Compression check question
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Junior User
Joined: Sep 2006
Posts: 57
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From: Tofino, BC
Compression check question
Hi all, thanks for all your help so far. One last question (for now)
I had a truck inspected today that I might buy. My question is regarding the comression check. They did it in a way that I am not famialiar with. It is in percentages........relative to the highest one I think. The truck is a 2000 F-250 SD 4X4 with the 5.4 litre V8 and 138,000 kilometers on it. The report reads as follows- 1-100% 2-97% 3-99% 4-98% 5-96% 6-98% 7-95% 8-97%
From what I remember about the last time I got the compression checked on my old Nissan, was that if they were within 10psi of each other, then you were ok. So if the highest one was around 150psi, then that would put the lowest one at 142.5 psi. Is this right?
Thanks,
Karma
I had a truck inspected today that I might buy. My question is regarding the comression check. They did it in a way that I am not famialiar with. It is in percentages........relative to the highest one I think. The truck is a 2000 F-250 SD 4X4 with the 5.4 litre V8 and 138,000 kilometers on it. The report reads as follows- 1-100% 2-97% 3-99% 4-98% 5-96% 6-98% 7-95% 8-97%
From what I remember about the last time I got the compression checked on my old Nissan, was that if they were within 10psi of each other, then you were ok. So if the highest one was around 150psi, then that would put the lowest one at 142.5 psi. Is this right?
Thanks,
Karma
FTE Leadership Emeritus
Joined: Sep 2000
Posts: 23,174
Likes: 1,678
From: Stephensville WI
The reason they do it this way instead of giving you the PSI for each cylinder is because of how they measure compression.
Instead of taking off each spark plug and getting the compression tool down there, they hook your engine up to a computer. The computer turn off the spark plugs one at a time and measures the parasitic drag from that cylinder on the engine. More drag (measured in idle RPM dropping) means a tighter seal on your cylinder.
Instead of taking off each spark plug and getting the compression tool down there, they hook your engine up to a computer. The computer turn off the spark plugs one at a time and measures the parasitic drag from that cylinder on the engine. More drag (measured in idle RPM dropping) means a tighter seal on your cylinder.
