Compression check question 4.0L
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Model: Cherokee
Compression check question 4.0L
Planning to check the cylinder compression when I pull the plugs on my 2000 XJ 4.0L and have a question. Here is the how to from the service manual:
(1) Clean the spark plug recesses with compressed
air.
(2) Remove the spark plugs.
(3) Secure the throttle in the wide-open position.
(4) Disable the fuel system. (Refer to Group 14,
Fuel System for the correct procedure)
(5) Disconnect the ignition coil.
(6) Insert a compression pressure gauge and rotate
the engine with the engine starter motor for three
revolutions.
(7) Record the compression pressure on the 3rd
revolution. Continue the test for the remaining cylinders.
Refer to Engine Specifications for the correct
engine compression pressures.
I will be doing this on my own so I won't be able to visually watch for the piston in an adjacent cylinder when I crank the engine over so how critical is the 3 revolutions?
Also, does anyone know the compression pressures off the top of their head so I don't have to do a search for them?
Thanks - Mike
(1) Clean the spark plug recesses with compressed
air.
(2) Remove the spark plugs.
(3) Secure the throttle in the wide-open position.
(4) Disable the fuel system. (Refer to Group 14,
Fuel System for the correct procedure)
(5) Disconnect the ignition coil.
(6) Insert a compression pressure gauge and rotate
the engine with the engine starter motor for three
revolutions.
(7) Record the compression pressure on the 3rd
revolution. Continue the test for the remaining cylinders.
Refer to Engine Specifications for the correct
engine compression pressures.
I will be doing this on my own so I won't be able to visually watch for the piston in an adjacent cylinder when I crank the engine over so how critical is the 3 revolutions?
Also, does anyone know the compression pressures off the top of their head so I don't have to do a search for them?
Thanks - Mike
CF Veteran
Joined: Aug 2010
Posts: 8,357
Likes: 103
From: Canton, MI
Year: 1999
Model: Cherokee
Engine: 4.0
Planning to check the cylinder compression when I pull the plugs on my 2000 XJ 4.0L and have a question. Here is the how to from the service manual:
(1) Clean the spark plug recesses with compressed
air.
(2) Remove the spark plugs.
(3) Secure the throttle in the wide-open position.
(4) Disable the fuel system. (Refer to Group 14, Fuel System for the correct procedure)
(5) Disconnect the ignition coil.
For items (4) and (5), pull the ASD relay and the Fuel Pump Relay out of the PDC. The ASD Relay will disable the ignition and the fuel injectors.
(6) Insert a compression pressure gauge and rotate
the engine with the engine starter motor for three
revolutions.
Three revolutions is BS. 720 degrees crank rotation will bring a cylinder back up on its compression stroke. Three revolutions would only bring the cylinder up on compression one and one half times. It would take six revolutions to get a cylinder on compression three times.
(7) Record the compression pressure on the 3rd
revolution. Continue the test for the remaining cylinders.
Refer to Engine Specifications for the correct
engine compression pressures.
Again, BS.
I will be doing this on my own so I won't be able to visually watch for the piston in an adjacent cylinder when I crank the engine over so how critical is the 3 revolutions?
Let it crank. Most mechanics will let it pump up until the highest reading is seen. If you can crank it precisely six times (three compression strokes) then do that.
I use a remote starter switch so I can control the starter/compression stokes.
Do not exceed starter duty time of over 15 seconds. Let it cool off at least five minutes between duty cycles.
Also, does anyone know the compression pressures off the top of their head so I don't have to do a search for them?
The FSM calls for 120 to 150 psi as normal compression. Letting it crank will get you to 170 to 180 psi.
Thanks - Mike
(1) Clean the spark plug recesses with compressed
air.
(2) Remove the spark plugs.
(3) Secure the throttle in the wide-open position.
(4) Disable the fuel system. (Refer to Group 14, Fuel System for the correct procedure)
(5) Disconnect the ignition coil.
For items (4) and (5), pull the ASD relay and the Fuel Pump Relay out of the PDC. The ASD Relay will disable the ignition and the fuel injectors.
(6) Insert a compression pressure gauge and rotate
the engine with the engine starter motor for three
revolutions.
Three revolutions is BS. 720 degrees crank rotation will bring a cylinder back up on its compression stroke. Three revolutions would only bring the cylinder up on compression one and one half times. It would take six revolutions to get a cylinder on compression three times.
(7) Record the compression pressure on the 3rd
revolution. Continue the test for the remaining cylinders.
Refer to Engine Specifications for the correct
engine compression pressures.
Again, BS.
I will be doing this on my own so I won't be able to visually watch for the piston in an adjacent cylinder when I crank the engine over so how critical is the 3 revolutions?
Let it crank. Most mechanics will let it pump up until the highest reading is seen. If you can crank it precisely six times (three compression strokes) then do that.
I use a remote starter switch so I can control the starter/compression stokes.
Do not exceed starter duty time of over 15 seconds. Let it cool off at least five minutes between duty cycles.
Also, does anyone know the compression pressures off the top of their head so I don't have to do a search for them?
The FSM calls for 120 to 150 psi as normal compression. Letting it crank will get you to 170 to 180 psi.
Thanks - Mike
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