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Old 02-05-2009, 01:28 PM   #1 (permalink)
Poseur, Rice Boi
 
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Default How to: Compression test?

Can anyone give me a walk through on how to do a compression test? Ive got everything needed, but I have never had to do one before, so Im really gonna need someone to break it down barney style, and give me a step by step. Also, what are ideal numbers that Im looking for? Its gonna be done on a 97' Disco I, with 131XXX on her.

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Old 02-05-2009, 02:05 PM   #2 (permalink)
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I've never done one on a car or truck but have on a motorcycle.

Simply pull the plug wires, insert the tester in the plug spot on the cylinder being tested, crank motor, check readings.

What you want to do is get the pistons to move but not actually run the motor. That's the reason behind disconnecting the plug wires.
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Old 02-05-2009, 05:03 PM   #3 (permalink)
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Do I need to disconnect any grounds, wires or the computer or anything like that?
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Old 02-05-2009, 07:51 PM   #4 (permalink)
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Pull plugs, cut off fuel pump/injectors, and insert compression tester and crank... I believe numbers are supposed to be around 170'ish ... and should be around +/- 10% of each other.
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Old 02-06-2009, 01:12 AM   #5 (permalink)
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First off, why are you doing a compression test?
Warm up the motor to running temp and then switch it off. Pull all the plugs and disconnect the power to the coil. Put the compression gauge into each cylinder and get someone to turn over the motor, while holding the throttle flat to the floor. Watch how the gauge increases its reading. It should get close to its maximum within 2 to 3 turns of the motor. Keep on turning over the motor until the gauge stops increasing.

The compression reading will change depending on whether you have a high compression or low compression motor and a number of other factors. Anything above 130 psi is serviceable. But as already pointed out, you are mainly looking for differences between the cylinders.

A vacuum gauge will tell you more about your motor than a compression gauge.
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Old 02-06-2009, 05:13 AM   #6 (permalink)
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Additionally, remove adjacent plugs together as a head gasket leak between cylinders can be masked if you don't.
If you have a low reading, a couple of squirts in to the cylinder can help isolate the problem. If the pressure comes up retesting after adding oil, it points to worn rings/cylinder wall. No change points towards a bad valve.
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Old 02-06-2009, 07:01 AM   #7 (permalink)
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Well p76rangie, Ive been have problems with power with my rig. I only really experience it at speeds above 50 to 55, and sometimes are better then others. im not loosing any coolent(if I am, its very slow), and I dont believe that I have any coolant in the oil. Ive already cleaned the MAF and butterfly valve, as Im pulling code P1194. Im pretty sure the power lose is due to either a bad o2 heater or o2 sensor. Im pretty sure the cat is now blown as a result. Im just having the entire exhaust system replaced... It was needing it anyways and I dont feel like trying to solve the mystery. I just wanna do a compression test to be sure its not due to a head gasket leak or something similar.

Thank you everyone for your advice! I have to tell ya, this is probably my favorite forum, def the most responses.
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Old 02-06-2009, 07:04 AM   #8 (permalink)
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Here's a couple good links on compression & vacuum testing.
Compression Testing
Using A Vacuum Gauge

Hypothetically speaking... say your "friend" did a compression test and forgot to hold the throttle open, how would this effect the readings???
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Old 02-06-2009, 08:36 AM   #9 (permalink)
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Quote:
Originally Posted by Kevin88RRC View Post
Hypothetically speaking... say your "friend" did a compression test and forgot to hold the throttle open, how would this effect the readings???
As the main thing you look for is the difference between the cylinders, not having the throttle open for any of the readings would not make that much difference.
If you actually want to get the proper PSI, not having the throttle open will restrict air into the cylinders and will give a slightly lower reading.

Putting oil in the cylinder can increase the compression when it is a valve as well and in a V8 it is difficult for it to have the right impact on the rings due to the piston being at a angle.

As already indicated, a vacuum gauge will tell you a lot more about the state of your motor than a compression gauge.
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