Low compression?

2002 NISSAN SENTRA
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JUSTINCPOWELL
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okay we fixed a few problems its had and the such but it says according to my step-dad that it has low compression in all the cylinders, like half of what it should be.. what causes it? how do you fix it? and how much would it cost on average? if it helps i think the previous owner camped a lot and when outdoors(ing). please help. I'm looking into modding the motor as when as turning it into a true ricer by looks lol but i need to know how good this motor will be with this problem and how i can fix it and help it. or if i need a lot of money saved up repairs. thanks
Oct 14, 2008 at 7:39 PM
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SQM
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Hello,

You can have low compression for few different reasons. The most common one is faulty piston rings.
First you need to confirm the compression number for the cylinders.
This engine should have minimum of 168 PSI of compression. With the highest to lowest cylinder should not have a variance more than 14 PSI. So if the highest cylinder is at 180, the lowest one should not be less than 166.

https://www.2carpros.com/articles/symptoms-of-low-compression
https://www.2carpros.com/articles/how-to-test-engine-compression

After you find the compression and if it is low then you will have to do some diagnostics of the reason.

Here is a helpful guide of what to look for when you have engine with low compression:

https://www.2carpros.com/articles/the-reasons-for-low-compression

I have attached diagrams for your reference.

Please let me know of any questions.
Thank you.
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Sep 19, 2021 at 7:53 PM
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JACOBANDNICKOLAS
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Hi,

There are a few things that can cause low engine compression. For example, a bad head gasket, worn rings, and leaky valves. What you need to do is perform a couple checks to see what the cause may be.

To start, take a look through this link. It explains in detail the things I mentioned.

https://www.2carpros.com/articles/the-reasons-for-low-compression

Now, the first thing to check for is a bad head gasket. Take a look through this link:

https://www.2carpros.com/articles/head-gasket-blown-test

If that all checks out good, then we need to determine if it is a piston ring issue or a valve issue.

Next, follow this link to perform a compression test. While doing it, write down the compression found in each cylinder.

https://www.2carpros.com/articles/how-to-test-engine-compression

When you finish, take approximately 1 tablespoon of motor oil and place it in one cylinder at a time. This is called a wet compression test. Retake the compression of that cylinder and write down the results. Continue this with each of the remaining cylinders.

Here is what you need to focus on. If the compression has substantially increased under the wet test, the rings are likely worn and need to be replaced. If there is no chance, then you likely have an issue with the intake/exhaust valves in the cylinder head.

I hope this helps. Let me know if you have other questions.

Take care,

Joe
Sep 19, 2021 at 7:55 PM
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