A leakdown test can reveal more than a compression test.
Compression Check Question
Thread Starter
Burning Brakes
Joined: Oct 2009
Posts: 782
Likes: 1
From: Magic Valley ID
Compression Check Question
Is there a way to check the compression on a used engine that is NOT installed in a vehicle? I know what the preferred method is for one that is in the car, and it requires that the engine be ran until warm and then checked. But there has to be a way to check out an engine that has been pulled in order to be sure that there are no issues before going through the trouble of completely installing a motor...only to find out AFTER you put it in that there is a problem. Any help here?
And please...no "make sure the engine is completely rebuilt" responses....because that's not an option at this time, even though that may be the plan down the road.
Deja
And please...no "make sure the engine is completely rebuilt" responses....because that's not an option at this time, even though that may be the plan down the road.
Deja
Last edited by mydejavooo; Jan 26, 2010 at 02:46 AM.
Le Mans Master
Joined: Jun 2004
Posts: 6,231
Likes: 65
From: Seattle WA
St. Jude Donor '14
A leakdown test really tells you more in this situation than a compression test. It tells you if there are issues, and if so - with a good ear you can tell where the problems are. It's a very useful test that for some reason folks seem reluctant to do - the tool is cheap (I'm sure there are better ones, but...) and it doesn't take long.
Harbor Freight Leak Down Tester
Harbor Freight Leak Down Tester
Race Director
Joined: May 2005
Posts: 10,674
Likes: 122
From: Millington Illinois
St. Jude Donor '08-'09
To check a used engine look at the exhaust ports. They should all be the same shade of tan to almost white with no oily residue or excessive build up. Then remove the plugs. Again they should look all the same with similar coloration and no buildup or wet plugs. Check the engine for excessive build up of oil gunk on the outside showing bad gaskets and look under the valve cover or shine a flashlight down in the oil fill cap hole and check for sludge buildup inside. Pull the dipstick and check the condition of the oil. This is a very good indication of the overall condition of the engine.
Then you can hook a battery to the starter lug with the engine laying on the floor and hook a ground cable to the block and jump the starter terminals to crank the engine to check each cylinders compression with a compression guage. It is not as accurate as a leakdown test or if the engine is warm but all readings within 10% is a good sign the engine is a good one. I used to work at a salvage yard and we fired the engines up on an engine stand that was purpose built for this. This was back before computers were in cars (1976,77).
Then you can hook a battery to the starter lug with the engine laying on the floor and hook a ground cable to the block and jump the starter terminals to crank the engine to check each cylinders compression with a compression guage. It is not as accurate as a leakdown test or if the engine is warm but all readings within 10% is a good sign the engine is a good one. I used to work at a salvage yard and we fired the engines up on an engine stand that was purpose built for this. This was back before computers were in cars (1976,77).
Last edited by 63mako; Jan 26, 2010 at 02:39 PM.
Thread Starter
Burning Brakes
Joined: Oct 2009
Posts: 782
Likes: 1
From: Magic Valley ID
Sounds like the leakdown test is the way to go.......so how does one do that? I realize that's probably a dumb question, but I have never done one before...always the compression check. Any pointers on that procedure?
PS...thanks everyone for helping out a feller in a tight spot!
PS...thanks everyone for helping out a feller in a tight spot!
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Thread Starter
Burning Brakes
Joined: Oct 2009
Posts: 782
Likes: 1
From: Magic Valley ID
Looking on YouTube answered my question on how to use the Harbor Freight leakdown tester..... http://www.youtube.com/watch?v=UOHYr...eature=related
Racer
Joined: Nov 2004
Posts: 485
Likes: 1
From: Oakville ON
Looking on YouTube answered my question on how to use the Harbor Freight leakdown tester..... http://www.youtube.com/watch?v=UOHYr...eature=related
Racer
Joined: Mar 2008
Posts: 428
Likes: 0
mydejavooo,
63mako gave you some very good advice to start with. For people that really understand what a compression test is for, watching the gauge during the first couple of puffs tells an very important story of the condition of the rings when combined with a wet test. The leakdown test shows both overall valve condition and leakage past the rings. If a valve is leaking, the condition of the rings in that cylinder could still be in question depending on your experience level. Buying an engine out of a car doesn't give you the advantage of checking on the actual mileage (if its the original motor). Are you looking to put this motor directly into a vehicle or are you going to rebuild it? If it's going directly into a car, the minimum is a dry and wet compression test even if it's cold. Otherwise, try to borrow a good leakdown tester. My very first step would be what 63mako said, you may walk away after that.
63mako gave you some very good advice to start with. For people that really understand what a compression test is for, watching the gauge during the first couple of puffs tells an very important story of the condition of the rings when combined with a wet test. The leakdown test shows both overall valve condition and leakage past the rings. If a valve is leaking, the condition of the rings in that cylinder could still be in question depending on your experience level. Buying an engine out of a car doesn't give you the advantage of checking on the actual mileage (if its the original motor). Are you looking to put this motor directly into a vehicle or are you going to rebuild it? If it's going directly into a car, the minimum is a dry and wet compression test even if it's cold. Otherwise, try to borrow a good leakdown tester. My very first step would be what 63mako said, you may walk away after that.
