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Old 03-28-2011, 05:50 PM   #1
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Compression test results

The car vibrates at a noticeable rate but despite changing the plugs, wires, cap, rotor and coil module, it still feels off. A strange sort of uneven sound can be heard with the hood open or closed; Ive had this problem since I started working on her last June. My mechanic recommended I do a compression test to help discover if the problem was a bad lifter, valves, etc.

I was told to crank the car over, record the value, clear the gauge, and repeat (5 times total) then remove the tester, squirt a little bit of motor oil into the hole, plug the tester gauge again in the same hole and crank the car over, record the value, clear the gauge, and repeat (5 times total again).

Here are the values: (all in PSI)

Cyl #2

89 98 89 90 115

Cyl #2 after oil squirt

89 90 90 90 90



Cyl #4

99 98 98 99 98

Cyl #4 after oil squirt

130 109 109 110 141



Cyl #6

90 110 127 95 94

Cyl #6 after oil squirt

100 116 105 107 130



Cyl #8

110 124 95 91 92

Cyl #8 after oil squirt

129 95 95 95 135



Cyl #1

106 137 106 141 116

Cyl #1 after oil squirt

111 140 135 112 133



Cyl #3

110 145 111 145 125

Cyl #3 after oil squirt

119 114 145 111 115



Cyl #5

109 130 105 105 114

Cyl #5 after oil squirt

140 105 107 115 115



Cyl #7

115 110 114 116 140

Cyl #7 after oil squirt

109 110 110 103 110




What do you guys think?
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Old 03-28-2011, 05:56 PM   #2
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Re: Compression test results

I think you didn't have your throttle blocked wide open.
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Old 04-09-2011, 11:02 PM   #3
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Re: Compression test results

those numbers just don't seem right to me, they're all over the place. i myself just did a compression test and they ranged from 40-65 psi (no oil). I'm replacing my rings next weekend! I hope someone here can help you though.
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Old 04-10-2011, 12:46 AM   #4
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Re: Compression test results

A proper compression needs needs the throttle at WOT while cranking over the engine. Based on those wild numbers and ignoring tests 4-5, it looks like #2 has some problems but nothing that's too serious enough to cause an uneven sound. There was no change in compression when oil was added to the cylinder but it's still building compression. Since the oil seals up the rings and the compression didn't change, I'd guess you have a valve problem.

How did the #2 plug look compared to the others?

If none of the plugs are fouled up, reinstall everything so that the engine can be started up. Use an inductive type of timing light and put the plug wire clamp onto each plug wire. As the plug fires, the timing light will flash. If one hole isn't firing, the light won't flash.

Check for vacuum leaks around the intake manifold and carb or throttle body. Many people like to use WD40 but it can be hard to isolate a leak with a fluid. Using a propane torch, turn the bottle on and allow the propane gas to flow along the intake and base of the carb. If there's a vacuum leak, the propane will increase the rpms of the engine.

Right now all your compression test says is that #2 is a little more worn than the others which are not all in great shape. A typical engine should have around 145 psi of compression with a dry test. As long as all the cylinders are roughly the same no matter what the reading, there's nothing seriously wrong other than the engine is getting worn if they're all consistently low. If you did a compression test and all were in the 120-140 range and one was less than 80 then you can suspect that cylinder to be causing problems.

The last step if you can't get a good compression test is to do a leakdown test. With a leakdown tester, regulated shop air is put into the cylinders and the amount of leakdown is recorded. Leaks into the exhaust, intake or crankcase can better pinpoint a potential problem better than a basic compression test can do. Chances are you'll hear air escaping into the exhaust or intake indicating a bad valve. Could be bent or burnt. Only pulling the head off will determine the exact valve problem if that's what the tests point to.
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Old 04-10-2011, 10:58 AM   #5
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Re: Compression test results

Sorry, not trying to highjack Wiggles thread but i have a question. What would make a dry compression test read 40-65psi on a motor with 20,000 miles?
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Old 04-10-2011, 01:38 PM   #6
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Re: Compression test results

A dry compression test gives you a baseline reading as an engine would normally operate. A wet test simply puts a little bit of oil in the cylinders to seal up the rings. If the rings get oil on them and the compression test increases then you can assume the dry rings are not sealing. If the wet test fails to increase the compression numbers, there are other issues such as bad valves, broken rings, head gasket etc.

A compression test is only one part of diagnostic testing. You can't do a simple compression test and determine an exact problem from the results. It only gives you an indication of where to start looking for the cause of the problem.

Low compression in a low mileage engine can be any number of things. Maybe the rings were not gapped properly. Maybe one got broken during assembly. Maybe the cylinder wall surface didn't have the correct honing for the type of rings used and they didn't seat. Maybe a valve isn't seating properly. Maybe a cam lobe or lifer wore out and the valves are not opening enough.
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Old 04-11-2011, 07:54 PM   #7
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Re: Compression test results

wow, thanks. i did have some valve issues, but the motor is disassembled now i'll just change the rings while i'm at it.
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Old 04-12-2011, 03:47 PM   #8
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Re: Compression test results

Quote:
Originally Posted by AlkyIROC View Post
How did the #2 plug look compared to the others?
Plug #2 looked identical to the other plugs; a tiny amount of oil and carbon on the electrode. I also checked the gap on each plug which said .45

Thanks for all the replies. Yeah, those numbers jumping around inconsistently did seem to be a bit strange to me, but this was my first time doing a compression test and I didn't know any better. Being an amateur doesn't help any, but I'm slowly learning.

In a few weeks, my friend is going to help me remove the heads and we'll go from there. I keep you guys posted with the results at some point.
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Old 04-12-2011, 03:47 PM
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