How fast should a compression test rise?

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djwarner
Posts: 407
Joined: Sat Jan 23, 2010 6:07 pm
Car: 1971 240Z Series I
2006 350Z
Location: Central Florida

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I bought a $20 compression test rig from Harbor Freight to test my L24 with 168K miles.

My son helped me set up the test. Pulled all the plugs after warming the engine. It took almost 15 seconds of cranking (3-5 seconds at a time) for cylinder #1 to max out a reading of 150 psi. My son said it should have maxed out in as little as 3 or 4 revolutions. Tried a wet test and again it was slow to come up but finally got close to 160 psi.

Second cylinder was also slow to come up so we stopped fearing an overheat of the starter or draining the battery.

The compression kit has a quick disconnect and a total combined hose length of about 37" I was thinking that there is substantial volume in the hose to compress causing the long cranking interval.

Have I got something else goin on here? :gotme


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evildky
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Joined: Sat Jan 31, 2004 9:23 pm
Car: 71 Datsun 240ZT
87 Nissan 300ZX N/A-T
06 Nissan 350Z GT
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did you wedge the carb pistons open and hold the throttle open? in order to do a compression test you need all the plugs out and unrestricted air coming in

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djwarner
Posts: 407
Joined: Sat Jan 23, 2010 6:07 pm
Car: 1971 240Z Series I
2006 350Z
Location: Central Florida

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I believe so. All plugs were out and we pushed the gas pedal to the floor while cranking.

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evildky
Posts: 14225
Joined: Sat Jan 31, 2004 9:23 pm
Car: 71 Datsun 240ZT
87 Nissan 300ZX N/A-T
06 Nissan 350Z GT
Toyota Tundra TRD RW
Location: Louisville, KY
Contact:

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Pushing the gas pedal to the floor opens the throttle blades, the pistons rely on vacuum which is why they must be wedged open to do a compression test.


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