for if it is too tight or too loose on the threads. Always inspect the plug threads for signs of the threads in the head failing.

Always tighten the terminal on the plug. I add a split washer when not supplied with the plug.

Use a little AntiSieze on the threads. Do not slather it all over the place.

Upon reinstalling the sparkplug, be very careful that it starts properly and doesn’t get “cross-threaded.”

When reinstalling the plug connectors, get them on the terminal. In some situations it is possible for the connector to be alongside the plug and you think it is on the terminal.

Don’t forget to put everything else back in its place. We once had a 911 towed in because the owner forgot to re-connect the CDI.


The cranking compression test and cylinder leak test, particularly as a combination, are some of the best diagnostics of the internal workings of your engine.

Best,
Grady

[Grady Clay]

Derek,

How about you take the CMA procedure:
Check Measure Adjust (CMA)
http://www.scatliff.mb.ca/pelican/MFI_Check_Measure_Adjust.pdf
and the extensions I posted here:
“MFI thermostat spacers”
p. 1 end has CMA extension, p. 3 has nozzle, fuel flow, and fuel pressure.
MFI thermostat spacers
and post a CMA Plan for yourself. Let us critique and expand it for you. Having written a plan, you will find it even more useful and others will benefit.

Give us a run-down on the MFI tools you have accumulated.
How is the engine and MFI now?


Yes, the part throttle is necessary. It isn’t free-rev with no load and it isn’t heavy throttle acceleration. It is best to repeat in 2nd, 3rd, and 4th gears. Second will tell you the most as 3rd and 4th are more throttle at the same RPM. You should do this on a long, flat road at constant throttle (not accelerating or decelerating.) If running too rich – the usual case - between tests a little sprint may be in order.

Best,
Grady