Engine Compression Spec
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Junior User
Joined: Jan 2008
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From: USA - New Mexico
Engine Compression Spec
I did a engine compression test on my 1992 Aerostar 4.0L and came up with the following numbers:
#1 - 130
#2 - 142
#3 - 135
#4 - 135
#5 - 145
#6 - 130
While I can see that its well balanced I'd like to know what the original spec for the engine was so I have something to compare my numbers against. I haven't been able to find anything in any of the books that I have so if anyone knows please let me know. I'd also be interested in what anyone else has seen on there Aerostar's.
#1 - 130
#2 - 142
#3 - 135
#4 - 135
#5 - 145
#6 - 130
While I can see that its well balanced I'd like to know what the original spec for the engine was so I have something to compare my numbers against. I haven't been able to find anything in any of the books that I have so if anyone knows please let me know. I'd also be interested in what anyone else has seen on there Aerostar's.
Posting Guru
Joined: Jul 2004
Posts: 2,416
Likes: 1
From: Mitchell, SD
Ford does not give specs of compression for the engine, but here is what they advise doing.
Compression Readings — Interpreting
It is recommended the Compression Pressure Limit Chart be used when checking cylinder compression so that the lowest reading number is 75 percent of the highest reading.
If one or more cylinders reads low, squirt approximately one tablespoon of SAE 50 weight or equivalent engine oil on top of the pistons (6108) in the low reading cylinders. Repeat compression pressure check on these cylinders.
If, after checking the compression pressures in all cylinders, it was found that the highest reading obtained was 1351 kPa (196 psi) and the lowest pressure reading was 1069 kPa (155 psi), the engine (6007) is within specification and the compression is considered satisfactory.
Compression Readings — Interpreting
It is recommended the Compression Pressure Limit Chart be used when checking cylinder compression so that the lowest reading number is 75 percent of the highest reading.
If one or more cylinders reads low, squirt approximately one tablespoon of SAE 50 weight or equivalent engine oil on top of the pistons (6108) in the low reading cylinders. Repeat compression pressure check on these cylinders.
- If compression improves considerably, piston rings (6149) are at fault.
- If compression does not improve, valves are sticking or seating poorly.
- If two adjacent cylinders indicate low compression pressures and squirting oil on pistons does not increase compression, cause may be a head gasket leak between cylinders. Engine oil and/or coolant in cylinders could result from this problem.
If, after checking the compression pressures in all cylinders, it was found that the highest reading obtained was 1351 kPa (196 psi) and the lowest pressure reading was 1069 kPa (155 psi), the engine (6007) is within specification and the compression is considered satisfactory.
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