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Power Balance Test
02-11-2012, 04:29 PM
Post: #1
Power Balance Test
1990 Cadillac Deville 4.5 L FWD 180,500 mi.

Power Balance Test
1. Disconnect all spark plug wires at the spark plugs.
2. Let plug wires sit loose atop the spark plugs.
3. Attach a Accurate Tachometer
Engine warm – measure normal idle rpm
Disconnect and ground one of the loose spark plug wires to ground.
One Plug at a time. Continue with remaining plugs.
Test indications :
Good Cylinder 200 rpm drop
Comparison is the relative difference of each cylinder
No drop in rpm indicates a Poor Cylinder
50 rpm - ? rpm between Cylinders may indicate some trouble.

When I performed this Test on my vehicle it appears that the ECM
corrects for any Spark Plug deviation almost immediately, rendering
this test use-less on some vehicle's.
Have you found this to be true ?
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02-11-2012, 05:52 PM
Post: #2
RE: Power Balance Test
Yup, the ECM corrects for idle speed automatically until you reach about 1200 RPM or the throttle switch is no longer showing a closed throttle. At that speed it's difficult to get a proper reading for a power balance so the test is not really valid.

You could try disconnecting the IAC for another test but that will set a code. I also suspect the extra fuel in the exhaust will trigger some correction via the O2 sensors.... and the same goes for the MAP sensor seeing different manifold readings. OTOH, each "dead" cylinder should be roughly the same so it may point you in the right direction.

A word of caution: Don't leave a plug wire off for more than 10 seconds and allow 20 - 30 seconds for the engine to stabilize between cyl checks. The idea is to not get a build up of fuel in the cat, which may cause it to over heat.

Let us know what you find.
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