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Old 10-14-2010, 06:26 PM   #1
ShawnV
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Default How-To: Compression testing w/ engine out of car

I recently swapped an engine into a saturn only to find out it didn't run correctly. I took the advice from others on here and compression tested it. The results clearly showed that the engine was toast and I had wasted my time and money installing it.

Richpin has a video on compression testing your engine when it's in the car, but I needed to know how to test when the engine was outside of the car. OldNuc explained how to perform a compression test on an engine that is out of the car and below is a video on how to do this. I won't buy another used engine without compression testing it first!

I bought a cheap compression tester from harbor freight that works for now.

Some essentials:
Make sure there's oil in your engine before you start spinning it over.
Your compression values might be a bit low until you squirt some oil into the spark plug wells and some WD-40 into the exhaust ports.

The junkyard compression tested this engine and reported 2 cylinders at 160 and 2 at 170psi. After the oil and WD I had 180-185psi for all 4.


http://www.youtube.com/watch?v=z6jGloCPv_o
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Old 10-14-2010, 07:18 PM   #2
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Default Re: How-To: Compression testing w/ engine out of car

As for starter-rigging to the battery, I actually learned that in my preventative maintenance diesel class recently! Where exactly is the positive lead on the starter?

Good video all the same.
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Old 10-14-2010, 07:37 PM   #3
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Default Re: How-To: Compression testing w/ engine out of car

The positive on the starter is the larger post with the 13mm nut.
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Old 10-14-2010, 07:55 PM   #4
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Default Re: How-To: Compression testing w/ engine out of car

For a DOHC the service limit is 180.. I would personally pass on that engine. I found one with 220 even on all 4.. just keep looking.
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Old 10-14-2010, 08:54 PM   #5
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Default Re: How-To: Compression testing w/ engine out of car

When testing engines that are in a junk yard there are several considerations that you do not run into when the engine is is daily use. Do not immediately reject a low but even compression reading in all cylinders as worn out. Without spending the time to check for any valve/valve seat corrosion and absolutely dry cylinders you are looking for even and at least reasonable. If you are picking through a big pile of engines priced at 100.00 ea. then be picky. If you happen to be in a part of the country where a core engine is worth 200.00 and the half life of a running engine that does not blow oil, priced at 550.00, is measured in days then picky can mean no engine for you.

In some parts of the country the intake and exhaust valve seats and faces will oxidize slightly and give you low readings. This can happen rather rapidly.
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Old 10-14-2010, 11:23 PM   #6
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Default Re: How-To: Compression testing w/ engine out of car

I guess I should have noted that I paid $600 + tax for that engine. When I backed the truck into their warehouse I saw a familiar face. It was a previous student of mine from a couple years ago. We remembered each other, as we talked about cars before and after class occasionally. He said the yard was his family's and to call him next time I need parts. I'm hoping I've got a nice hookup here.

I will report back with the compression figures once I get this engine installed and tuned up.
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Old 10-14-2010, 11:42 PM   #7
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Default Re: How-To: Compression testing w/ engine out of car

Probably a good connection. you should be able to get something for all of those cores you have cluttering up the place. There is an actual market for cores but it is not something that the general public can get at.
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