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[JamesV6CDX]

Just a few questions on compression testing methods.

Am I right in thinking that, do to a compression test, I need to

1) Remove ALL the spark plugs at the same time, and leave them ALL out when testing each cylinder?
2) When cranking the engine to check compression, crank it with WOT
3) Disable the engine from firing (obviously!)

Anything else to look out for?

[JamesV6CDX]

Also, what size plug hole does the meega have - 14mm?

[Dazzler]

Warm engine up

Take out all spark plugs

Disconnect coil wire so you don't get a spark

Open throttle slightly to allow air into the cylinders

Screw gauge in

Spin engine over on starter for maybe 6 revolutions until gauge reading peaks

Ensure same no of revs for all 6 cylinders

Take readings. Note also if needle goes up quickly on first revolution (good).

Ideal readings vary according to nmgine type but for the 3.0 12v look for

160 - 180 psi -very good engine. No discernible fuming. Throttle housing will be dry.

150 psi OK Very slight fuming may be discernible. Some oil deposits in throttle housing.

130 psi getting badly worn. There will also be significant "fuming" when the engine is running hot -ie exhaust fumes and burnt oil escaping from oil filler cap as a fine greyish mist. Oil deposits draining into air filter box.

Below 90 - bad, maybe rings broken, burnt valve
Fuming will be very obvious, puffing out like a steam train. Oil everwhere.

No reading or struggling to get to 10 psi -hole in piston or similar catastrophe. Oil all over engine bay.

On a good engine look for consistent readings across all six cylinders but at the upper end of the scale if they vary by say 20 psi max it is nothing to be unduly concerned about, although it is not ideal. The greater the difference, the less evenly it will idle and run.

Abnormally low compressions on adjacent cylinders suggest head gasket leaking between the two.

My cheap Gunson gauge is just as accurate as my over-priced Snap-on unit, much to my surprise.

Borrowed How To Guide, hope this helps a bit

[Kevin Wood]

Only thing I'd add is to use a wide open throttle, otherwise, it can take quiite a few revolutions to build pressure and this might give you a false reading. Also, wide open throttle will disable fuel injection so you don't run the risk of flooding the engine. Having all the plugs out helps to get a good, consistent cranking speed and will give you a more accurate reading.

Kevin