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post #1 of 3 (permalink) Old 03-01-10, 08:53 AM Thread Starter
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Location: Texas
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COmpression Test

Performed the compression test and got these numbers:
1-145 5-135
2-145 6-130
3-180 7-155
4-190 8-190
Number eight is the one that was giving the code for misfiring. but my numbers are just all over the place. on the wet test i average a 20psi rise. ex cyl 8-dry190, wet 205
3-dry180, wet 205

I imagine i need new rings on the entire engine. but could it be more to the problem. kinda short on the economic side right now but definitely need my vehicle.
any help trying to make sense of this numbers is appreciated. currently researching on the web.

oh and its a 1995 grand marquis 4.8L

Last edited by mex2texzz; 03-01-10 at 08:55 AM. Reason: forgot model name
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post #2 of 3 (permalink) Old 03-01-10, 11:06 AM
2002 F150 XLT
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Location: Oregon
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Re: COmpression Test

If one or more cylinders reads low, squirt approximately one tablespoon of clean engine oil. Repeat compression pressure check on these cylinders.

Compression Readings -- Interpreting

1. If compression improves considerably, piston rings are faulty.

2. If compression does not improve, valves are sticking or seating poorly.

3. If two adjacent cylinders indicate low compression pressures and squirting oil on pistons does not increase compression, a head gasket may be leaking between cylinders. Engine oil and/or coolant in cylinders could result.

When checking cylinder compression so that the lowest reading number is 75 percent of the highest reading.

Example Readings
If, after checking the compression pressures in all cylinders, the highest reading was 1351 kPa (196 psi) and the lowest pressure reading was 1069 kPa (155 psi), the engine is within specification and the compression is considered satisfactory.

Is it running like it has a mifire? A friend of mine gets a lot of used P71s and if they set for awhile, sometimes when he starts them, a rocker rm will fall off. It produces no noise and it runs rough. Also the 95s wer proen to have valve seals leak acusing oil consumption. They could carbon up and cause mixed readings like that. The rings haven't been known to wear out for at least 400,000 miles with good servicing.

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post #3 of 3 (permalink) Old 03-02-10, 10:39 AM Thread Starter
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Re: COmpression Test

will check the valves this weekend. I saw a bit of oil leaking from the bottom of the oil pan. but read the procedures and seem like i need to do some lifting.
I did notice about two weeks ago that my pcv was out about 1/4" out of its seat.
Been driving the car like this on 400 mile trips. it handles it but i am burning oil plus that leak is quite sure a big contributor.
Well will try to post an update as i do a closer inspection. Thanks for the info.

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